JANUS ASPEN SERIES
497, 1999-05-06
Previous: EUROWEB INTERNATIONAL CORP, S-3, 1999-05-06
Next: NTL COMMUNICATIONS CORP, 424B3, 1999-05-06



<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              Aggressive Growth Portfolio
                              Capital Appreciation Portfolio
                              International Growth Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
                              Flexible Income Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes seven mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital, current income and a combination of growth and income.
                Each Portfolio of Janus Aspen Series currently offers two
                classes of shares. The Institutional Shares, (the "Shares"), are
                sold under the name of "Janus Aspen Series" and are offered by
                this prospectus in connection with investment in and payments
                under variable annuity contracts and variable life insurance
                contracts, as well as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    7
                   Flexible Income Portfolio................................    9
                   Fees and expenses........................................   11
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................   12
                   Balanced Portfolio.......................................   14
                   Flexible Income Portfolio................................   15
                   General portfolio policies...............................   16
                   Risks for Growth, Global Growth and Balanced
                   Portfolios...............................................   19
                   Risks for Flexible Income Portfolios.....................   20
                   Risks Common to All Portfolios...........................   21
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   23
                   Management expenses and expense limits...................   23
                   Investment personnel.....................................   24
                OTHER INFORMATION...........................................   26
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   27
                   Taxes....................................................   27
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   28
                   Purchases................................................   28
                   Redemptions..............................................   28
                   Shareholder communications...............................   29
                FINANCIAL HIGHLIGHTS........................................   30
                GLOSSARY
                   Glossary of investment terms.............................   37
                RATING CATEGORIES
                   Explanation of rating categories.........................   41
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          DOMESTIC GROWTH PORTFOLIOS
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          - AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO
            seek long-term growth of capital.
 
          GLOBAL GROWTH PORTFOLIOS
 
          - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks
          selected for their growth potential, and normally invests at least 50%
          of its equity assets in medium-sized companies.
 
          CAPITAL APPRECIATION PORTFOLIO invests primarily in common stocks
          selected for their growth potential. The Portfolio may invest in
          companies of any size, from larger, well-established companies to
          smaller, emerging growth companies.
 
          INTERNATIONAL GROWTH PORTFOLIO normally invests at least 65% of its
          total assets in securities of issuers from at least five different
          countries, excluding the United States. Although the Portfolio intends
          to invest substantially all of its assets in issuers located outside
          the United States, it may invest in U.S. issuers and it may at times
          invest all of its assets in fewer than five countries, or even a
          single country.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
 2 Janus Aspen Series
<PAGE>
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO may have
          significant exposure to foreign markets. As a result, their returns
          and NAV may be affected to a large degree by fluctuations in currency
          exchange rates or political or economic conditions in a particular
          country.
 
          AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO are
          nondiversified. In other words, they may hold larger positions in a
          smaller number of securities than a diversified fund. As a result, a
          single security's increase or decrease in value may have a greater
          impact on a Portfolio's NAV and total return.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides some indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses, but do not include fees and expenses. The
          tables compare the average annual returns for the Shares of each
          Portfolio for the periods indicated to a broad-based securities market
          index.
 
                                                          Risk return summary  3
<PAGE>
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Aggressive Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                     16.33%   27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998
           
           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Aggressive Growth Portfolio - Institutional Shares        34.26%    19.35%         21.96%
                S&P 400 Mid Cap Index*                                    18.25%    18.67%         18.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic
             stocks chosen for their market size, liquidity and industry group
             representation.
 
 4 Janus Aspen Series
<PAGE>
 
           CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Capital Appreciation
           Portfolio - Institutional Shares for 1998:

           Annual returns for periods ended 12/31

                                                                         58.11%
                                                                          1998
           
           The percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  33.98%  Worst Quarter 3rd-1998 (9.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/1/97)
                <S>                                                              <C>       <C>           
                Capital Appreciation Portfolio - Institutional Shares            58.11%        51.65%
                S&P 500 Index*                                                   28.74%        31.38%
                                                                             -----------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for International Growth 
           Portfolio - Institutional Shares from 1995 through 1998:

           Annual returns for periods ended 12/31

                                              23.15%   34.71%   18.51%   17.23%
                                               1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  16.63%  Worst Quarter 3rd-1998 (17.76%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/2/94)
                <S>                                                              <C>       <C>           
                International Growth Portfolio - Institutional Shares            17.23%        18.87%
                Morgan Stanley Capital International EAFE Index*                 20.00%         8.11%
                                                                             -----------------------------
</TABLE>
 
           * The Morgan Stanley Capital International EAFE Index is a market
             capitalization weighted index composed of companies representative
             of the market structure of 20 Developed Market countries in Europe,
             Australasia and the Far East.
 
                                                          Risk return summary  5
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
 6 Janus Aspen Series
<PAGE>
 
BALANCED PORTFOLIO
 
          The Balanced Portfolio is designed for investors who primarily seek
          growth of capital with current income. It is not designed for
          investors who desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If the portfolio manager is unable to find such investments,
          much of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
                                                          Risk return summary  7
<PAGE>
 
          The following information provides some indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of the
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998
           
           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
 8 Janus Aspen Series
<PAGE>
 
FLEXIBLE INCOME PORTFOLIO
 
          Flexible Income Portfolio is designed for long-term investors who
          primarily seek current income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF FLEXIBLE INCOME PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - FLEXIBLE INCOME PORTFOLIO seeks to obtain maximum total return,
            consistent with preservation of capital.
 
          The Trustees may change the objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether it remains an appropriate investment for
          you. There is no guarantee that the Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF FLEXIBLE INCOME PORTFOLIO?
 
          In addition to considering economic factors such as the effect of
          interest rates on the Portfolio's investments, the portfolio manager
          applies a "bottom up" approach in choosing investments. In other
          words, he looks mostly for income-producing securities that meet his
          investment criteria one at a time. If the portfolio manager is unable
          to find such investments, the Portfolio's assets may be in cash or
          similar investments.
 
          Flexible Income Portfolio invests primarily in a wide variety of
          income-producing securities such as corporate bonds and notes,
          government securities and preferred stock. As a fundamental policy,
          the Portfolio will invest at least 80% of its assets in
          income-producing securities. The Portfolio may own an unlimited amount
          of high-yield/high-risk fixed-income securities, and these securities
          may be a big part of the portfolio.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN FLEXIBLE INCOME PORTFOLIO?
 
          Although Flexible Income Portfolio may be less volatile than funds
          that invest most of their assets in common stocks, the Portfolio's
          returns and yields will vary, and you could lose money.
 
          The Portfolio invests in a variety of fixed-income securities. A
          fundamental risk is that the value of these securities will fall if
          interest rates rise. Generally, the value of a fixed-income portfolio
          will decrease when interest rates rise, which means the Portfolio's
          NAV will likewise decrease. Another fundamental risk associated with
          fixed-income funds is credit risk, which is the risk that an issuer
          will be unable to make principal and interest payments when due.
 
          Flexible Income Portfolio may invest an unlimited amount of its assets
          in high-yield/high-risk securities, also known as "junk" bonds which
          may be sensitive to economic changes, political changes, or adverse
          developments specific to the company that issued the bond. These
          securities generally have a greater credit risk than other types of
          fixed-income securities. Because of these factors, the performance and
          NAV of the Portfolio may vary significantly, depending upon its
          holdings of junk bonds.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
                                                          Risk return summary  9
<PAGE>
 
          The following information provides some indication of the risks of
          investing in Flexible Income Portfolio by showing how Flexible Income
          Portfolio's performance has varied over time. The bar chart depicts
          the change in performance from year-to-year during the period
          indicated, but does not include charges and expenses attributable to
          any insurance product which would lower the performance illustrated.
          The Portfolio does not impose any sales or other charges that would
          affect total return computations. Total return figures include the
          effect of the Portfolio's expenses. The table compares the average
          annual returns for the Shares of the Portfolio for the periods
          indicated to a broad-based securities market index.
 
           FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Flexible Income
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                    (0.91%)  23.86%    9.19%   11.76%    9.11%
                                      1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 2nd-1995   6.71%  Worst Quarter 1st-1996 (1.08%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                                Since Inception
                                                                           1 year    5 years       (9/13/93)
                <S>                                                        <C>       <C>        <C>           
                Flexible Income Portfolio - Institutional Shares           9.11%     10.32%          9.87%
                Lehman Brothers Gov't/Corp Bond Index*                     9.47%      7.30%          6.90%
                                                                       ----------------------------------------
</TABLE>
 
           * Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Flexible Income Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
 10 Janus Aspen Series
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%                 N/A                  0.75%
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%               0.05%                  0.92%
    International Growth Portfolio           0.75%          0.20%          0.95%               0.09%                  0.86%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
    Flexible Income Portfolio                0.65%          0.08%          0.73%                 N/A                  0.73%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth, Aggressive
     Growth, Capital Appreciation, International Growth, Worldwide Growth and
     Balanced Portfolios reduce the Management Fee to the level of the
     corresponding Janus retail fund. Other waivers, if applicable, are first
     applied against the Management Fee and then against Other Expenses.
     Janus Capital has agreed to continue the waivers and fee reductions
     until at least the next annual renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>  
    Growth Portfolio                                               $ 77       $240      $  417      $  930
    Aggressive Growth Portfolio                                    $ 77       $240      $  417      $  930
    Capital Appreciation Portfolio                                 $ 99       $309      $  536      $1,190
    International Growth Portfolio                                 $ 97       $303      $  526      $1,166
    Worldwide Growth Portfolio                                     $ 76       $237      $  411      $  918
    Balanced Portfolio                                             $ 76       $237      $  411      $  918
    Flexible Income Portfolio                                      $ 75       $233      $  406      $  906
</TABLE>
 
                                                         Risk return summary  11
<PAGE>
Investment objectives, principal investment
           strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                              <C>
                  Growth Portfolio                                                 Janus Fund
                  Aggressive Growth Portfolio                           Janus Enterprise Fund
                  Capital Appreciation Portfolio                            Janus Twenty Fund*
                  International Growth Portfolio                          Janus Overseas Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
                  Flexible Income Portfolio                        Janus Flexible Income Fund
</TABLE>
 
          * Prior to May 1, 1999 Capital Appreciation Portfolio was managed in a
            similar manner to Janus Olympus Fund.
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 19-22 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
DOMESTIC GROWTH PORTFOLIOS
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO
          Aggressive Growth Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential, and normally invests at least 50% of its
          equity assets in medium-sized companies. Medium-sized companies are
          those whose market capitalizations fall within the range of companies
          in the S&P MidCap 400 Index. Market capitalization is a commonly used
          measure of the size and value of a company. The market capitalizations
          within the Index will vary, but as of December 31, 1998, they ranged
          from approximately $142 million to $73 billion.
 
 12 Janus Aspen Series
<PAGE>
 
          CAPITAL APPRECIATION PORTFOLIO
          Capital Appreciation Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential. The Portfolio may invest in companies of
          any size, from larger, well-established companies to smaller, emerging
          growth companies.
 
GLOBAL GROWTH PORTFOLIOS
 
          INTERNATIONAL GROWTH PORTFOLIO
          International Growth Portfolio seeks long-term growth of capital.
          Normally, the Portfolio pursues its objective by investing at least
          65% of its total assets in securities of issuers from at least five
          different countries, excluding the United States. Although the
          Portfolio intends to invest substantially all of its assets in issuers
          located outside the United States, it may at times invest in U.S.
          issuers and it may at times invest all of its assets in fewer than
          five countries or even a single country.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. As noted previously, market capitalization is
          an important investment criteria for
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
          Aggressive Growth Portfolio. Although the other Growth Portfolios
          offered by this Prospectus do not emphasize companies of any
          particular size, Portfolios with a larger asset base are more likely
          to invest in larger, more established issuers.
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in the Portfolio. Strategies and policies that are
          noted as "fundamental" cannot be changed without a shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 19-22 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
The following questions and answers are designed to help you better understand
Balanced Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of the Portfolio,
          the portfolio manager may consider dividend-paying characteristics to
          a greater degree in selecting common stocks for this Portfolio.
 
2. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of its holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, the Portfolio will place a greater emphasis on the growth
          component.
 
3. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of the Portfolio's investments is expected to
          consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
 14 Janus Aspen Series
<PAGE>
 
FLEXIBLE INCOME PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Flexible Income Portfolio, its principal investment strategies and
          certain risks of investing in the Portfolio. Strategies and policies
          that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 19-22 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          In addition to considering economic factors such as the effect of
          interest rates on the Portfolio's investments, the portfolio manager
          applies a "bottom up" approach in choosing investments. In other
          words, he looks mostly for income-producing securities that meet his
          investment criteria one at a time. If the portfolio manager is unable
          to find such investments, much of the Portfolio's assets may be in
          cash or similar investments.
 
          Flexible Income Portfolio seeks to obtain maximum total return,
          consistent with preservation of capital. It pursues its objective by
          primarily investing in a wide variety of income-producing securities
          such as corporate bonds and notes, government securities and preferred
          stock. As a fundamental policy, the Portfolio will invest at least 80%
          of its assets in income-producing securities. The Portfolio may own an
          unlimited amount of high-yield/high-risk securities, and these may be
          a big part of the portfolio. This Portfolio generates total return
          from a combination of current income and capital appreciation, but
          income is usually the dominant portion.
 
The following questions and answers are designed to help you better understand
Flexible Income Portfolio's principal investment strategies.
 
1. HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
 
          Generally, a fixed-income security will increase in value when
          interest rates fall and decrease in value when interest rates rise.
          Longer-term securities are generally more sensitive to interest rate
          changes than shorter-term securities, but they generally offer higher
          yields to compensate investors for the associated risks. High-yield
          bond prices are generally less directly responsive to interest rate
          changes than investment grade issues and may not always follow this
          pattern. A bond fund's average-weighted effective maturity and its
          duration are measures of how the fund may react to interest rate
          changes.
 
2. HOW DOES FLEXIBLE INCOME PORTFOLIO MANAGE INTEREST RATE RISK?
 
          The Portfolio may vary the average-weighted effective maturity of its
          assets to reflect its portfolio manager's analysis of interest rate
          trends and other factors. The Portfolio's average-weighted effective
          maturity will tend to be shorter when the portfolio manager expects
          interest rates to rise and longer when the portfolio manager expects
          interest rates to fall. The Portfolio may also use futures, options
          and other derivatives to manage interest rate risks.
 
3. WHAT IS MEANT BY THE PORTFOLIO'S "AVERAGE-WEIGHTED EFFECTIVE MATURITY"?
 
          The stated maturity of a bond is the date when the issuer must repay
          the bond's entire principal value to an investor. Some types of bonds
          may also have an "effective maturity" that is shorter than the stated
          date due to prepayment or call provisions. Securities without
          prepayment or call provisions generally have an effective maturity
          equal to their stated maturity. Dollar-weighted effective maturity is
          calculated by
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
 
          averaging the effective maturity of bonds held by the Portfolio with
          each effective maturity "weighted" according to the percentage of net
          assets that it represents.
 
4. WHAT IS MEANT BY THE PORTFOLIO'S "DURATION"?
 
          A bond's duration indicates the time it will take an investor to
          recoup his investment. Unlike average maturity, duration reflects both
          principal and interest payments. Generally, the higher the coupon rate
          on a bond, the lower its duration will be. The duration of a bond
          portfolio is calculated by averaging the duration of bonds held by a
          fund with each duration "weighted" according to the percentage of net
          assets that it represents. Because duration accounts for interest
          payments, the Portfolio's duration is usually shorter than its average
          maturity.
 
5. WHAT IS A HIGH-YIELD/HIGH-RISK SECURITY?
 
          A high-yield/high-risk security (also called a "junk" bond) is a debt
          security rated below investment grade by major rating agencies (i.e.,
          BB or lower by Standard & Poor's or Ba or lower by Moody's) or an
          unrated bond of similar quality. It presents greater risk of default
          (the failure to make timely interest and principal payments) than
          higher quality bonds.
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth and Global Growth Portfolios invest primarily in domestic
          and foreign equity securities, which may include preferred stocks,
          common stocks, warrants and securities convertible into common or
          preferred stocks. Balanced Portfolio also invest in domestic and
          foreign equity securities with varying degrees of emphasis on income.
          The Portfolios may also invest to a lesser degree in other types of
          securities. These securities (which are described in the Glossary) may
          include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
 16 Janus Aspen Series
<PAGE>
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          Flexible Income Portfolio invests primarily in fixed-income securities
          which may include corporate bonds and notes, government securities,
          preferred stock, high-yield/high-risk fixed-income securities and
          municipal obligations. The Portfolio may also invest to a lesser
          degree in other types of securities. These securities (which are
          described in the Glossary) may include:
 
          - common stocks
 
          - mortgage- and asset-backed securities
 
          - zero coupon, pay-in-kind and step coupon securities
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
            Investment objectives, principal investment strategies and risks  17
<PAGE>
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 18 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH, GLOBAL GROWTH AND BALANCED PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth, Global Growth and Balanced
Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL
   APPRECIATION PORTFOLIO AFFECT THEIR RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of a Portfolio.
 
            Investment objectives, principal investment strategies and risks  19
<PAGE>
 
RISKS FOR FLEXIBLE INCOME PORTFOLIO
 
          Because the Portfolio invests substantially all of its assets in
          fixed-income securities, it is subject to risks such as credit or
          default risks, and decreased value due to interest rate increases. The
          Portfolio's performance may also be affected by risks to certain types
          of investments, such as foreign securities and derivative instruments.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Flexible Income Portfolio.
 
1. WHAT IS MEANT BY "CREDIT QUALITY" AND WHAT ARE THE RISKS ASSOCIATED WITH IT?
 
          Credit quality measures the likelihood that the issuer will meet its
          obligations on a bond. One of the fundamental risks associated with
          all fixed-income funds is credit risk, which is the risk that an
          issuer will be unable to make principal and interest payments when
          due. U.S. government securities are generally considered to be the
          safest type of investment in terms of credit risk. Municipal
          obligations generally rank between U.S. government securities and
          corporate debt securities in terms of credit safety. Corporate debt
          securities, particularly those rated below investment grade, present
          the highest credit risk.
 
2. HOW IS CREDIT QUALITY MEASURED?
 
          Ratings published by nationally recognized statistical rating agencies
          such as Standard & Poor's Ratings Service and Moody's Investors
          Service, Inc. are widely accepted measures of credit risk. The lower a
          bond issue is rated by an agency, the more credit risk it is
          considered to represent. Lower rated bonds generally pay higher yields
          to compensate investors for the associated risk. Please refer to
          "Explanation of Rating Categories" on page 40 for a description of
          rating categories.
 
 20 Janus Aspen Series
<PAGE>
 
RISKS COMMON TO ALL PORTFOLIOS
 
The following questions and answers discuss risks that apply to all Portfolios.
 
1. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
2. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
          The junk bond market can experience sudden and sharp price swings.
          Because Flexible Income Portfolio may invest a significant portion of
          its assets in high-yield/high-risk securities, investors should be
          willing to tolerate a corresponding increase in the risk of
          significant and sudden changes in NAV.
 
          Please refer to "Explanation of Rating Categories" on page 40 for a
          description of bond rating categories.
 
            Investment objectives, principal investment strategies and risks  21
<PAGE>
 
3. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
4. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 22 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Growth Portfolio
     Aggressive Growth Portfolio                        First $300 Million        0.75               N/A(1)
     Capital Appreciation Portfolio                     Next $200 Million         0.70
     International Growth Portfolio                     Over $500 Million         0.65
     Worldwide Growth Portfolio
     Balanced Portfolio
- -------------------------------------------------------------------------------------------------------------
     Flexible Income Portfolio                          First $300 Million        0.65              1.00(2)
                                                        Over $300 Million         0.55
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive Growth, Capital
    Appreciation, International Growth, Worldwide Growth, and Balanced,
    Portfolio's management fee to the extent that such fee exceeds the effective
    rate of the Janus retail fund corresponding to such Portfolio. Janus Capital
    has agreed to continue such waivers until at least the next annual renewal
    of the advisory contracts. The effective rate is the management fee
    calculated by the corresponding retail fund as of the last day of each
    calendar quarter (expressed as an annual rate). The effective rates of Janus
    Fund, Janus Enterprise Fund, Janus Olympus Fund, Janus Overseas Fund, Janus
    Worldwide Fund and Janus Balanced Fund were 0.65%, 0.69%, 0.67%, 0.66%,
    0.65%, and 0.67%, respectively, for the quarter ended March 31, 1999.
 
(2) Janus Capital has agreed to limit the Portfolios' expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
                                                Management of the portfolios  23
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
            is Executive Vice President and co-manager of International
            Growth Portfolio and Janus Overseas Fund which he has co-managed
            since May 1998 and April 1998, respectively. He served as
            assistant portfolio manager for these funds since 1996. He is
            also assistant portfolio manager for Worldwide Growth Portfolio
            and Janus Worldwide Fund. Mr. Chang joined Janus Capital in 1993
            after receiving a Masters Degree in Political Science from
            Stanford University. He is a Chartered Financial Analyst.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Aggressive
            Growth Portfolio, which he has managed since inception. Mr. Goff
            joined Janus Capital in 1988 and has managed Janus Enterprise
            Fund since its inception. Mr. Goff co-managed or managed Janus
            Venture Fund from December 1993 to February 1, 1997. He holds a
            Bachelor of Arts in Economics from Yale University and is a
            Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
 24 Janus Aspen Series
<PAGE>
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Capital
            Appreciation Portfolio, which he has managed since its inception.
            He is portfolio manager of Janus Twenty Fund, which he has
            managed since August 1997. He previously managed Janus Olympus
            Fund from its inception to August 1997. Mr. Schoelzel joined
            Janus Capital in January 1994. He holds a Bachelor of Arts in
            Business from Colorado College.
 
RONALD V. SPEAKER
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Flexible
            Income Portfolio which he has managed or co-managed since its
            inception. He previously served as co-manager of High-Yield
            Portfolio, from its inception to May 1998. He managed Short-Term
            Bond Portfolio from its inception through April 1996. Mr. Speaker
            joined Janus Capital in 1986. He has managed or co-managed Janus
            Flexible Income Fund since December 1991 and previously managed
            both Janus Short-Term Bond Fund and Janus Federal Tax-Exempt Fund
            from inception through December 1995. He previously managed or
            co-managed Janus High-Yield Fund from its inception to February
            1998. He holds a Bachelor of Arts in Finance from the University
            of Colorado and is a Chartered Financial Analyst.
 
            In January 1997, Mr. Speaker settled an SEC administrative action
            involving two personal trades made by him in January of 1993.
            Without admitting or denying the allegations, Mr. Speaker agreed
            to civil money penalty, disgorgement, and interest payments
            totaling $37,199 and to a 90-day suspension which ended on April
            25, 1997.
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
RON SACHS
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Aggressive Growth Portfolio.
            Mr. Sachs joined Janus Capital in 1996 as a research analyst.
            Prior to coming to Janus, he worked as a consultant for Bain &
            Company and as an attorney for Willkie, Farr & Gallagher. Mr.
            Sachs graduated from Princeton cum laude with an undergraduate
            degree in economics. He obtained his law degree from the
            University of Michigan. Mr. Sachs is a Chartered Financial
            Analyst.
 
                                                Management of the portfolios  25
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 26 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
                                                     Distributions and taxes  27
<PAGE>
Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
 28 Janus Aspen Series
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
                                                         Shareholder's guide  29
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years or the life of the Portfolio if less than
          five years. Items 1 through 9 reflect financial results for a single
          Share. Total return in the tables represents the rate that an investor
          would have earned (or lost) on an investment in each of the
          Institutional Shares of the Portfolios (assuming reinvestment of all
          dividends and distributions) but does not include charges and expenses
          attributable to any insurance product. This information has been
          audited by PricewaterhouseCoopers LLP, whose report, along with the
          Portfolios' financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                     $18.48       $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                      0.05         0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                            6.36         3.34        2.29        2.90       0.20
  4. Total from investment operations                           6.41         3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    (0.05)      (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                           --          --          --          --         --
  7. Distributions (from capital gains)                        (1.30)      (0.37)      (0.23)          --         --
  8. Total distributions                                       (1.35)      (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                            $23.54      $18.48      $15.51      $13.45     $10.57
 10. Total return                                              35.66%      22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)              $1,103,549    $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)        $789,454    $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets              0.68%(6)    0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets                0.68%       0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                     0.26%       0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                      73%        122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 30 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- --------------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                             1998        1997        1996        1995        1994
<S>                                                        <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                    $20.55        $18.24      $17.08      $13.62     $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                       --            --          --        0.24       0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                           7.09          2.31        1.36        3.47       1.82
  4. Total from investment operations                          7.09          2.31        1.36        3.71       1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      --            --          --      (0.25)     (0.11)
  6. Tax return of capital distributions                         --            --      (0.01)          --         --
  7. Distributions (from capital gains)                          --            --      (0.19)          --         --
  8. Total distributions                                         --            --      (0.20)      (0.25)     (0.11)
  9. NET ASSET VALUE, END OF PERIOD                          $27.64        $20.55      $18.24      $17.08     $13.62
 10. Total return                                            34.26%        12.66%       7.95%      27.48%     16.33%
 11. Net assets, end of period (in thousands)              $772,943      $508,198    $383,693    $185,911    $41,289
 12. Average net assets for the period (in thousands)      $576,444      $418,464    $290,629    $107,582    $14,152
 13. Ratio of gross expenses to average net assets            0.75%(6)      0.76%(5)    0.76%(4)    0.86%(3)     N/A
 14. Ratio of net expenses to average net assets              0.75%         0.76%       0.76%       0.84%      1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 (0.36%)       (0.10%)     (0.27%)      0.58%       2.18%
 16. Portfolio turnover rate                                   132%          130%         88%       155%        259%
- --------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  31
<PAGE>
 
<TABLE>
<CAPTION>
               CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------
                                                                   Periods ending
                                                                    December 31
                                                                  1998      1997(1)
<S>                                                             <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $12.62      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.01        0.05
  3. Net gains or losses on securities (both realized and
     unrealized)                                                   7.32        2.61
  4. Total from investment operations                              7.33        2.66
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.01)      (0.04)
  6. Tax return of capital distributions                             --          --
  7. Distributions (from capital gains)                              --          --
  8. Total distributions                                         (0.01)      (0.04)
  9. NET ASSET VALUE, END OF PERIOD                              $19.94      $12.62
 10. Total return*                                               58.11%      26.60%
 11. Net assets, end of period (in thousands)                   $74,187      $6,833
 12. Average net assets for the period (in thousands)           $25,964      $2,632
 13. Ratio of gross expenses to average net assets**              0.92%(3)    1.26%(2)
 14. Ratio of net expenses to average net assets**                0.91%       1.25%
 15. Ratio of net investment income to average net assets**       0.27%       1.43%
 16. Portfolio turnover rate**                                      91%        101%
- ------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 2.19% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
(3) The ratio was 0.97% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
 
 32 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                 INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995      1994(1)
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $18.48     $15.72      $11.95       $9.72      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.13       0.11        0.05        0.09      (0.09)
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    3.07       2.80        4.06        2.16      (0.19)
  4. Total from investment operations                               3.20       2.91        4.11        2.25      (0.28)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.14)     (0.11)      (0.11)      (0.02)          --
  6. Dividends (in excess of net investment income)                   --         --          --          --          --
  7. Tax return of capital distributions                              --         --          --          --          --
  8. Distributions (from capital gains)                               --     (0.01)      (0.23)          --          --
  9. Distributions (in excess of realized gains)                  (0.27)     (0.03)          --          --          --
 10. Total distributions                                          (0.41)     (0.15)      (0.34)      (0.02)          --
 11. NET ASSET VALUE, END OF PERIOD                               $21.27     $18.48      $15.72      $11.95       $9.72
 12. Total return*                                                17.23%     18.51%      34.71%      23.15%     (2.80%)
 13. Net assets, end of period (in thousands)                   $311,110   $161,091     $27,192      $1,608      $1,353
 14. Average net assets for the period (in thousands)           $234,421    $96,164      $7,437      $1,792      $1,421
 15. Ratio of gross expenses to average net assets**               0.86%(6)   0.96%(5)    1.26%(4)    2.69%(3)      N/A
 16. Ratio of net expenses to average net assets**                 0.86%      0.96%       1.25%       2.50%       2.50%(2)
 17. Ratio of net investment income to average net assets**        0.73%      0.70%       0.62%     (0.80%)     (1.30%)
 18. Portfolio turnover rate**                                       93%        86%         65%        211%        275%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1994 (inception) through December 31, 1994.
(2) The ratio was 4.67% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(3) The ratio was 3.57% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(4) The ratio was 2.21% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(5) The ratio was 1.08% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(6) The ratio was 0.95% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
 
                                                        Financial highlights  33
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
 34 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
                                                        Financial highlights  35
<PAGE>
 
<TABLE>
<CAPTION>
                                    FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $11.78     $11.24      $11.11       $9.48       $9.97
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.64       0.67        0.74        0.53        0.47
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    0.41       0.62        0.24        1.70      (0.56)
  4. Total from investment operations                               1.05       1.29        0.98        2.23      (0.09)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.67)     (0.64)      (0.72)      (0.60)      (0.40)
  6. Tax return of capital distributions                              --         --          --          --          --
  7. Distributions (from capital gains)                           (0.11)     (0.11)      (0.13)          --          --
  8. Total distributions                                          (0.78)     (0.75)      (0.85)      (0.60)      (0.40)
  9. NET ASSET VALUE, END OF PERIOD                               $12.05     $11.78      $11.24      $11.11       $9.48
 10. Total return                                                  9.11%     11.76%       9.19%      23.86%     (0.91%)
 11. Net assets, end of period (in thousands)                   $129,582    $54,098     $25,315     $10,831      $1,924
 12. Average net assets for the period (in thousands)            $86,627    $36,547     $17,889      $5,556      $1,636
 13. Ratio of gross expenses to average net assets                 0.73%      0.75%       0.84%       1.07%         N/A
 14. Ratio of net expenses to average net assets                   0.73%      0.75%       0.83%       1.00%       1.00%(1)
 15. Ratio of net investment income to average net assets          6.36%      6.90%       7.31%       7.46%       5.49%
 16. Portfolio turnover rate                                        145%       119%        250%        236%        234%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) The ratio was 1.35% before waiver of certain fees incurred by the Portfolio.
 
 36 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
                                                Glossary of investment terms  37
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
 38 Janus Aspen Series
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
                                                Glossary of investment terms  39
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 40 Janus Aspen Series
<PAGE>
                                                Explanation of rating categories
 
          The following is a description of credit ratings issued by two of the
          major credit ratings agencies. Credit ratings evaluate only the safety
          of principal and interest payments, not the market value risk of lower
          quality securities. Credit rating agencies may fail to change credit
          ratings to reflect subsequent events on a timely basis. Although Janus
          Capital considers security ratings when making investment decisions,
          it also performs its own investment analysis and does not rely solely
          on the ratings assigned by credit agencies.
 
STANDARD & POOR'S
RATINGS SERVICES
 
<TABLE>
                <S>                          <C>
                BOND RATING                  EXPLANATION
                -----------------------------------------------------------------------------------------
                Investment Grade
                AAA......................... Highest rating; extremely strong capacity to pay principal
                                             and interest.
                AA.......................... High quality; very strong capacity to pay principal and
                                             interest.
                A........................... Strong capacity to pay principal and interest; somewhat more
                                             susceptible to the adverse effects of changing circumstances
                                             and economic conditions.
                BBB......................... Adequate capacity to pay principal and interest; normally
                                             exhibit adequate protection parameters, but adverse economic
                                             conditions or changing circumstances more likely to lead to
                                             a weakened capacity to pay principal and interest than for
                                             higher rated bonds.
                Non-Investment Grade
                BB, B, CCC, CC, C........... Predominantly speculative with respect to the issuer's
                                             capacity to meet required interest and principal payments.
                                             BB - lowest degree of speculation; C - the highest degree of
                                             speculation. Quality and protective characteristics
                                             outweighed by large uncertainties or major risk exposure to
                                             adverse conditions.
                D........................... In default.
</TABLE>
 
                                            Explanation of rating categories  41
<PAGE>
 
MOODY'S INVESTORS SERVICE, INC.
 
<TABLE>
                <S>                          <C>
                BOND RATING                  EXPLANATION
                -----------------------------------------------------------------------------------------
                Investment Grade
                Aaa......................... Highest quality, smallest degree of investment risk.
                Aa.......................... High quality; together with Aaa bonds, they compose the
                                             high-grade bond group.
                A........................... Upper-medium grade obligations; many favorable investment
                                             attributes.
                Baa......................... Medium-grade obligations; neither highly protected nor
                                             poorly secured. Interest and principal appear adequate for
                                             the present but certain protective elements may be lacking
                                             or may be unreliable over any great length of time.
                Non-Investment Grade
                Ba.......................... More uncertain, with speculative elements. Protection of
                                             interest and principal payments not well safeguarded during
                                             good and bad times.
                B........................... Lack characteristics of desirable investment; potentially
                                             low assurance of timely interest and principal payments or
                                             maintenance of other contract terms over time.
                Caa......................... Poor standing, may be in default; elements of danger with
                                             respect to principal or interest payments.
                Ca.......................... Speculative in a high degree; could be in default or have
                                             other marked shortcomings.
                C........................... Lowest-rated; extremely poor prospects of ever attaining
                                             investment standing.
</TABLE>
 
          Unrated securities will be treated as noninvestment grade securities
          unless a portfolio manager determines that such securities are the
          equivalent of investment grade securities. Securities that have
          received ratings from more than one agency are considered investment
          grade if at least one agency has rated the security investment grade.
 
 42 Janus Aspen Series
<PAGE>
 
SECURITIES HOLDINGS BY RATING CATEGORY
 
          During the fiscal period ended December 31, 1998, the percentage of
          securities holdings for Flexible Income Portfolio by rating category
          based upon a weighted monthly average was:
 
<TABLE>
<CAPTION>
                FLEXIBLE INCOME PORTFOLIO
                ----------------------------------------------------------------------------------------
                <S>                                                           <C>
                    BONDS-S&P RATING:
                 AAA                                                                       24%
                 AA                                                                         4%
                 A                                                                         13%
                 BBB                                                                       18%
                 BB                                                                        13%
                 B                                                                         15%
                 CCC                                                                        1%
                 CC                                                                         0%
                 C                                                                          0%
                 Preferred Stock                                                            2%
                 Cash and Options                                                          10%
                 TOTAL                                                                    100%
                ----------------------------------------------------------------------------------------
</TABLE>
 
          No other Portfolio described in this Prospectus held 5% or more of its
          assets in bonds rated below investment grade for the fiscal year ended
          December 31, 1998.
 
                                            Explanation of rating categories  43
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              Aggressive Growth Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
                              Flexible Income Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes five mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital, current income and a combination of growth and income.
                Each Portfolio of Janus Aspen Series currently offers two
                classes of shares. The Institutional Shares, (the "Shares"), are
                sold under the name of "Janus Aspen Series" and are offered by
                this prospectus in connection with investment in and payments
                under variable annuity contracts and variable life insurance
                contracts, as well as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    6
                   Flexible Income Portfolio................................    8
                   Fees and expenses........................................   10
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................   11
                   Balanced Portfolio.......................................   12
                   Flexible Income Portfolio................................   13
                   General portfolio policies...............................   15
                   Risks for Growth and Balanced Portfolios.................   17
                   Risks for Flexible Income Portfolio......................   18
                   Risks Common to All Portfolios...........................   19
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   21
                   Management expenses and expense limits...................   21
                   Investment personnel.....................................   22
                OTHER INFORMATION...........................................   24
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   25
                   Taxes....................................................   25
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   26
                   Purchases................................................   26
                   Redemptions..............................................   26
                   Shareholder communications...............................   27
                FINANCIAL HIGHLIGHTS........................................   28
                GLOSSARY
                   Glossary of investment terms.............................   33
                RATING CATEGORIES
                   Explanation of rating categories.........................   37
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          - AGGRESSIVE GROWTH PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks
          selected for their growth potential, and normally invests at least 50%
          of its equity assets in medium-sized companies.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          WORLDWIDE GROWTH PORTFOLIO may have significant exposure to foreign
          markets. As a result, its returns and NAV may be affected to a large
          degree by fluctuations in currency exchange rates or political or
          economic conditions in a particular country.
 
 2 Janus Aspen Series
<PAGE>
 
          AGGRESSIVE GROWTH PORTFOLIO is nondiversified. In other words, it may
          hold larger positions in a smaller number of securities than a
          diversified fund. As a result, a single security's increase or
          decrease in value may have a greater impact on the Portfolio's NAV and
          total return.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides an indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performances illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
                                                          Risk return summary  3
<PAGE>
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Aggressive Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                     16.33%   27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Aggressive Growth Portfolio - Institutional Shares        34.26%    19.35%         21.96%
                S&P 400 Mid Cap Index*                                    18.25%    18.67%         18.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic
             stocks chosen for their market size, liquidity and industry group
             representation.
 
 4 Janus Aspen Series
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
                                                          Risk return summary  5
<PAGE>
 
BALANCED PORTFOLIO
 
          The Balanced Portfolio is designed for investors who primarily seek
          growth of capital with current income. It is not designed for
          investors who desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If the portfolio manager is unable to find such investments,
          much of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
 6 Janus Aspen Series
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of the
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
                                                          Risk return summary  7
<PAGE>
 
FLEXIBLE INCOME PORTFOLIO
 
          Flexible Income Portfolio is designed for long-term investors who
          primarily seek current income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF FLEXIBLE INCOME PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - FLEXIBLE INCOME PORTFOLIO seeks to obtain maximum total return,
            consistent with preservation of capital.
 
          The Trustees may change the objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether it remains an appropriate investment for
          you. There is no guarantee that the Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF FLEXIBLE INCOME PORTFOLIOS?
 
          In addition to considering economic factors such as the effect of
          interest rates on the Portfolio's investments, the portfolio manager
          applies a "bottom up" approach in choosing investments. In other
          words, he looks mostly for income-producing securities that meet his
          investment criteria one at a time. If the portfolio manager is unable
          to find such investments, the Portfolio's assets may be in cash or
          similar investments.
 
          Flexible Income Portfolio invests primarily in a wide variety of
          income-producing securities such as corporate bonds and notes,
          government securities and preferred stock. As a fundamental policy,
          the Portfolio will invest at least 80% of its assets in
          income-producing securities. The Portfolio may own an unlimited amount
          of high-yield/high-risk fixed-income securities, and these securities
          may be a big part of the portfolio.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN FLEXIBLE INCOME PORTFOLIO?
 
          Although Flexible Income Portfolio may be less volatile than funds
          that invest most of their assets in common stocks, the Portfolio's
          returns and yields will vary, and you could lose money.
 
          The Portfolio invests in a variety of fixed-income securities. A
          fundamental risk is that the value of these securities will fall if
          interest rates rise. Generally, the value of a fixed-income portfolio
          will decrease when interest rates rise, which means the Portfolio's
          NAV will likewise decrease. Another fundamental risk associated with
          fixed-income funds is credit risk, which is the risk that an issuer
          will be unable to make principal and interest payments when due.
 
          Flexible Income Portfolio may invest an unlimited amount of its assets
          in high-yield/high-risk securities, also known as "junk" bonds which
          may be sensitive to economic changes, political changes, or adverse
          developments specific to the company that issued the bond. These
          securities generally have a greater credit risk than other types of
          fixed-income securities. Because of these factors, the performance and
          NAV of the Portfolio may vary significantly, depending upon its
          holdings of junk bonds.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
 8 Janus Aspen Series
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Flexible Income Portfolio by showing how Flexible Income
          Portfolio's performance has varied over time. The bar chart depicts
          the change in performance from year-to-year during the period
          indicated, but does not include charges and expenses attributable to
          any insurance product which would lower the performance illustrated.
          The Portfolio does not impose any sales or other charges that would
          affect total return computations. Total return figures include the
          effect of the Portfolio's expenses. The table compares the average
          annual returns for the Shares of the Portfolio for the periods
          indicated to a broad-based securities market index.
 
           FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Flexible Income    
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                    (0.91%)  23.86%    9.19%   11.76%    9.11%
                                      1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 2nd-1995   6.71%  Worst Quarter 1st-1996 (1.08%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                                Since Inception
                                                                           1 year    5 years       (9/13/93)
                <S>                                                        <C>       <C>        <C>           
                Flexible Income Portfolio - Institutional Shares           9.11%     10.32%          9.87%
                Lehman Brothers Gov't/Corp Bond Index*                     9.47%      7.30%          6.90%
                                                                       ----------------------------------------
</TABLE>
 
           * Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Flexible Income Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
                                                          Risk return summary  9
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%                 N/A                  0.75%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
    Flexible Income Portfolio                0.65%          0.08%          0.73%                 N/A                  0.73%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth, Aggressive
     Growth, Worldwide Growth and Balanced Portfolios reduce the Management
     Fee to the level of the corresponding Janus retail fund. Other waivers,
     if applicable, are first applied against the Management Fee and then
     against Other Expenses. Janus Capital has agreed to continue the waivers
     and fee reductions until at least the next annual renewal of the
     advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>   
    Growth Portfolio                                                $77       $240       $417        $930
    Aggressive Growth Portfolio                                     $77       $240       $417        $930
    Worldwide Growth Portfolio                                      $76       $237       $411        $918
    Balanced Portfolio                                              $76       $237       $411        $918
    Flexible Income Portfolio                                       $75       $233       $406        $906
</TABLE>
 
 10 Janus Aspen Series
<PAGE>
                                     Investment objectives, principal investment
                                                strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                              <C>
                  Growth Portfolio                                                 Janus Fund
                  Aggressive Growth Portfolio                           Janus Enterprise Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
                  Flexible Income Portfolio                        Janus Flexible Income Fund
</TABLE>
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 17-18 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO
          Aggressive Growth Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential, and normally invests at least 50% of its
          equity assets in medium-sized companies. Medium-sized companies are
          those whose market capitalizations fall within the range of companies
          in the S&P MidCap 400 Index. Market capitalization is a commonly used
          measure of the size and value of a company. The market capitalizations
          within the Index will vary, but as of December 31, 1998, they ranged
          from approximately $142 million to $73 billion.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
          countries, including the United States. The Portfolio may at times
          invest in fewer than five countries or even a single country.
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. As noted previously, market capitalization is
          an important investment criteria for Aggressive Growth Portfolio.
          Although the other Growth Portfolios offered by this Prospectus do not
          emphasize companies of any particular size, Portfolios with a larger
          asset base are more likely to invest in larger, more established
          issuers.
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in the Portfolio. Strategies and policies that are
          noted as "fundamental" cannot be changed without a shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 17-18 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
 12 Janus Aspen Series
<PAGE>
 
The following questions and answers are designed to help you better understand
Balanced Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of the Portfolio,
          the portfolio manager may consider dividend-paying characteristics to
          a greater degree in selecting common stocks for this Portfolio.
 
2. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of its holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, the Portfolio will place a greater emphasis on the growth
          component.
 
3. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of the Portfolio's investments is expected to
          consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
FLEXIBLE INCOME PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Flexible Income Portfolio, its principal investment strategies and
          certain risks of investing in the Portfolio. Strategies and policies
          that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 17-18 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          In addition to considering economic factors such as the effect of
          interest rates on the Portfolio's investments, the portfolio manager
          applies a "bottom up" approach in choosing investments. In other
          words, he looks mostly for income-producing securities that meet his
          investment criteria one at a time. If the portfolio manager is unable
          to find such investments, much of the Portfolio's assets may be in
          cash or similar investments.
 
          Flexible Income Portfolio seeks to obtain maximum total return,
          consistent with preservation of capital. It pursues its objective by
          primarily investing in a wide variety of income-producing securities
          such as corporate bonds and notes, government securities and preferred
          stock. As a fundamental policy, the Portfolio will invest at least 80%
          of its assets in income-producing securities. The Portfolio may own an
          unlimited amount of high-yield/high-risk securities, and these may be
          a big part of the portfolio. This Portfolio generates total return
          from a combination of current income and capital appreciation, but
          income is usually the dominant portion.
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
The following questions and answers are designed to help you better understand
Flexible Income Portfolio's principal investment strategies.
 
1. HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
 
          Generally, a fixed-income security will increase in value when
          interest rates fall and decrease in value when interest rates rise.
          Longer-term securities are generally more sensitive to interest rate
          changes than shorter-term securities, but they generally offer higher
          yields to compensate investors for the associated risks. High-yield
          bond prices are generally less directly responsive to interest rate
          changes than investment grade issues and may not always follow this
          pattern. A bond fund's average-weighted effective maturity and its
          duration are measures of how the fund may react to interest rate
          changes.
 
2. HOW DOES FLEXIBLE INCOME PORTFOLIO MANAGE INTEREST RATE RISK?
 
          The Portfolio may vary the average-weighted effective maturity of its
          assets to reflect its portfolio manager's analysis of interest rate
          trends and other factors. The Portfolio's average-weighted effective
          maturity will tend to be shorter when the portfolio manager expects
          interest rates to rise and longer when the portfolio manager expects
          interest rates to fall. The Portfolio may also use futures, options
          and other derivatives to manage interest rate risks.
 
3. WHAT IS MEANT BY THE PORTFOLIO'S "AVERAGE-WEIGHTED EFFECTIVE MATURITY"?
 
          The stated maturity of a bond is the date when the issuer must repay
          the bond's entire principal value to an investor. Some types of bonds
          may also have an "effective maturity" that is shorter than the stated
          date due to prepayment or call provisions. Securities without
          prepayment or call provisions generally have an effective maturity
          equal to their stated maturity. Dollar-weighted effective maturity is
          calculated by averaging the effective maturity of bonds held by the
          Portfolio with each effective maturity "weighted" according to the
          percentage of net assets that it represents.
 
4. WHAT IS MEANT BY THE PORTFOLIO'S "DURATION"?
 
          A bond's duration indicates the time it will take an investor to
          recoup his investment. Unlike average maturity, duration reflects both
          principal and interest payments. Generally, the higher the coupon rate
          on a bond, the lower its duration will be. The duration of a bond
          portfolio is calculated by averaging the duration of bonds held by a
          fund with each duration "weighted" according to the percentage of net
          assets that it represents. Because duration accounts for interest
          payments, the Portfolio's duration is usually shorter than its average
          maturity.
 
5. WHAT IS A HIGH-YIELD/HIGH-RISK SECURITY?
 
          A high-yield/high-risk security (also called a "junk" bond) is a debt
          security rated below investment grade by major rating agencies (i.e.,
          BB or lower by Standard & Poor's or Ba or lower by Moody's) or an
          unrated bond of similar quality. It presents greater risk of default
          (the failure to make timely interest and principal payments) than
          higher quality bonds.
 
 14 Janus Aspen Series
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          Balanced Portfolio also invests in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          Flexible Income Portfolio invests primarily in fixed-income securities
          which may include corporate bonds and notes, government securities,
          preferred stock, high-yield/high-risk fixed-income securities and
          municipal obligations. The Portfolio may also invest to a lesser
          degree in other types of securities. These securities (which are
          described in the Glossary) may include:
 
          - common stocks
 
          - mortgage- and asset-backed securities
 
          - zero coupon, pay-in-kind and step coupon securities
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 16 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH AND BALANCED PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in these Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AFFECT ITS
   RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of the Portfolio.
 
            Investment objectives, principal investment strategies and risks  17
<PAGE>
 
RISKS FOR FLEXIBLE INCOME PORTFOLIO
 
          Because the Portfolio invests substantially all of its assets in
          fixed-income securities, it is subject to risks such as credit or
          default risks, and decreased value due to interest rate increases. The
          Portfolio's performance may also be affected by risks to certain types
          of investments, such as foreign securities and derivative instruments.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Flexible Income Portfolio.
 
1. WHAT IS MEANT BY "CREDIT QUALITY" AND WHAT ARE THE RISKS ASSOCIATED WITH IT?
 
          Credit quality measures the likelihood that the issuer will meet its
          obligations on a bond. One of the fundamental risks associated with
          all fixed-income funds is credit risk, which is the risk that an
          issuer will be unable to make principal and interest payments when
          due. U.S. government securities are generally considered to be the
          safest type of investment in terms of credit risk. Municipal
          obligations generally rank between U.S. government securities and
          corporate debt securities in terms of credit safety. Corporate debt
          securities, particularly those rated below investment grade, present
          the highest credit risk.
 
2. HOW IS CREDIT QUALITY MEASURED?
 
          Ratings published by nationally recognized statistical rating agencies
          such as Standard & Poor's Ratings Service and Moody's Investors
          Service, Inc. are widely accepted measures of credit risk. The lower a
          bond issue is rated by an agency, the more credit risk it is
          considered to represent. Lower rated bonds generally pay higher yields
          to compensate investors for the associated risk. Please refer to
          "Explanation of Rating Categories" on page 36 for a description of
          rating categories.
 
 18 Janus Aspen Series
<PAGE>
 
RISKS COMMON TO ALL PORTFOLIOS
 
The following questions and answers discuss risks that apply to all Portfolios.
 
1. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
2. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
          The junk bond market can experience sudden and sharp price swings.
          Because Flexible Income Portfolio may invest a significant portion of
          its assets in high-yield/high-risk securities, investors should be
          willing to tolerate a corresponding increase in the risk of
          significant and sudden changes in NAV.
 
          Please refer to "Explanation of Rating Categories" on page 36 for a
          description of bond rating categories.
 
            Investment objectives, principal investment strategies and risks  19
<PAGE>
 
3. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
4. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 20 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Growth,                                            First $300 Million        0.75               N/A(1)
     Aggressive Growth,                                 Next $200 Million         0.70
     Worldwide Growth and                               Over $500 Million         0.65
     Balanced Portfolios
- -------------------------------------------------------------------------------------------------------------
     Flexible Income Portfolio                          First $300 Million        0.65              1.00(2)
                                                        Over $300 Million         0.55
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive Growth, Worldwide
    Growth and Balanced Portfolio's management fee to the extent that such fee
    exceeds the effective rate of the Janus retail fund corresponding to such
    Portfolio. Janus Capital has agreed to continue such waivers until at least
    the next annual renewal of the advisory contracts. The effective rate is the
    management fee calculated by the corresponding retail fund as of the last
    day of each calendar quarter (expressed as an annual rate). The effective
    rates of Janus Fund, Janus Enterprise Fund, Janus Worldwide Fund and Janus
    Balanced Fund were 0.65%, 0.69%, 0.65%, and 0.67%, respectively, for the
    quarter ended March 31, 1999.
 
(2) Janus Capital has agreed to limit the Portfolio's expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
                                                Management of the portfolios  21
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Aggressive
            Growth Portfolio, which he has managed since inception. Mr. Goff
            joined Janus Capital in 1988 and has managed Janus Enterprise
            Fund since its inception. Mr. Goff co-managed or managed Janus
            Venture Fund from December 1993 to February 1, 1997. He holds a
            Bachelor of Arts in Economics from Yale University and is a
            Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
 22 Janus Aspen Series
<PAGE>
 
RONALD V. SPEAKER
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Flexible
            Income Portfolio which he has managed or co-managed since its
            inception. He previously served as co-manager of High-Yield
            Portfolio, from its inception to May 1998. He managed Short-Term
            Bond Portfolio from its inception through April 1996. Mr. Speaker
            joined Janus Capital in 1986. He has managed or co-managed Janus
            Flexible Income Fund since December 1991 and previously managed
            both Janus Short-Term Bond Fund and Janus Federal Tax-Exempt Fund
            from inception through December 1995. He previously managed or
            co-managed Janus High-Yield Fund from its inception to February
            1998. He holds a Bachelor of Arts in Finance from the University
            of Colorado and is a Chartered Financial Analyst.
 
            In January 1997, Mr. Speaker settled an SEC administrative action
            involving two personal trades made by him in January of 1993.
            Without admitting or denying the allegations, Mr. Speaker agreed
            to civil money penalty, disgorgement, and interest payments
            totaling $37,199 and to a 90-day suspension which ended on April
            25, 1997.
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
RON SACHS
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Aggressive Growth Portfolio.
            Mr. Sachs joined Janus Capital in 1996 as a research analyst.
            Prior to coming to Janus, he worked as a consultant for Bain &
            Company and as an attorney for Willkie, Farr & Gallagher. Mr.
            Sachs graduated from Princeton cum laude with an undergraduate
            degree in economics. He obtained his law degree from the
            University of Michigan. Mr. Sachs is a Chartered Financial
            Analyst.
 
                                                Management of the portfolios  23
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 24 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
                                                     Distributions and taxes  25
<PAGE>
Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
 26 Janus Aspen Series
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
                                                         Shareholder's guide  27
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years. Items 1 through 9 reflect financial results
          for a single Share. Total return in the tables represents the rate
          that an investor would have earned (or lost) on an investment in each
          of the Institutional Shares of the Portfolios (assuming reinvestment
          of all dividends and distributions) but does not include charges and
          expenses attributable to any insurance product. This information has
          been audited by PricewaterhouseCoopers LLP, whose report, along with
          the Portfolios' financial statements, is included in the Annual
          Report, which is available upon request and incorporated by reference
          into the SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $18.48          $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.05            0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                         6.36            3.34        2.29        2.90       0.20
  4. Total from investment operations                        6.41            3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.05)          (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                       --              --          --          --         --
  7. Distributions (from capital gains)                    (1.30)          (0.37)      (0.23)          --         --
  8. Total distributions                                   (1.35)          (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                        $23.54          $18.48      $15.51      $13.45     $10.57
 10. Total return                                          35.66%          22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)          $1,103,549        $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)    $789,454        $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets          0.68%(6)        0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets            0.68%           0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 0.26%           0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                  73%            122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 28 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- -------------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                             1998        1997        1996        1995        1994
<S>                                                        <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $20.55        $18.24      $17.08      $13.62     $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                     --            --          --        0.24       0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                         7.09          2.31        1.36        3.47       1.82
  4. Total from investment operations                        7.09          2.31        1.36        3.71       1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    --            --          --      (0.25)     (0.11)
  6. Tax return of capital distributions                       --            --      (0.01)          --         --
  7. Distributions (from capital gains)                        --            --      (0.19)          --         --
  8. Total distributions                                       --            --      (0.20)      (0.25)     (0.11)
  9. NET ASSET VALUE, END OF PERIOD                        $27.64        $20.55      $18.24      $17.08     $13.62
 10. Total return                                          34.26%        12.66%       7.95%      27.48%     16.33%
 11. Net assets, end of period (in thousands)            $772,943      $508,198    $383,693    $185,911    $41,289
 12. Average net assets for the period (in thousands)    $576,444      $418,464    $290,629    $107,582    $14,152
 13. Ratio of gross expenses to average net assets          0.75%(6)      0.76%(5)    0.76%(4)    0.86%(3)     N/A
 14. Ratio of net expenses to average net assets            0.75%         0.76%       0.76%       0.84%      1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                               (0.36%)       (0.10%)     (0.27%)       0.58%      2.18%
 16. Portfolio turnover rate                                 132%          130%         88%        155%       259%
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  29
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
 30 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
                                                        Financial highlights  31
<PAGE>
 
<TABLE>
<CAPTION>
                                    FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $11.78     $11.24      $11.11       $9.48       $9.97
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.64       0.67        0.74        0.53        0.47
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    0.41       0.62        0.24        1.70      (0.56)
  4. Total from investment operations                               1.05       1.29        0.98        2.23      (0.09)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.67)     (0.64)      (0.72)      (0.60)      (0.40)
  6. Tax return of capital distributions                              --         --          --          --          --
  7. Distributions (from capital gains)                           (0.11)     (0.11)      (0.13)          --          --
  8. Total distributions                                          (0.78)     (0.75)      (0.85)      (0.60)      (0.40)
  9. NET ASSET VALUE, END OF PERIOD                               $12.05     $11.78      $11.24      $11.11       $9.48
 10. Total return                                                  9.11%     11.76%       9.19%      23.86%     (0.91%)
 11. Net assets, end of period (in thousands)                   $129,582    $54,098     $25,315     $10,831      $1,924
 12. Average net assets for the period (in thousands)            $86,627    $36,547     $17,889      $5,556      $1,636
 13. Ratio of gross expenses to average net assets                 0.73%      0.75%       0.84%       1.07%         N/A
 14. Ratio of net expenses to average net assets                   0.73%      0.75%       0.83%       1.00%       1.00%(1)
 15. Ratio of net investment income to average net assets          6.36%      6.90%       7.31%       7.46%       5.49%
 16. Portfolio turnover rate                                        145%       119%        250%        236%        234%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) The ratio was 1.35% before waiver of certain fees incurred by the Portfolio.
 
 32 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
                                                Glossary of investment terms  33
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
 34 Janus Aspen Series
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
                                                Glossary of investment terms  35
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 36 Janus Aspen Series
<PAGE>
                                                Explanation of rating categories
 
          The following is a description of credit ratings issued by two of the
          major credit ratings agencies. Credit ratings evaluate only the safety
          of principal and interest payments, not the market value risk of lower
          quality securities. Credit rating agencies may fail to change credit
          ratings to reflect subsequent events on a timely basis. Although Janus
          Capital considers security ratings when making investment decisions,
          it also performs its own investment analysis and does not rely solely
          on the ratings assigned by credit agencies.
 
STANDARD & POOR'S
RATINGS SERVICES
 
<TABLE>
                <S>                          <C>
                BOND RATING                  EXPLANATION
                -----------------------------------------------------------------------------------------
                Investment Grade
                AAA......................... Highest rating; extremely strong capacity to pay principal
                                             and interest.
                AA.......................... High quality; very strong capacity to pay principal and
                                             interest.
                A........................... Strong capacity to pay principal and interest; somewhat more
                                             susceptible to the adverse effects of changing circumstances
                                             and economic conditions.
                BBB......................... Adequate capacity to pay principal and interest; normally
                                             exhibit adequate protection parameters, but adverse economic
                                             conditions or changing circumstances more likely to lead to
                                             a weakened capacity to pay principal and interest than for
                                             higher rated bonds.
                Non-Investment Grade
                BB, B, CCC, CC, C........... Predominantly speculative with respect to the issuer's
                                             capacity to meet required interest and principal payments.
                                             BB - lowest degree of speculation; C - the highest degree of
                                             speculation. Quality and protective characteristics
                                             outweighed by large uncertainties or major risk exposure to
                                             adverse conditions.
                D........................... In default.
</TABLE>
 
                                            Explanation of rating categories  37
<PAGE>
 
MOODY'S INVESTORS SERVICE, INC.
 
<TABLE>
                <S>                          <C>
                BOND RATING                  EXPLANATION
                -----------------------------------------------------------------------------------------
                Investment Grade
                Aaa......................... Highest quality, smallest degree of investment risk.
                Aa.......................... High quality; together with Aaa bonds, they compose the
                                             high-grade bond group.
                A........................... Upper-medium grade obligations; many favorable investment
                                             attributes.
                Baa......................... Medium-grade obligations; neither highly protected nor
                                             poorly secured. Interest and principal appear adequate for
                                             the present but certain protective elements may be lacking
                                             or may be unreliable over any great length of time.
                Non-Investment Grade
                Ba.......................... More uncertain, with speculative elements. Protection of
                                             interest and principal payments not well safeguarded during
                                             good and bad times.
                B........................... Lack characteristics of desirable investment; potentially
                                             low assurance of timely interest and principal payments or
                                             maintenance of other contract terms over time.
                Caa......................... Poor standing, may be in default; elements of danger with
                                             respect to principal or interest payments.
                Ca.......................... Speculative in a high degree; could be in default or have
                                             other marked shortcomings.
                C........................... Lowest-rated; extremely poor prospects of ever attaining
                                             investment standing.
</TABLE>
 
          Unrated securities will be treated as noninvestment grade securities
          unless a portfolio manager determines that such securities are the
          equivalent of investment grade securities. Securities that have
          received ratings from more than one agency are considered investment
          grade if at least one agency has rated the security investment grade.
 
 38 Janus Aspen Series
<PAGE>
 
SECURITIES HOLDINGS BY RATING CATEGORY
 
          During the fiscal period ended December 31, 1998, the percentage of
          securities holdings for Flexible Income Portfolio by rating category
          based upon a weighted monthly average was:
 
<TABLE>
<CAPTION>
                                               FLEXIBLE INCOME PORTFOLIO
                ----------------------------------------------------------------------------------------
                <S>                                                                        <C>
                    BONDS-S&P RATING:
                 AAA                                                                       24%
                 AA                                                                         4%
                 A                                                                         13%
                 BBB                                                                       18%
                 BB                                                                        13%
                 B                                                                         15%
                 CCC                                                                        1%
                 CC                                                                         0%
                 C                                                                          0%
                 Preferred Stock                                                            2%
                 Cash and Options                                                          10%
                 TOTAL                                                                    100%
                ----------------------------------------------------------------------------------------
</TABLE>
 
          No other Portfolio described in this Prospectus held 5% or more of its
          assets in bonds rated below investment grade for the fiscal year ended
          December 31, 1998.
 
                                            Explanation of rating categories  39
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              Aggressive Growth Portfolio
                              Capital Appreciation Portfolio
                              International Growth Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes six mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital and a combination of growth and income. Each Portfolio
                of Janus Aspen Series currently offers two classes of shares.
                The Institutional Shares, (the "Shares"), are sold under the
                name of "Janus Aspen Series" and are offered by this prospectus
                in connection with investment in and payments under variable
                annuity contracts and variable life insurance contracts, as well
                as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    7
                   Fees and expenses........................................    9
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................   10
                   Balanced Portfolio.......................................   12
                   General portfolio policies...............................   13
                   Risks for Growth, Global Growth and Balanced
                   Portfolios...............................................   15
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   17
                   Management expenses and expense limits...................   17
                   Investment personnel.....................................   18
                OTHER INFORMATION...........................................   20
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   21
                   Taxes....................................................   21
                SHAREHOLDER'S GUIDE
                   Purchases................................................   22
                   Redemptions..............................................   22
                   Shareholder communications...............................   23
                FINANCIAL HIGHLIGHTS........................................   24
                GLOSSARY
                   Glossary of investment terms.............................   30
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          DOMESTIC GROWTH PORTFOLIOS
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          - AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO
            seek long-term growth of capital.
 
          GLOBAL GROWTH PORTFOLIOS
 
          - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks
          selected for their growth potential, and normally invests at least 50%
          of its equity assets in medium-sized companies.
 
          CAPITAL APPRECIATION PORTFOLIO invests primarily in common stocks
          selected for their growth potential. The Portfolio may invest in
          companies of any size, from larger, well-established companies to
          smaller, emerging growth companies.
 
          INTERNATIONAL GROWTH PORTFOLIO normally invests at least 65% of its
          total assets in securities of issuers from at least five different
          countries, excluding the United States. Although the Portfolio intends
          to invest substantially all of its assets in issuers located outside
          the United States, it may invest in U.S. issuers and it may at times
          invest all of its assets in fewer than five countries, or even a
          single country.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
 2 Janus Aspen Series
<PAGE>
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO may have
          significant exposure to foreign markets. As a result, their returns
          and NAV may be affected to a large degree by fluctuations in currency
          exchange rates or political or economic conditions in a particular
          country.
 
          AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO are
          nondiversified. In other words, they may hold larger positions in a
          smaller number of securities than a diversified fund. As a result, a
          single security's increase or decrease in value may have a greater
          impact on a Portfolio's NAV and total return.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides an indication of the risks of
          investing in the Growth Portfolio by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
                                                          Risk return summary  3
<PAGE>
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Aggressive Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                     16.33%   27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Aggressive Growth Portfolio - Institutional Shares        34.26%    19.35%         21.96%
                S&P 400 Mid Cap Index*                                    18.25%    18.67%         18.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic
             stocks chosen for their market size, liquidity and industry group
             representation.
 
 4 Janus Aspen Series
<PAGE>
 
           CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Capital Appreciation 
           Portfolio - Institutional Shares for 1998:

           Annual returns for periods ended 12/31

                                                                         58.11%
                                                                          1998

           The percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  33.98%  Worst Quarter 3rd-1998 (9.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/1/97)
                <S>                                                              <C>       <C>           
                Capital Appreciation Portfolio - Institutional Shares            58.11%        51.65%
                S&P 500 Index*                                                   28.74%        31.38%
                                                                             -----------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for International Growth 
           Portfolio - Institutional Shares from 1995 through 1998:

           Annual returns for periods ended 12/31

                                              23.15%   34.71%   18.51%   17.23%
                                               1995     1996     1997     1998
           
           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  16.63%  Worst Quarter 3rd-1998 (17.76%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/2/94)
                <S>                                                              <C>       <C>           
                International Growth Portfolio - Institutional Shares            17.23%        18.87%
                Morgan Stanley Capital International EAFE Index*                 20.00%         8.11%
                                                                             -----------------------------
</TABLE>
 
           * The Morgan Stanley Capital International EAFE Index is a market
             capitalization weighted index composed of companies representative
             of the market structure of 20 Developed Market countries in Europe,
             Australasia and the Far East.
 
                                                          Risk return summary  5
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
 6 Janus Aspen Series
<PAGE>
 
BALANCED PORTFOLIO
 
          The Balanced Portfolio is designed for investors who primarily seek
          growth of capital with current income. It is not designed for
          investors who desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If the portfolio manager is unable to find such investments,
          much of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
                                                          Risk return summary  7
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of the
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
 8 Janus Aspen Series
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%                 N/A                  0.75%
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%               0.05%                  0.92%
    International Growth Portfolio           0.75%          0.20%          0.95%               0.09%                  0.86%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth, Aggressive
     Growth, Capital Appreciation, International Growth, Worldwide Growth and
     Balanced Portfolios reduce the Management Fee to the level of the
     corresponding Janus retail fund. Other waivers, if applicable, are first
     applied against the Management Fee and then against Other Expenses.
     Janus Capital has agreed to continue the waivers and fee reductions
     until at least the next annual renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>    
    Growth Portfolio                                               $ 77       $240      $  417      $  930
    Aggressive Growth Portfolio                                    $ 77       $240      $  417      $  930
    Capital Appreciation Portfolio                                 $ 99       $309      $  536      $1,190
    International Growth Portfolio                                 $ 97       $303      $  526      $1,166
    Worldwide Growth Portfolio                                     $ 76       $237      $  411      $  918
    Balanced Portfolio                                             $ 76       $237      $  411      $  918
</TABLE>
 
                                                          Risk return summary  9
<PAGE>
Investment objectives, principal investment
           strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                 <S>                                                    <C>
                  Growth Portfolio                                                 Janus Fund
                  Aggressive Growth Portfolio                           Janus Enterprise Fund
                  Capital Appreciation Portfolio                           Janus Twenty Fund*
                  International Growth Portfolio                          Janus Overseas Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
</TABLE>
 
          *Prior to May 1, 1999 Capital Appreciation Portfolio was managed in a
           similar manner to Janus Olympus Fund.
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 15-16 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
DOMESTIC GROWTH PORTFOLIOS
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO
          Aggressive Growth Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential, and normally invests at least 50% of its
          equity assets in medium-sized companies. Medium-sized companies are
          those whose market capitalizations fall within the range of companies
          in the S&P MidCap 400 Index. Market capitalization is a commonly used
          measure of the size and value of a company. The market capitalizations
          within the Index will vary, but as of December 31, 1998, they ranged
          from approximately $142 million to $73 billion.
 
 10 Janus Aspen Series
<PAGE>
 
          CAPITAL APPRECIATION PORTFOLIO
          Capital Appreciation Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential. The Portfolio may invest in companies of
          any size, from larger, well-established companies to smaller, emerging
          growth companies.
 
GLOBAL GROWTH PORTFOLIOS
 
          INTERNATIONAL GROWTH PORTFOLIO
          International Growth Portfolio seeks long-term growth of capital.
          Normally, the Portfolio pursues its objective by investing at least
          65% of its total assets in securities of issuers from at least five
          different countries, excluding the United States. Although the
          Portfolio intends to invest substantially all of its assets in issuers
          located outside the United States, it may at times invest in U.S.
          issuers and it may at times invest all of its assets in fewer than
          five countries or even a single country.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. As noted previously, market capitalization is
          an important investment criteria for
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
          Aggressive Growth Portfolio. Although the other Growth Portfolios
          offered by this Prospectus do not emphasize companies of any
          particular size, Portfolios with a larger asset base are more likely
          to invest in larger, more established issuers.
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in the Portfolio. Strategies and policies that are
          noted as "fundamental" cannot be changed without a shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 15-16 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
The following questions and answers are designed to help you better understand
Balanced Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of the Portfolio,
          the portfolio manager may consider dividend-paying characteristics to
          a greater degree in selecting common stocks for this Portfolio.
 
2. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of its holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, the Portfolio will place a greater emphasis on the growth
          component.
 
3. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of the Portfolio's investments is expected to
          consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
 12 Janus Aspen Series
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          Balanced Portfolio also invest in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 14 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH, GLOBAL GROWTH AND BALANCED PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth, Global Growth and Balanced
Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL
   APPRECIATION PORTFOLIO AFFECT THEIR RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of a Portfolio.
 
3. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
5. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
6. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 16 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>         
     Growth Portfolio
     Aggressive Growth Portfolio                        First $300 Million        0.75               N/A(1)
     Capital Appreciation Portfolio                     Next $200 Million         0.70
     International Growth Portfolio                     Over $500 Million         0.65
     Worldwide Growth Portfolio
     Balanced Portfolio
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive Growth, Capital
    Appreciation, International Growth, Worldwide Growth, and Balanced
    Portfolio's management fee to the extent that such fee exceeds the effective
    rate of the Janus retail fund corresponding to such Portfolio. Janus Capital
    has agreed to continue such waivers until at least the next annual renewal
    of the advisory contracts. The effective rate is the management fee
    calculated by the corresponding retail fund as of the last day of each
    calendar quarter (expressed as an annual rate). The effective rates of Janus
    Fund, Janus Enterprise Fund, Janus Olympus Fund, Janus Overseas Fund, Janus
    Worldwide Fund and Janus Balanced Fund were 0.65%, 0.69%, 0.67%, 0.66%,
    0.65%, and 0.67%, respectively, for the quarter ended March 31, 1999.
 
                                                Management of the portfolios  17
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
            is Executive Vice President and co-manager of International
            Growth Portfolio and Janus Overseas Fund which he has co-managed
            since May 1998 and April 1998, respectively. He served as
            assistant portfolio manager for these funds since 1996. He is
            also assistant portfolio manager for Worldwide Growth Portfolio
            and Janus Worldwide Fund. Mr. Chang joined Janus Capital in 1993
            after receiving a Masters Degree in Political Science from
            Stanford University. He is a Chartered Financial Analyst.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Aggressive
            Growth Portfolio, which he has managed since inception. Mr. Goff
            joined Janus Capital in 1988 and has managed Janus Enterprise
            Fund since its inception. Mr. Goff co-managed or managed Janus
            Venture Fund from December 1993 to February 1, 1997. He holds a
            Bachelor of Arts in Economics from Yale University and is a
            Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
 18 Janus Aspen Series
<PAGE>
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Capital
            Appreciation Portfolio, which he has managed since its inception.
            He is portfolio manager of Janus Twenty Fund, which he has
            managed since August 1997. He previously managed Janus Olympus
            Fund from its inception to August 1997. Mr. Schoelzel joined
            Janus Capital in January 1994. He holds a Bachelor of Arts in
            Business from Colorado College.
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
RON SACHS
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Aggressive Growth Portfolio.
            Mr. Sachs joined Janus Capital in 1996 as a research analyst.
            Prior to coming to Janus, he worked as a consultant for Bain &
            Company and as an attorney for Willkie, Farr & Gallagher. Mr.
            Sachs graduated from Princeton cum laude with an undergraduate
            degree in economics. He obtained his law degree from the
            University of Michigan. Mr. Sachs is a Chartered Financial
            Analyst.
 
                                                Management of the portfolios  19
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 20 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
                                                     Distributions and taxes  21
<PAGE>
Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
 22 Janus Aspen Series
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
                                                         Shareholder's guide  23
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years or the life of the Portfolio if less than
          five years. Items 1 through 9 reflect financial results for a single
          Share. Total return in the tables represents the rate that an investor
          would have earned (or lost) on an investment in each of the
          Institutional Shares of the Portfolios (assuming reinvestment of all
          dividends and distributions) but does not include charges and expenses
          attributable to any insurance product. This information has been
          audited by PricewaterhouseCoopers LLP, whose report, along with the
          Portfolios' financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $18.48          $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.05            0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                         6.36            3.34        2.29        2.90       0.20
  4. Total from investment operations                        6.41            3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.05)          (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                       --              --          --          --         --
  7. Distributions (from capital gains)                    (1.30)          (0.37)      (0.23)          --         --
  8. Total distributions                                   (1.35)          (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                        $23.54          $18.48      $15.51      $13.45     $10.57
 10. Total return                                          35.66%          22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)          $1,103,549        $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)    $789,454        $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets          0.68%(6)        0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets            0.68%           0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 0.26%           0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                  73%            122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 24 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- -------------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                             1998        1997        1996        1995        1994
<S>                                                        <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $20.55        $18.24      $17.08      $13.62     $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                     --            --          --        0.24       0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                         7.09          2.31        1.36        3.47       1.82
  4. Total from investment operations                        7.09          2.31        1.36        3.71       1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    --            --          --      (0.25)     (0.11)
  6. Tax return of capital distributions                       --            --      (0.01)          --         --
  7. Distributions (from capital gains)                        --            --      (0.19)          --         --
  8. Total distributions                                       --            --      (0.20)      (0.25)     (0.11)
  9. NET ASSET VALUE, END OF PERIOD                        $27.64        $20.55      $18.24      $17.08     $13.62
 10. Total return                                          34.26%        12.66%       7.95%      27.48%     16.33%
 11. Net assets, end of period (in thousands)            $772,943      $508,198    $383,693    $185,911    $41,289
 12. Average net assets for the period (in thousands)    $576,444      $418,464    $290,629    $107,582    $14,152
 13. Ratio of gross expenses to average net assets          0.75%(6)      0.76%(5)    0.76%(4)    0.86%(3)     N/A
 14. Ratio of net expenses to average net assets            0.75%         0.76%       0.76%       0.84%      1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                               (0.36%)       (0.10%)     (0.27%)       0.58%      2.18%
 16. Portfolio turnover rate                                 132%          130%         88%        155%       259%
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  25
<PAGE>
 
<TABLE>
<CAPTION>
               CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------
                                                                   Periods ending
                                                                    December 31
                                                                  1998      1997(1)
<S>                                                             <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $12.62      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.01        0.05
  3. Net gains or losses on securities (both realized and
     unrealized)                                                   7.32        2.61
  4. Total from investment operations                              7.33        2.66
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.01)      (0.04)
  6. Tax return of capital distributions                             --          --
  7. Distributions (from capital gains)                              --          --
  8. Total distributions                                         (0.01)      (0.04)
  9. NET ASSET VALUE, END OF PERIOD                              $19.94      $12.62
 10. Total return*                                               58.11%      26.60%
 11. Net assets, end of period (in thousands)                   $74,187      $6,833
 12. Average net assets for the period (in thousands)           $25,964      $2,632
 13. Ratio of gross expenses to average net assets**              0.92%(3)    1.26%(2)
 14. Ratio of net expenses to average net assets**                0.91%       1.25%
 15. Ratio of net investment income to average net assets**       0.27%       1.43%
 16. Portfolio turnover rate**                                      91%        101%
- ------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 2.19% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
(3) The ratio was 0.97% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
 
 26 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                 INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995      1994(1)
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $18.48     $15.72      $11.95       $9.72      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.13       0.11        0.05        0.09      (0.09)
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    3.07       2.80        4.06        2.16      (0.19)
  4. Total from investment operations                               3.20       2.91        4.11        2.25      (0.28)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.14)     (0.11)      (0.11)      (0.02)          --
  6. Dividends (in excess of net investment income)                   --         --          --          --          --
  7. Tax return of capital distributions                              --         --          --          --          --
  8. Distributions (from capital gains)                               --     (0.01)      (0.23)          --          --
  9. Distributions (in excess of realized gains)                  (0.27)     (0.03)          --          --          --
 10. Total distributions                                          (0.41)     (0.15)      (0.34)      (0.02)          --
 11. NET ASSET VALUE, END OF PERIOD                               $21.27     $18.48      $15.72      $11.95       $9.72
 12. Total return*                                                17.23%     18.51%      34.71%      23.15%     (2.80%)
 13. Net assets, end of period (in thousands)                   $311,110    $161,091    $27,192      $1,608      $1,353
 14. Average net assets for the period (in thousands)           $234,421    $96,164      $7,437      $1,792      $1,421
 15. Ratio of gross expenses to average net assets**               0.86%(6)   0.96%(5)    1.26%(4)    2.69%(3)      N/A
 16. Ratio of net expenses to average net assets**                 0.86%      0.96%       1.25%       2.50%       2.50%(2)
 17. Ratio of net investment income to average net assets**        0.73%      0.70%       0.62%     (0.80%)     (1.30%)
 18. Portfolio turnover rate**                                       93%        86%         65%        211%        275%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1994 (inception) through December 31, 1994.
(2) The ratio was 4.67% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(3) The ratio was 3.57% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(4) The ratio was 2.21% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(5) The ratio was 1.08% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(6) The ratio was 0.95% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
 
                                                        Financial highlights  27
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
 28 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
                                                        Financial highlights  29
<PAGE>
Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
 30 Janus Aspen Series
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
                                                Glossary of investment terms  31
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
 32 Janus Aspen Series
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
                                                Glossary of investment terms  33
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
                              Growth and Income Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes four mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital and a combination of growth and income. Each Portfolio
                of Janus Aspen Series currently offers two classes of shares.
                The Institutional Shares, (the "Shares"), are sold under the
                name of "Janus Aspen Series" and are offered by this prospectus
                in connection with investment in and payments under variable
                annuity contracts and variable life insurance contracts, as well
                as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Combination Portfolios...................................    5
                   Fees and expenses........................................    7
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................    8
                   Combination Portfolios...................................    9
                   General portfolio policies...............................   11
                   Risks for Growth and Combination Portfolios..............   13
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   15
                   Management expenses and expense limits...................   15
                   Investment personnel.....................................   16
                OTHER INFORMATION...........................................   18
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   19
                   Taxes....................................................   19
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   20
                   Purchases................................................   20
                   Redemptions..............................................   20
                   Shareholder communications...............................   21
                FINANCIAL HIGHLIGHTS........................................   22
                GLOSSARY
                   Glossary of investment terms.............................   26
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF THE PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO seek long-term
            growth of capital in a manner consistent with the preservation of
            capital.
 
          The Portfolios' Trustees may change the objective without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in either of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          WORLDWIDE GROWTH PORTFOLIO may have significant exposure to foreign
          markets. As a result, its returns and NAV may be affected to a large
          degree by fluctuations in currency exchange rates or political or
          economic conditions in a particular country.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
 2 Janus Aspen Series
<PAGE>
 
          The following information provides some indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
                                                          Risk return summary  3
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
     
           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994     1995     1996     1997     1998

           East percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
 4 Janus Aspen Series
<PAGE>
 
COMBINATION PORTFOLIOS
 
          The Combination Portfolios are designed for investors who primarily
          seek growth of capital with varying degrees of emphasis on income.
          They are not designed for investors who desire a consistent level of
          income.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE COMBINATION PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          - GROWTH AND INCOME PORTFOLIO seeks long-term capital growth and
            current income.
 
          The Trustees may change these objectives without a shareholder vote
          and the Portfolios will notify you of any changes that are material.
          If there is a material change to a Portfolio's objective or policies,
          you should consider whether that Portfolio remains an appropriate
          investment for you. There is no guarantee that a Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE COMBINATION PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look mostly for equity and
          income-producing securities that meet their investment criteria one at
          a time. If a portfolio manager is unable to find such investments,
          much of a Portfolio's assets may be in cash or similar investments.
 
          BALANCED PORTFOLIO normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
          GROWTH AND INCOME PORTFOLIO normally emphasizes investments in common
          stocks. It will normally invest up to 75% of its assets in equity
          securities selected primarily for their growth potential, and at least
          25% of its assets in securities the portfolio manager believes have
          income potential. Equity securities may make up part of this income
          component if they currently pay dividends or the portfolio manager
          believes they have potential for increasing or commencing dividend
          payments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE COMBINATION PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in either of the
          Combination Portfolios, remember that they are each designed for
          long-term investors who can accept the risks of investing in a
          portfolio with significant common stock holdings. Common stocks tend
          to be more volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in a Portfolio you would get back less money.
 
          The income component of the Portfolios' holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
                                                          Risk return summary  5
<PAGE>
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides some indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolios do not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of each
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%) 
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
           GROWTH AND INCOME PORTFOLIO -- INSTITUTIONAL SHARES
 
          Growth and Income Portfolio does not have a full calendar year return
          because the Portfolio commenced operations on May 1, 1998.
 
          The Combination Portfolios' past performance does not necessarily
          indicate how they will perform in the future.
 
 6 Janus Aspen Series
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
    Growth and Income Portfolio              0.75%          2.31%          3.06%               1.81%                  1.25%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth, Worldwide
     Growth, Balanced and Growth and Income Portfolios reduce the Management
     Fee to the level of the corresponding Janus retail fund. Other waivers,
     if applicable, are first applied against the Management Fee and then
     against Other Expenses. Janus Capital has agreed to continue the waivers
     and fee reductions until at least the next annual renewal of the
     advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>   
    Growth Portfolio                                               $ 77       $240      $  417      $  930
    Worldwide Growth Portfolio                                     $ 76       $237      $  411      $  918
    Balanced Portfolio                                             $ 76       $237      $  411      $  918
    Growth and Income Portfolio                                    $309       $945      $1,606      $3,374
</TABLE>
 
                                                          Risk return summary  7
<PAGE>
Investment objectives, principal investment
           strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                            <C>
                  Growth Portfolio                                                 Janus Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
                  Growth and Income Portfolio                    Janus Growth and Income Fund
</TABLE>
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 24-27 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
 8 Janus Aspen Series
<PAGE>
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. Although the Portfolios offered by this
          Prospectus do not emphasize companies of any particular size,
          Portfolios with a larger asset base are more likely to invest in
          larger, more established issuers.
 
COMBINATION PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Combination Portfolios, their principal investment strategies
          and certain risks of investing in the Combination Portfolios.
          Strategies and policies that are noted as "fundamental" cannot be
          changed without a shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 13-14 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          BALANCED PORTFOLIO
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
             Investment objectives, principal investment strategies and risks  9
<PAGE>
 
          GROWTH AND INCOME PORTFOLIO
          Growth and Income Portfolio seeks long-term capital growth and current
          income. It normally emphasizes investments in common stocks. It will
          normally invest up to 75% of its assets in equity securities selected
          primarily for their growth potential, and at least 25% of its assets
          in securities the portfolio manager believes have income potential.
          Because of this investment strategy, the Portfolio is not designed for
          investors who need consistent income.
 
The following questions and answers are designed to help you better understand
the Combination Portfolios' principal investment strategies.
 
1. HOW DO THE COMBINATION PORTFOLIOS DIFFER FROM EACH OTHER?
 
          Growth and Income Portfolio places a greater emphasis on aggressive
          growth stocks and may derive a greater portion of its income from
          dividend-paying common stocks. Because of these factors, its NAV can
          be expected to fluctuate more than Balanced Portfolio. Balanced
          Portfolio places a greater emphasis on the income component of its
          portfolio and invests to a greater degree in securities selected
          primarily for their income potential. As a result it is expected to be
          less volatile than Growth and Income Portfolio.
 
2. HOW ARE COMMON STOCKS SELECTED FOR THE COMBINATION PORTFOLIOS IN COMPARISON
   TO THE GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of the
          Combination Portfolios, a portfolio manager may consider
          dividend-paying characteristics to a greater degree in selecting
          common stocks for these Portfolios.
 
3. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S AND GROWTH AND INCOME PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio and Growth and Income Portfolio shift assets
          between the growth and income components of their holdings based on
          the portfolio managers' analysis of relevant market, financial and
          economic conditions. If a portfolio manager believes that growth
          securities will provide better returns than the yields then available
          or expected on income-producing securities, that Portfolio will place
          a greater emphasis on the growth component.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF THE COMBINATION
   PORTFOLIOS' INVESTMENTS?
 
          The growth component of the Combination Portfolios' investments is
          expected to consist primarily of common stocks, but may also include
          warrants, preferred stocks or convertible securities selected
          primarily for their growth potential.
 
5. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   AND GROWTH AND INCOME PORTFOLIO'S HOLDINGS?
 
          The income component of Balanced Portfolio and Growth and Income
          Portfolio will consist of securities that the portfolio managers
          believe have income potential. Such securities may include equity
          securities, convertible securities and all types of debt securities.
          Equity securities may be included in the income component of a
          Portfolio if they currently pay dividends or the portfolio manager
          believes they have the potential for either increasing their dividends
          or commencing dividends, if none are currently paid.
 
 10 Janus Aspen Series
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          The Combination Portfolios also invest in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis.
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 12 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH AND COMBINATION PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth and Combination Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
3. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
4. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
5. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 14 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- --------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Growth,                                            First $300 Million        0.75               N/A(1)
     Worldwide Growth and                               Next $200 Million         0.70
     Balanced Portfolios                                Over $500 Million         0.65
- --------------------------------------------------------------------------------------------------------------
     Growth and Income Portfolio                        First $300 Million        0.75
                                                        Next $200 Million         0.70              1.25(1)(2)
                                                        Over $500 Million         0.65
- --------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Worldwide Growth, Balanced and
    Growth and Income Portfolio's management fee to the extent that such fee
    exceeds the effective rate of the Janus retail fund corresponding to such
    Portfolio. Janus Capital has agreed to continue such waivers until at least
    the next annual renewal of the advisory contracts. The effective rate is the
    management fee calculated by the corresponding retail fund as of the last
    day of each calendar quarter (expressed as an annual rate). The effective
    rates of Janus Fund, Janus Worldwide Fund, Janus Balanced Fund and Janus
    Growth and Income Fund were 0.65%, 0.65%, 0.67%, and 0.66%, respectively,
    for the quarter ended March 31, 1999.
 
(2) Janus Capital has agreed to limit the Portfolio's expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
                                                Management of the portfolios  15
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
DAVID J. CORKINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Growth and
            Income Portfolio which he has managed since its inception. He is
            Executive Vice President and portfolio manager of Janus Growth
            and Income Fund which he has managed since August 1997. He
            previously served as an assistant portfolio manager of Janus
            Mercury Fund. He joined Janus in 1995 as a research analyst
            specializing in domestic financial services companies and a
            variety of foreign industries. Prior to joining Janus, he was the
            Chief Financial Officer of Chase U.S. Consumer Services, Inc., a
            Chase Manhattan mortgage business. He holds a Bachelor of Arts in
            English and Russian from Dartmouth and received his Master of
            Business Administration from Columbia University in 1993.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
 16 Janus Aspen Series
<PAGE>
 
ASSISTANT PORTFOLIO MANAGER
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
                                                Management of the portfolios  17
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services
          provided. Because the expenses of each class may differ, the
          performance of each class is expected to differ. If you would like
          additional information about the Retirement Shares, please call
          1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 18 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
                                                     Distributions and taxes  19
<PAGE>
Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
 20 Janus Aspen Series
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
                                                         Shareholder's guide  21
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years or the life of the Portfolio if less than
          five years. Items 1 through 9 reflect financial results for a single
          Share. Total return in the tables represents the rate that an investor
          would have earned (or lost) on an investment in each of the
          Institutional Shares of the Portfolios (assuming reinvestment of all
          dividends and distributions) but does not include charges and expenses
          attributable to any insurance product. This information has been
          audited by PricewaterhouseCoopers LLP, whose report, along with the
          Portfolios' financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $18.48          $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.05            0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                         6.36            3.34        2.29        2.90       0.20
  4. Total from investment operations                        6.41            3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.05)          (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                       --              --          --          --         --
  7. Distributions (from capital gains)                    (1.30)          (0.37)      (0.23)          --         --
  8. Total distributions                                   (1.35)          (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                        $23.54          $18.48      $15.51      $13.45     $10.57
 10. Total return                                          35.66%          22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)          $1,103,549        $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)    $789,454        $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets          0.68%(6)        0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets            0.68%           0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 0.26%           0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                  73%            122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 22 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
                                                        Financial highlights  23
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
 24 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
             GROWTH AND INCOME PORTFOLIO - INSTITUTIONAL SHARES
- -----------------------------------------------------------------------------
                                                                Period ending
                                                                 December 31
                                                                   1998(1)
<S>                                                             <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.02
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                      1.96
  4. Total from investment operations                                 1.98
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.02)
  6. Tax return of capital distributions                                --
  7. Distributions (from capital gains)                                 --
  8. Total distributions                                            (0.02)
  9. NET ASSET VALUE, END OF PERIOD                                 $11.96
 10. Total return*                                                  19.80%
 11. Net assets, end of period (in thousands)                       $6,413
 12. Average net assets for the period (in thousands)               $2,883
 13. Ratio of gross expenses to average net assets**                 1.25%(2)
 14. Ratio of net expenses to average net assets**                   1.25%
 15. Ratio of net investment income to average net assets**          0.66%
 16. Portfolio turnover rate**                                         62%
- -----------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized.
**  Annualized.
(1) May 1, 1998 (inception) through December 31, 1998.
(2) The ratio was 3.06% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Growth and Income Fund.
 
                                                        Financial highlights  25
<PAGE>
Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
 26 Janus Aspen Series
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
                                                Glossary of investment terms  27
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
 28 Janus Aspen Series
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
                                                Glossary of investment terms  29
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>

 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Aggressive Growth Portfolio
                              Capital Appreciation Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes four mutual funds (the "Portfolios")
                with different investment objectives, including growth of
                capital and a combination of growth and income. Each Portfolio
                of Janus Aspen Series currently offers two classes of shares.
                The Institutional Shares, (the "Shares"), are sold under the
                name of "Janus Aspen Series" and are offered by this prospectus
                in connection with investment in and payments under variable
                annuity contracts and variable life insurance contracts, as well
                as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    6
                   Fees and expenses........................................    8
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................    9
                   Balanced Portfolio.......................................   11
                   General portfolio policies...............................   12
                   Risks for the Growth and Balanced Portfolios.............   14
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   16
                   Management expenses and expense limits...................   16
                   Investment personnel.....................................   17
                OTHER INFORMATION...........................................   19
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   20
                   Taxes....................................................   20
                SHAREHOLDER'S GUIDE
                   Pricing of Portfolio Shares..............................   21
                   Purchases................................................   21
                   Redemptions..............................................   21
                   Shareholder communications...............................   22
                FINANCIAL HIGHLIGHTS........................................   23
                GLOSSARY
                   Glossary of investment terms.............................   27
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO
            seek long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks
          selected for their growth potential, and normally invests at least 50%
          of its equity assets in medium-sized companies.
 
          CAPITAL APPRECIATION PORTFOLIO invests primarily in common stocks
          selected for their growth potential. The Portfolio may invest in
          companies of any size, from larger, well-established companies to
          smaller, emerging growth companies.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          WORLDWIDE GROWTH PORTFOLIO may have significant exposure to foreign
          markets. As a result, its returns and NAV may be affected to a large
          degree by fluctuations in currency exchange rates or political or
          economic conditions in a particular country.
 
 2 Janus Aspen Series
<PAGE>
 
          AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO are
          nondiversified. In other words, they may hold larger positions in a
          smaller number of securities than a diversified fund. As a result, a
          single security's increase or decrease in value may have a greater
          impact on a Portfolio's NAV and total return.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides an indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
                                                          Risk return summary  3
<PAGE>
 
           AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Aggressive Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                     16.33%   27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Aggressive Growth Portfolio - Institutional Shares        34.26%    19.35%         21.96%
                S&P 400 Mid Cap Index*                                    18.25%    18.67%         18.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic
             stocks chosen for their market size, liquidity and industry group
             representation.
 
           CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Capital Appreciation 
           Portfolio - Institutional Shares for 1998:

           Annual returns for periods ended 12/31

                                                                         58.11%
                                                                          1998

           The percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.           

           Best Quarter 4th-1998  33.98%  Worst Quarter 3rd-1998 (9.98%) 

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/1/97)
                <S>                                                              <C>       <C>           
                Capital Appreciation Portfolio - Institutional Shares            58.11%        51.65%
                S&P 500 Index*                                                   28.74%        31.38%
                                                                             -----------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
 4 Janus Aspen Series
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
                                                          Risk return summary  5
<PAGE>
 
BALANCED PORTFOLIO
 
          The Balanced Portfolio is designed for investors who primarily seek
          growth of capital with current income. It is not designed for
          investors who desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If the portfolio manager is unable to find such investments,
          much of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in this Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
 6 Janus Aspen Series
<PAGE>
 
          The following information provides some indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of the
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%) 

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolios' past performance does not necessarily indicate
          how it will perform in the future.
 
                                                          Risk return summary  7
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%                 N/A                  0.75%
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%               0.05%                  0.92%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Aggressive Growth,
     Capital Appreciation, Worldwide Growth and Balanced Portfolios reduce
     the Management Fee to the level of the corresponding Janus retail fund.
     Other waivers, if applicable, are first applied against the Management
     Fee and then against Other Expenses. Janus Capital has agreed to
     continue the waivers and fee reductions until at least the next annual
     renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>    
    Aggressive Growth Portfolio                                    $ 77       $240      $  417      $  930
    Capital Appreciation Portfolio                                 $ 99       $309      $  536      $1,190
    Worldwide Growth Portfolio                                     $ 76       $237      $  411      $  918
    Balanced Portfolio                                             $ 76       $237      $  411      $  918
</TABLE>
 
 8 Janus Aspen Series
<PAGE>
                                     Investment objectives, principal investment
                                                strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                                   <C>
                  Aggressive Growth Portfolio                           Janus Enterprise Fund
                  Capital Appreciation Portfolio                           Janus Twenty Fund*
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
</TABLE>
 
                        * Prior to May 1, 1999, Capital Appreciation Portfolio
                          was managed in a similar manner to Janus Olympus Fund.
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 14-15 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          AGGRESSIVE GROWTH PORTFOLIO
          Aggressive Growth Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential, and normally invests at least 50% of its
          equity assets in medium-sized companies. Medium-sized companies are
          those whose market capitalizations fall within the range of companies
          in the S&P MidCap 400 Index. Market capitalization is a commonly used
          measure of the size and value of a company. The market capitalizations
          within the Index will vary, but as of December 31, 1998, they ranged
          from approximately $142 million to $73 billion.
 
          CAPITAL APPRECIATION PORTFOLIO
          Capital Appreciation Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential. The Portfolio may invest in companies of
          any size, from larger, well-established companies to smaller, emerging
          growth companies.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different
 
             Investment objectives, principal investment strategies and risks  9
<PAGE>
 
          countries, including the United States. The Portfolio may at times
          invest in fewer than five countries or even a single country.
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. As noted previously, market capitalization is
          an important investment criteria for Aggressive Growth Portfolio.
          Although the other Growth Portfolios offered by this Prospectus do not
          emphasize companies of any particular size, Portfolios with a larger
          asset base are more likely to invest in larger, more established
          issuers.
 
 10 Janus Aspen Series
<PAGE>
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in Balanced Portfolio. Strategies and policies that
          are noted as "fundamental" cannot be changed without a shareholder
          vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 14-15 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
The following questions and answers are designed to help you better understand
the Combination Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of Balanced
          Portfolio, the portfolio manager may consider dividend-paying
          characteristics to a greater degree in selecting common stocks for
          this Portfolio.
 
2. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of its holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, the Portfolio will place a greater emphasis on the growth
          component.
 
3. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of Balanced Portfolio's investments is expected
          to consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          Balanced Portfolio also invests in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
 12 Janus Aspen Series
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
RISKS FOR THE GROWTH AND BALANCED PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth and Balanced Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL
   APPRECIATION PORTFOLIO AFFECT THEIR RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of a Portfolio.
 
3. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
 14 Janus Aspen Series
<PAGE>
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
5. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
6. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Aggressive Growth,                                 First $300 Million        0.75               N/A(1)
     Capital Appreciation,                              Next $200 Million         0.70
     Worldwide Growth and                               Over $500 Million         0.65
     Balanced Portfolios
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Aggressive Growth, Capital Appreciation,
    Worldwide Growth and Balanced Portfolio's management fee to the extent that
    such fee exceeds the effective rate of the Janus retail fund corresponding
    to such Portfolio. Janus Capital has agreed to continue such waivers until
    at least the next annual renewal of the advisory contracts. The effective
    rate is the management fee calculated by the corresponding retail fund as of
    the last day of each calendar quarter (expressed as an annual rate). The
    effective rates of Janus Enterprise Fund, Janus Olympus Fund, Janus
    Worldwide Fund and Janus Balanced Fund were 0.69%, 0.67%, 0.65%, and 0.67%,
    respectively, for the quarter ended March 31, 1999.
 
 16 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Aggressive
            Growth Portfolio, which he has managed since inception. Mr. Goff
            joined Janus Capital in 1988 and has managed Janus Enterprise
            Fund since its inception. Mr. Goff co-managed or managed Janus
            Venture Fund from December 1993 to February 1, 1997. He holds a
            Bachelor of Arts in Economics from Yale University and is a
            Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Capital
            Appreciation Portfolio, which he has managed since its inception.
            He is portfolio manager of Janus Twenty Fund, which he has
            managed since August 1997. He previously managed Janus Olympus
            Fund from its inception to August 1997. Mr. Schoelzel joined
            Janus Capital in January 1994. He holds a Bachelor of Arts in
            Business from Colorado College.
 
                                                Management of the portfolios  17
<PAGE>
 
ASSISTANT PORTFOLIO MANAGER
 
RON SACHS
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Aggressive Growth Portfolio.
            Mr. Sachs joined Janus Capital in 1996 as a research analyst.
            Prior to coming to Janus, he worked as a consultant for Bain &
            Company and as an attorney for Willkie, Farr & Gallagher. Mr.
            Sachs graduated from Princeton cum laude with an undergraduate
            degree in economics. He obtained his law degree from the
            University of Michigan. Mr. Sachs is a Chartered Financial
            Analyst.
 
 18 Janus Aspen Series
<PAGE>
                                                               Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
                                                           Other information  19
<PAGE>
Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Aggressive Growth Portfolio declared a dividend in the amount of $0.25
          per share. If the price of Aggressive Growth Portfolio's Shares was
          $10.00 on December 30, the share price on December 31 would be $9.75,
          barring market fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
 20 Janus Aspen Series
<PAGE>
                                                             Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
                                                         Shareholder's guide  21
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
 22 Janus Aspen Series
<PAGE>
                                                            Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years or the life of the Portfolio if less than
          five years. Items 1 through 9 reflect financial results for a single
          Share. Total return in the tables represents the rate that an investor
          would have earned (or lost) on an investment in each of the
          Institutional Shares of the Portfolios (assuming reinvestment of all
          dividends and distributions) but does not include charges and expenses
          attributable to any insurance product. This information has been
          audited by PricewaterhouseCoopers LLP, whose report, along with the
          Portfolios' financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<TABLE>
<CAPTION>
                                AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- -------------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                             1998        1997        1996        1995        1994
<S>                                                        <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $20.55        $18.24      $17.08      $13.62     $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                     --            --          --        0.24       0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                         7.09          2.31        1.36        3.47       1.82
  4. Total from investment operations                        7.09          2.31        1.36        3.71       1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    --            --          --      (0.25)     (0.11)
  6. Tax return of capital distributions                       --            --      (0.01)          --         --
  7. Distributions (from capital gains)                        --            --      (0.19)          --         --
  8. Total distributions                                       --            --      (0.20)      (0.25)     (0.11)
  9. NET ASSET VALUE, END OF PERIOD                        $27.64        $20.55      $18.24      $17.08     $13.62
 10. Total return                                          34.26%        12.66%       7.95%      27.48%     16.33%
 11. Net assets, end of period (in thousands)            $772,943      $508,198    $383,693    $185,911    $41,289
 12. Average net assets for the period (in thousands)    $576,444      $418,464    $290,629    $107,582    $14,152
 13. Ratio of gross expenses to average net assets          0.75%(6)      0.76%(5)    0.76%(4)    0.86%(3)     N/A
 14. Ratio of net expenses to average net assets            0.75%         0.76%       0.76%       0.84%      1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                               (0.36%)       (0.10%)     (0.27%)       0.58%      2.18%
 16. Portfolio turnover rate                                 132%          130%         88%        155%       259%
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  23
<PAGE>
 
<TABLE>
<CAPTION>
               CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------
                                                                   Periods ending
                                                                    December 31
                                                                  1998      1997(1)
<S>                                                             <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $12.62      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.01        0.05
  3. Net gains or losses on securities (both realized and
     unrealized)                                                   7.32        2.61
  4. Total from investment operations                              7.33        2.66
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.01)      (0.04)
  6. Tax return of capital distributions                             --          --
  7. Distributions (from capital gains)                              --          --
  8. Total distributions                                         (0.01)      (0.04)
  9. NET ASSET VALUE, END OF PERIOD                              $19.94      $12.62
 10. Total return*                                               58.11%      26.60%
 11. Net assets, end of period (in thousands)                   $74,187      $6,833
 12. Average net assets for the period (in thousands)           $25,964      $2,632
 13. Ratio of gross expenses to average net assets**              0.92%(3)    1.26%(2)
 14. Ratio of net expenses to average net assets**                0.91%       1.25%
 15. Ratio of net investment income to average net assets**       0.27%       1.43%
 16. Portfolio turnover rate**                                      91%        101%
- ------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 2.19% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
(3) The ratio was 0.97% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
 
 24 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
                                                        Financial highlights  25
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
 26 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
                                                Glossary of investment terms  27
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
 28 Janus Aspen Series
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
                                                Glossary of investment terms  29
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 30 Janus Aspen Series
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 

                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              Aggressive Growth Portfolio
                              International Growth Portfolio
                              Worldwide Growth Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes four mutual funds (the "Portfolios")
                with different investment objectives. Each Portfolio of Janus
                Aspen Series currently offers two classes of shares. The
                Institutional Shares, (the "Shares"), are sold under the name of
                "Janus Aspen Series" and are offered by this prospectus in
                connection with investment in and payments under variable
                annuity contracts and variable life insurance contracts, as well
                as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Portfolio summaries......................................    2
                   Fees and expenses........................................    6
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objectives and principal investment
                   strategies...............................................    7
                   General portfolio policies...............................    9
                   Risks for Growth and Global Growth Portfolios............   11
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   13
                   Management expenses and expense limits...................   13
                   Investment personnel.....................................   14
                OTHER INFORMATION...........................................   16
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   17
                   Taxes....................................................   17
                SHAREHOLDER'S GUIDE
                   Purchases................................................   18
                   Redemptions..............................................   18
                   Shareholder communications...............................   19
                FINANCIAL HIGHLIGHTS........................................   20
                GLOSSARY
                   Glossary of investment terms.............................   24
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
          The Portfolios are designed for long-term investors who seek growth of
          capital and who can tolerate the greater risks associated with common
          stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          DOMESTIC GROWTH PORTFOLIOS
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          - AGGRESSIVE GROWTH PORTFOLIO seeks long-term growth of capital.
 
          GLOBAL GROWTH PORTFOLIOS
 
          - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks
          selected for their growth potential, and normally invests at least 50%
          of its equity assets in medium-sized companies.
 
          INTERNATIONAL GROWTH PORTFOLIO normally invests at least 65% of its
          total assets in securities of issuers from at least five different
          countries, excluding the United States. Although the Portfolio intends
          to invest substantially all of its assets in issuers located outside
          the United States, it may invest in U.S. issuers and it may at times
          invest all of its assets in fewer than five countries, or even a
          single country.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
 2 Janus Aspen Series
<PAGE>
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO may have
          significant exposure to foreign markets. As a result, their returns
          and NAV may be affected to a large degree by fluctuations in currency
          exchange rates or political or economic conditions in a particular
          country.
 
          AGGRESSIVE GROWTH PORTFOLIO is nondiversified. In other words, it may
          hold larger positions in a smaller number of securities than a
          diversified fund. As a result, a single security's increase or
          decrease in value may have a greater impact on a Portfolio's NAV and
          total return.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides an indication of the risks of
          investing in the Portfolios by showing how each of the Portfolios'
          performance has varied over time. The bar charts depict the change in
          performance from year-to-year during the period indicated, but do not
          include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolios do not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of each
          Portfolio's expenses. The tables compare the average annual returns
          for the Shares of each Portfolio for the periods indicated to a
          broad-based securities market index.
 
                                                          Risk return summary  3
<PAGE>
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Aggressive Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                     16.33%   27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Aggressive Growth Portfolio - Institutional Shares        34.26%    19.35%         21.96%
                S&P 400 Mid Cap Index*                                    18.25%    18.67%         18.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic
             stocks chosen for their market size, liquidity and industry group
             representation.
 
 4 Janus Aspen Series
<PAGE>
 
           INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for International Growth 
           Portfolio - Institutional Shares from 1995 through 1998:

           Annual returns for periods ended 12/31

                                              23.15%   34.71%   18.51%   17.23%
                                               1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  16.63%  Worst Quarter 3rd-1998 (17.76%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/2/94)
                <S>                                                              <C>       <C>           
                International Growth Portfolio - Institutional Shares            17.23%        18.87%
                Morgan Stanley Capital International EAFE Index*                 20.00%         8.11%
                                                                             -----------------------------
</TABLE>
 
           * The Morgan Stanley Capital International EAFE Index is a market
             capitalization weighted index composed of companies representative
             of the market structure of 20 Developed Market countries in Europe,
             Australasia and the Far East.
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998
          
           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Portfolios' past performance does not necessarily indicate how
          they will perform in the future.
 
                                                          Risk return summary  5
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%                 N/A                  0.75%
    International Growth Portfolio           0.75%          0.20%          0.95%               0.09%                  0.86%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth, Aggressive
     Growth, International Growth and Worldwide Growth Portfolios reduce the
     Management Fee to the level of the corresponding Janus retail fund.
     Other waivers, if applicable, are first applied against the Management
     Fee and then against Other Expenses. Janus Capital has agreed to
     continue the waivers and fee reductions until at least the next annual
     renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>    
    Growth Portfolio                                                $77       $240       $417       $  930
    Aggressive Growth Portfolio                                     $77       $240       $417       $  930
    International Growth Portfolio                                  $97       $303       $526       $1,166
    Worldwide Growth Portfolio                                      $76       $237       $411       $  918
</TABLE>
 
 6 Janus Aspen Series
<PAGE>
                                     Investment objectives, principal investment
                                                strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                                   <C>
                  Growth Portfolio                                                 Janus Fund
                  Aggressive Growth Portfolio                           Janus Enterprise Fund
                  International Growth Portfolio                          Janus Overseas Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
</TABLE>
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
          This section takes a closer look at the investment objectives of each
          of the Portfolios, their principal investment strategies and certain
          risks of investing in the Portfolios. Strategies and policies that are
          noted as "fundamental" cannot be changed without a shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 11-12 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
DOMESTIC GROWTH PORTFOLIOS
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          AGGRESSIVE GROWTH PORTFOLIO
          Aggressive Growth Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential, and normally invests at least 50% of its
          equity assets in medium-sized companies. Medium-sized companies are
          those whose market capitalizations fall within the range of companies
          in the S&P MidCap 400 Index. Market capitalization is a commonly used
          measure of the size and value of a company. The market capitalizations
          within the Index will vary, but as of December 31, 1998, they ranged
          from approximately $142 million to $73 billion.
 
             Investment objectives, principal investment strategies and risks  7
<PAGE>
 
GLOBAL GROWTH PORTFOLIOS
 
          INTERNATIONAL GROWTH PORTFOLIO
          International Growth Portfolio seeks long-term growth of capital.
          Normally, the Portfolio pursues its objective by investing at least
          65% of its total assets in securities of issuers from at least five
          different countries, excluding the United States. Although the
          Portfolio intends to invest substantially all of its assets in issuers
          located outside the United States, it may at times invest in U.S.
          issuers and it may at times invest all of its assets in fewer than
          five countries or even a single country.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
The following questions and answers are designed to help you better understand
the Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. As noted previously, market capitalization is
          an important investment criteria for Aggressive Growth Portfolio.
          Although the other Portfolios offered by this Prospectus do not
          emphasize companies of any particular size, Portfolios with a larger
          asset base are more likely to invest in larger, more established
          issuers.
 
 8 Janus Aspen Series
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth and Global Growth Portfolios invest primarily in domestic
          and foreign equity securities, which may include preferred stocks,
          common stocks, warrants and securities convertible into common or
          preferred stocks. The Portfolios may also invest to a lesser degree in
          other types of securities. These securities (which are described in
          the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
             Investment objectives, principal investment strategies and risks  9
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 10 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH AND GLOBAL GROWTH PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AFFECT ITS
   RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of the Portfolio.
 
3. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
5. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
6. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 12 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Growth, Aggressive Growth,                         First $300 Million        0.75               N/A(1)
     International Growth and                           Next $200 Million         0.70
     Worldwide Growth Portfolios                        Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive Growth, International
    Growth and Worldwide Growth Portfolio's management fee to the extent that
    such fee exceeds the effective rate of the Janus retail fund corresponding
    to such Portfolio. Janus Capital has agreed to continue such waivers until
    at least the next annual renewal of the advisory contracts. The effective
    rate is the management fee calculated by the corresponding retail fund as of
    the last day of each calendar quarter (expressed as an annual rate). The
    effective rates of Janus Fund, Janus Enterprise Fund, Janus Overseas Fund
    and Janus Worldwide Fund were 0.65%, 0.69%, 0.66%, and 0.65%, respectively,
    for the quarter ended March 31, 1999.
 
                                                Management of the portfolios  13
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
            is Executive Vice President and co-manager of International
            Growth Portfolio and Janus Overseas Fund which he has co-managed
            since May 1998 and April 1998, respectively. He served as
            assistant portfolio manager for these funds since 1996. He is
            also assistant portfolio manager for Worldwide Growth Portfolio
            and Janus Worldwide Fund. Mr. Chang joined Janus Capital in 1993
            after receiving a Masters Degree in Political Science from
            Stanford University. He is a Chartered Financial Analyst.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Aggressive
            Growth Portfolio, which he has managed since inception. Mr. Goff
            joined Janus Capital in 1988 and has managed Janus Enterprise
            Fund since its inception. Mr. Goff co-managed or managed Janus
            Venture Fund from December 1993 to February 1, 1997. He holds a
            Bachelor of Arts in Economics from Yale University and is a
            Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
 14 Janus Aspen Series
<PAGE>
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
RON SACHS
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Aggressive Growth Portfolio.
            Mr. Sachs joined Janus Capital in 1996 as a research analyst.
            Prior to coming to Janus, he worked as a consultant for Bain &
            Company and as an attorney for Willkie, Farr & Gallagher. Mr.
            Sachs graduated from Princeton cum laude with an undergraduate
            degree in economics. He obtained his law degree from the
            University of Michigan. Mr. Sachs is a Chartered Financial
            Analyst.
 
                                                Management of the portfolios  15
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 16 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
                                                     Distributions and taxes  17
<PAGE>
Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
 18 Janus Aspen Series
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
                                                         Shareholder's guide  19
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years. Items 1 through 9 reflect financial results
          for a single Share. Total return in the tables represents the rate
          that an investor would have earned (or lost) on an investment in each
          of the Institutional Shares of the Portfolios (assuming reinvestment
          of all dividends and distributions) but does not include charges and
          expenses attributable to any insurance product. This information has
          been audited by PricewaterhouseCoopers LLP, whose report, along with
          the Portfolios' financial statements, is included in the Annual
          Report, which is available upon request and incorporated by reference
          into the SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $18.48          $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.05            0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                         6.36            3.34        2.29        2.90       0.20
  4. Total from investment operations                        6.41            3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.05)          (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                       --              --          --          --         --
  7. Distributions (from capital gains)                    (1.30)          (0.37)      (0.23)          --         --
  8. Total distributions                                   (1.35)          (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                        $23.54          $18.48      $15.51      $13.45     $10.57
 10. Total return                                          35.66%          22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)          $1,103,549        $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)    $789,454        $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets          0.68%(6)        0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets            0.68%           0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 0.26%           0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                  73%            122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 20 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- -------------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                             1998        1997        1996        1995        1994
<S>                                                        <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $20.55        $18.24      $17.08      $13.62     $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                     --            --          --        0.24       0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                         7.09          2.31        1.36        3.47       1.82
  4. Total from investment operations                        7.09          2.31        1.36        3.71       1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    --            --          --      (0.25)     (0.11)
  6. Tax return of capital distributions                       --            --      (0.01)          --         --
  7. Distributions (from capital gains)                        --            --      (0.19)          --         --
  8. Total distributions                                       --            --      (0.20)      (0.25)     (0.11)
  9. NET ASSET VALUE, END OF PERIOD                        $27.64        $20.55      $18.24      $17.08     $13.62
 10. Total return                                          34.26%        12.66%       7.95%      27.48%     16.33%
 11. Net assets, end of period (in thousands)            $772,943      $508,198    $383,693    $185,911    $41,289
 12. Average net assets for the period (in thousands)    $576,444      $418,464    $290,629    $107,582    $14,152
 13. Ratio of gross expenses to average net assets          0.75%(6)      0.76%(5)    0.76%(4)    0.86%(3)     N/A
 14. Ratio of net expenses to average net assets            0.75%         0.76%       0.76%       0.84%      1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                               (0.36%)       (0.10%)     (0.27%)       0.58%      2.18%
 16. Portfolio turnover rate                                 132%          130%         88%        155%       259%
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  21
<PAGE>
 
<TABLE>
<CAPTION>
                                 INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995      1994(1)
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $18.48     $15.72      $11.95       $9.72      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.13       0.11        0.05        0.09      (0.09)
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    3.07       2.80        4.06        2.16      (0.19)
  4. Total from investment operations                               3.20       2.91        4.11        2.25      (0.28)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.14)     (0.11)      (0.11)      (0.02)          --
  6. Dividends (in excess of net investment income)                   --         --          --          --          --
  7. Tax return of capital distributions                              --         --          --          --          --
  8. Distributions (from capital gains)                               --     (0.01)      (0.23)          --          --
  9. Distributions (in excess of realized gains)                  (0.27)     (0.03)          --          --          --
 10. Total distributions                                          (0.41)     (0.15)      (0.34)      (0.02)          --
 11. NET ASSET VALUE, END OF PERIOD                               $21.27     $18.48      $15.72      $11.95       $9.72
 12. Total return*                                                17.23%     18.51%      34.71%      23.15%     (2.80%)
 13. Net assets, end of period (in thousands)                   $311,110    $161,091    $27,192      $1,608      $1,353
 14. Average net assets for the period (in thousands)           $234,421    $96,164      $7,437      $1,792      $1,421
 15. Ratio of gross expenses to average net assets**               0.86%(6)   0.96%(5)    1.26%(4)    2.69%(3)      N/A
 16. Ratio of net expenses to average net assets**                 0.86%      0.96%       1.25%       2.50%       2.50%(2)
 17. Ratio of net investment income to average net assets**        0.73%      0.70%       0.62%     (0.80%)     (1.30%)
 18. Portfolio turnover rate**                                       93%        86%         65%        211%        275%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1994 (inception) through December 31, 1994.
(2) The ratio was 4.67% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(3) The ratio was 3.57% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(4) The ratio was 2.21% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(5) The ratio was 1.08% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(6) The ratio was 0.95% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
 
 22 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
                                                        Financial highlights  23
<PAGE>
Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
 24 Janus Aspen Series
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
                                                Glossary of investment terms  25
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
 26 Janus Aspen Series
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
                                                Glossary of investment terms  27
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Growth Portfolio
                              International Growth Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes four mutual funds (the "Portfolios")
                with different investment objectives, including growth of
                capital and a combination of growth and income. Each Portfolio
                of Janus Aspen Series currently offers two classes of shares.
                The Institutional Shares, (the "Shares"), are sold under the
                name of "Janus Aspen Series" and are offered by this prospectus
                in connection with investment in and payments under variable
                annuity contracts and variable life insurance contracts, as well
                as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    6
                   Fees and expenses........................................    8
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................    9
                   Balanced Portfolio.......................................   11
                   General portfolio policies...............................   12
                   Risks for the Portfolios.................................   14
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   16
                   Management expenses and expense limits...................   16
                   Investment personnel.....................................   17
                OTHER INFORMATION...........................................   19
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   20
                   Taxes....................................................   20
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   21
                   Purchases................................................   21
                   Redemptions..............................................   21
                   Shareholder communications...............................   22
                FINANCIAL HIGHLIGHTS........................................   23
                GLOSSARY
                   Glossary of investment terms.............................   27
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          GROWTH PORTFOLIO invests primarily in common stocks selected for their
          growth potential. Although the Portfolio can invest in companies of
          any size, it generally invests in larger, more established companies.
 
          INTERNATIONAL GROWTH PORTFOLIO normally invests at least 65% of its
          total assets in securities of issuers from at least five different
          countries, excluding the United States. Although the Portfolio intends
          to invest substantially all of its assets in issuers located outside
          the United States, it may invest in U.S. issuers and it may at times
          invest all of its assets in fewer than five countries, or even a
          single country.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in any of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
 2 Janus Aspen Series
<PAGE>
 
          INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO may have
          significant exposure to foreign markets. As a result, their returns
          and NAV may be affected to a large degree by fluctuations in currency
          exchange rates or political or economic conditions in a particular
          country.
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information provides an indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
                                                          Risk return summary  3
<PAGE>
 
           GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Growth Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Growth Portfolio - Institutional Shares                   35.66%    21.41%         20.91%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
           INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for International Growth 
           Portfolio - Institutional Shares from 1995 through 1998:

           Annual returns for periods ended 12/31

                                              23.15%   34.71%   18.51%   17.23%
                                               1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  16.63%  Worst Quarter 3rd-1998 (17.76%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/2/94)
                <S>                                                              <C>       <C>           
                International Growth Portfolio - Institutional Shares            17.23%        18.87%
                Morgan Stanley Capital International EAFE Index*                 20.00%         8.11%
                                                                             -----------------------------
</TABLE>
 
           * The Morgan Stanley Capital International EAFE Index is a market
             capitalization weighted index composed of companies representative
             of the market structure of 20 Developed Market countries in Europe,
             Australasia and the Far East.
 
 4 Janus Aspen Series
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
                                                          Risk return summary  5
<PAGE>
 
BALANCED PORTFOLIO
 
          The Balanced Portfolio is designed for investors who primarily seek
          growth of capital with current income. It is not designed for
          investors who desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If the portfolio manager is unable to find such investments,
          much of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in the Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
 6 Janus Aspen Series
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of each
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
                                                          Risk return summary  7
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
    International Growth Portfolio           0.75%          0.20%          0.95%               0.09%                  0.86%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth,
     International Growth, Worldwide Growth, and Balanced Portfolios reduce
     the Management Fee to the level of the corresponding Janus retail fund.
     Other waivers, if applicable, are first applied against the Management
     Fee and then against Other Expenses. Janus Capital has agreed to
     continue the waivers and fee reductions until at least the next annual
     renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>    
    Growth Portfolio                                                $77       $240       $417       $  930
    International Growth Portfolio                                  $97       $303       $526       $1,166
    Worldwide Growth Portfolio                                      $76       $237       $411       $  918
    Balanced Portfolio                                              $76       $237       $411       $  918
</TABLE>
 
 8 Janus Aspen Series
<PAGE>
                                     Investment objectives, principal investment
                                                strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                                    <C>
                  Growth Portfolio                                                 Janus Fund
                  International Growth Portfolio                          Janus Overseas Fund
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
</TABLE>
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 14-15 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          GROWTH PORTFOLIO
          Growth Portfolio seeks long-term growth of capital in a manner
          consistent with the preservation of capital. It pursues its objective
          by investing primarily in common stocks selected for their growth
          potential. Although the Portfolio can invest in companies of any size,
          it generally invests in larger, more established companies.
 
          INTERNATIONAL GROWTH PORTFOLIO
          International Growth Portfolio seeks long-term growth of capital.
          Normally, the Portfolio pursues its objective by investing at least
          65% of its total assets in securities of issuers from at least five
          different countries, excluding the United States. Although the
          Portfolio intends to invest substantially all of its assets in issuers
          located outside the United States, it may at times invest in U.S.
          issuers and it may at times invest all of its assets in fewer than
          five countries or even a single country.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
             Investment objectives, principal investment strategies and risks  9
<PAGE>
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. Although the Portfolios offered by this
          Prospectus do not emphasize companies of any particular size,
          Portfolios with a larger asset base are more likely to invest in
          larger, more established issuers.
 
 10 Janus Aspen Series
<PAGE>
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in Balanced Portfolio. Strategies and policies that
          are noted as "fundamental" cannot be changed without a shareholder
          vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 14-15 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
The following questions and answers are designed to help you better understand
the Balanced Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of Balanced
          Portfolio, the portfolio manager may consider dividend-paying
          characteristics to a greater degree in selecting common stocks for
          this Portfolio.
 
2. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of their holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, the Portfolio will place a greater emphasis on the growth
          component.
 
3. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of Balanced Portfolio's investments is expected
          to consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
4. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
GENERAL PORTFOLIO POLICIES
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios. The percentage limitations included in these
          policies and elsewhere in this Prospectus apply at the time of
          purchase of the security. So, for example, if a Portfolio exceeds a
          limit as a result of market fluctuations or the sale of other
          securities, it will not be required to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          The Balanced Portfolio also invests in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
 12 Janus Aspen Series
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
RISKS FOR THE PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in Growth, Worldwide Growth, International Growth
and Balanced Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
 14 Janus Aspen Series
<PAGE>
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
3. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
4. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
5. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Growth,
     International Growth,                              First $300 Million        0.75               N/A(1)
     Worldwide Growth and                               Next $200 Million         0.70
     Balanced Portfolios                                Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, International Growth, Worldwide
    Growth and Balanced Portfolio's management fee to the extent that such fee
    exceeds the effective rate of the Janus retail fund corresponding to such
    Portfolio. Janus Capital has agreed to continue such waivers until at least
    the next annual renewal of the advisory contracts. The effective rate is the
    management fee calculated by the corresponding retail fund as of the last
    day of each calendar quarter (expressed as an annual rate). The effective
    rates of Janus Fund, Janus Overseas Fund, Janus Worldwide Fund and Janus
    Balanced Fund were 0.65%, 0.66%, 0.65%, and 0.67%, respectively, for the
    quarter ended March 31, 1999.
 
 16 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
            is Executive Vice President and co-manager of International
            Growth Portfolio and Janus Overseas Fund which he has co-managed
            since May 1998 and April 1998, respectively. He served as
            assistant portfolio manager for these funds since 1996. He is
            also assistant portfolio manager for Worldwide Growth Portfolio
            and Janus Worldwide Fund. Mr. Chang joined Janus Capital in 1993
            after receiving a Masters Degree in Political Science from
            Stanford University. He is a Chartered Financial Analyst.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
            is Chief Investment Officer of Janus Capital. He is Executive
            Vice President and portfolio manager of Growth Portfolio, which
            he has managed since inception. He has managed Janus Fund since
            1986 and has co-managed Janus Venture Fund since February 1,
            1997. Mr. Craig previously managed Janus Venture Fund from its
            inception, to December 1993, Janus Balanced Fund from December
            1993 to December 1995 and Balanced Portfolio from September 1993
            through April 1996. He holds a Bachelor of Arts in Business from
            the University of Alabama and a Master of Arts in Finance from
            the Wharton School of the University of Pennsylvania.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
                                                Management of the portfolios  17
<PAGE>
 
ASSISTANT PORTFOLIO MANAGER
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
            is an assistant portfolio manager of Growth Portfolio. He is also
            an assistant portfolio manager of Janus Fund. He is Executive
            Vice President and portfolio manager of Janus Special Situations
            Fund. Mr. Decker received a Masters of Business Administration in
            Finance from the Fuqua School of Business at Duke University and
            a Bachelor's Degree in Economics and Political Science from Tufts
            University. He is a Chartered Financial Analyst.
 
 18 Janus Aspen Series
<PAGE>
                                                               Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
                                                           Other information  19
<PAGE>
Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
          Each class of each Portfolio makes semi-annual distributions in June
          and December of substantially all of its investment income and an
          annual distribution in June of its net realized gains, if any. All
          dividends and capital gains distributions from Shares of a Portfolio
          will automatically be reinvested into additional Shares of that
          Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions are paid to shareholders as of the record date of the
          distribution of a Portfolio, regardless of how long the shares have
          been held. Undistributed income and realized gains are included in the
          daily NAV of a Portfolio's Shares. The Share price of a Portfolio
          drops by the amount of the distribution, net of any subsequent market
          fluctuations. For example, assume that on December 31, the Shares of
          Growth Portfolio declared a dividend in the amount of $0.25 per share.
          If the price of Growth Portfolio's Shares was $10.00 on December 30,
          the share price on December 31 would be $9.75, barring market
          fluctuations.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
 20 Janus Aspen Series
<PAGE>
                                                             Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities are valued at market value or, if a
          market quotation is not readily available, at their fair value
          determined in good faith under procedures established by and under the
          supervision of the Trustees. Short-term instruments maturing within 60
          days are valued at amortized cost, which approximates market value.
          See the SAI for more detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
                                                         Shareholder's guide  21
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
 22 Janus Aspen Series
<PAGE>
                                                            Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years. Items 1 through 9 reflect financial results
          for a single Share. Total return in the tables represents the rate
          that an investor would have earned (or lost) on an investment in each
          of the Institutional Shares of the Portfolios (assuming reinvestment
          of all dividends and distributions) but does not include charges and
          expenses attributable to any insurance product. This information has
          been audited by PricewaterhouseCoopers LLP, whose report, along with
          the Portfolios' financial statements, is included in the Annual
          Report, which is available upon request and incorporated by reference
          into the SAI.
 
<TABLE>
<CAPTION>
                                       GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                              1998         1997        1996        1995        1994
<S>                                                        <C>           <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $18.48          $15.51      $13.45      $10.57     $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.05            0.15        0.17        0.28       0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                         6.36            3.34        2.29        2.90       0.20
  4. Total from investment operations                        6.41            3.49        2.46        3.18       0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.05)          (0.15)      (0.17)      (0.30)     (0.04)
  6. Tax return of capital distributions                       --              --          --          --         --
  7. Distributions (from capital gains)                    (1.30)          (0.37)      (0.23)          --         --
  8. Total distributions                                   (1.35)          (0.52)      (0.40)      (0.30)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                        $23.54          $18.48      $15.51      $13.45     $10.57
 10. Total return                                          35.66%          22.75%      18.45%      30.17%      2.76%
 11. Net assets, end of period (in thousands)          $1,103,549        $608,281    $325,789    $126,911    $43,549
 12. Average net assets for the period (in thousands)    $789,454        $477,914    $216,125     $77,344    $26,464
 13. Ratio of gross expenses to average net assets          0.68%(6)        0.70%(5)    0.69%(4)    0.78%(3)     N/A
 14. Ratio of net expenses to average net assets            0.68%           0.69%       0.69%       0.76%      0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                 0.26%           0.91%       1.39%       1.24%      1.45%
 16. Portfolio turnover rate                                  73%            122%         87%        185%       169%
- ---------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
                                                        Financial highlights  23
<PAGE>
 
<TABLE>
<CAPTION>
                                 INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995      1994(1)
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $18.48     $15.72      $11.95       $9.72      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.13       0.11        0.05        0.09      (0.09)
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    3.07       2.80        4.06        2.16      (0.19)
  4. Total from investment operations                               3.20       2.91        4.11        2.25      (0.28)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.14)     (0.11)      (0.11)      (0.02)          --
  6. Dividends (in excess of net investment income)                   --         --          --          --          --
  7. Tax return of capital distributions                              --         --          --          --          --
  8. Distributions (from capital gains)                               --     (0.01)      (0.23)          --          --
  9. Distributions (in excess of realized gains)                  (0.27)     (0.03)          --          --          --
 10. Total distributions                                          (0.41)     (0.15)      (0.34)      (0.02)          --
 11. NET ASSET VALUE, END OF PERIOD                               $21.27     $18.48      $15.72      $11.95       $9.72
 12. Total return*                                                17.23%     18.51%      34.71%      23.15%     (2.80%)
 13. Net assets, end of period (in thousands)                   $311,110    $161,091    $27,192      $1,608      $1,353
 14. Average net assets for the period (in thousands)           $234,421    $96,164      $7,437      $1,792      $1,421
 15. Ratio of gross expenses to average net assets**               0.86%(6)   0.96%(5)    1.26%(4)    2.69%(3)      N/A
 16. Ratio of net expenses to average net assets**                 0.86%      0.96%       1.25%       2.50%       2.50%(2)
 17. Ratio of net investment income to average net assets**        0.73%      0.70%       0.62%     (0.80%)     (1.30%)
 18. Portfolio turnover rate**                                       93%        86%         65%        211%        275%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1994 (inception) through December 31, 1994.
(2) The ratio was 4.67% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(3) The ratio was 3.57% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(4) The ratio was 2.21% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(5) The ratio was 1.08% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(6) The ratio was 0.95% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
 
 24 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
                                                        Financial highlights  25
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
 26 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
                                                Glossary of investment terms  27
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
 28 Janus Aspen Series
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
                                                Glossary of investment terms  29
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 30 Janus Aspen Series
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
<PAGE>
 
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Capital Appreciation Portfolio
                              Worldwide Growth Portfolio
                              Balanced Portfolio
                              Money Market Portfolio
 
                   THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED OR
                   DISAPPROVED OF THESE SECURITIES OR PASSED ON THE ACCURACY OR
                   ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE
                   CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes four mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital, current income and a combination of growth and income.
                Each Portfolio of Janus Aspen Series currently offers two
                classes of shares. The Institutional Shares, (the "Shares"), are
                sold under the name of "Janus Aspen Series" and are offered by
                this prospectus in connection with investment in and payments
                under variable annuity contracts and variable life insurance
                contracts, as well as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolios........................................    2
                   Balanced Portfolio.......................................    5
                   Money Market Portfolio...................................    7
                   Fees and expenses........................................    9
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................   10
                   Balanced Portfolio.......................................   12
                   General portfolio policies for Portfolios other than
                   Money Market Portfolio...................................   13
                   Risks for the Growth and Balanced Portfolios.............   15
                   Money Market Portfolio...................................   17
                   Investment techniques....................................   18
                MANAGEMENT OF THE PORTFOLIOS
                   Investment adviser.......................................   20
                   Management expenses and expense limits...................   20
                   Investment personnel.....................................   21
                OTHER INFORMATION...........................................   22
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   23
                   Money Market Portfolios..................................   23
                   Portfolios other than Money Market Portfolios............   23
                   Taxes....................................................   23
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   25
                   Purchases................................................   25
                   Redemptions..............................................   26
                   Shareholder communications...............................   26
                FINANCIAL HIGHLIGHTS........................................   27
                GLOSSARY
                   Glossary of investment terms.............................   31
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
          The Growth Portfolios are designed for long-term investors who seek
          growth of capital and who can tolerate the greater risks associated
          with common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
          - CAPITAL APPRECIATION PORTFOLIO seeks long-term growth of capital.
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolios' Trustees may change these objectives without a
          shareholder vote and the Portfolios will notify you of any changes
          that are material. If there is a material change to a Portfolio's
          objective or policies, you should consider whether that Portfolio
          remains an appropriate investment for you. There is no guarantee
          that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
          The portfolio managers apply a "bottom up" approach in choosing
          investments. In other words, they look for companies with earnings
          growth potential one at a time. If a portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of a Portfolio's assets may be in cash or similar investments.
 
          CAPITAL APPRECIATION PORTFOLIO invests primarily in common stocks
          selected for their growth potential. The Portfolio may invest in
          companies of any size, from larger, well-established companies to
          smaller, emerging growth companies.
 
          WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
          companies of any size throughout the world. The Portfolio normally
          invests in issuers from at least five different countries, including
          the United States. The Portfolio may at times invest in fewer than
          five countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
          The biggest risk is that the Portfolios' returns may vary, and you
          could lose money. If you are considering investing in either of the
          Growth Portfolios, remember that they are each designed for long-term
          investors who can accept the risks of investing in a portfolio with
          significant common stock holdings. Common stocks tend to be more
          volatile than other investment choices.
 
          The value of a Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of a Portfolio could
          also decrease if the stock market goes down. If the value of a
          Portfolio decreases, its net asset value (NAV) will also decrease,
          which means if you sell your shares in a Portfolio you would get back
          less money.
 
          WORLDWIDE GROWTH PORTFOLIO may have significant exposure to foreign
          markets. As a result, its returns and NAV may be affected to a large
          degree by fluctuations in currency exchange rates or political or
          economic conditions in a particular country.
 
          CAPITAL APPRECIATION PORTFOLIO is nondiversified. In other words, it
          may hold larger positions in a smaller number of securities than a
          diversified fund. As a result, a single security's increase or
          decrease in value may have a greater impact on the Portfolio's NAV and
          total return.
 
 2 Janus Aspen Series
<PAGE>
 
          An investment in these Portfolios is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
          The following information, provides an indication of the risks of
          investing in the Growth Portfolios by showing how each of the Growth
          Portfolios' performance has varied over time. The bar charts depict
          the change in performance from year-to-year during the period
          indicated, but do not include charges and expenses attributable to any
          insurance product which would lower the performance illustrated. The
          Portfolios do not impose any sales or other charges that would affect
          total return computations. Total return figures include the effect of
          each Portfolio's expenses. The tables compare the average annual
          returns for the Shares of each Portfolio for the periods indicated to
          a broad-based securities market index.
 
           CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Capital Appreciation
           Portfolio - Institutional Shares for 1998:

           Annual returns for periods ended 12/31

                                                                         58.11%
                                                                          1998

           The percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  33.98%  Worst Quarter 3rd-1998 (9.98%) 
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                           Since Inception
                                                                                 1 year       (5/1/97)
                <S>                                                              <C>       <C>           
                Capital Appreciation Portfolio - Institutional Shares            58.11%        51.65%
                S&P 500 Index*                                                   28.74%        31.38%
                                                                             -----------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
 
                                                          Risk return summary  3
<PAGE>
 
           WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Worldwide Growth 
           Portfolio - Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Worldwide Growth Portfolio - Institutional Shares         28.92%    21.32%         24.06%
                Morgan Stanley International Worldwide Index*             24.34%    15.68%         14.39%
                                                                      ----------------------------------------
</TABLE>
 
           * The Morgan Stanley International Worldwide Index is a market
             capitalization weighted index composed of countries representative
             of the market structure of 47 Developed and Emerging Markets.
 
          The Growth Portfolios' past performance does not necessarily indicate
          how they will perform in the future.
 
 4 Janus Aspen Series
<PAGE>
 
BALANCED PORTFOLIO
 
          Balanced Portfolio is designed for investors who primarily seek growth
          of capital with current income. It is not designed for investors who
          desire a consistent level of income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF BALANCED PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - BALANCED PORTFOLIO seeks long-term capital growth, consistent
            with preservation of capital and balanced by current income.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change to the Portfolio's objective or policies,
          you should consider whether the Portfolio remains an appropriate
          investment for you. There is no guarantee that the Portfolio will meet
          its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF BALANCED PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks mostly for equity and
          income-producing securities that meet his investment criteria one at a
          time. If a portfolio manager is unable to find such investments, much
          of the Portfolio's assets may be in cash or similar investments.
 
          Balanced Portfolio normally invests 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. The
          Portfolio will normally invest at least 25% of its assets in
          fixed-income securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN BALANCED PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Balanced
          Portfolio, remember that it is designed for long-term investors who
          can accept the risks of investing in a portfolio with significant
          common stock holdings. Common stocks tend to be more volatile than
          other investment choices.
 
          The value of the Portfolio may decrease if the value of an individual
          company in the portfolio decreases. The value of the Portfolio could
          also decrease if the stock market goes down. If the value of the
          Portfolio decreases, its NAV will also decrease, which means if you
          sell your shares in the Portfolio you would get back less money.
 
          The income component of the Portfolio's holdings includes fixed-income
          securities. A fundamental risk to the income component is that the
          value of these securities will fall if interest rates rise. Generally,
          the value of a fixed-income portfolio will decrease when interest
          rates rise, which means the Portfolio's NAV may likewise decrease.
          Another fundamental risk associated with fixed-income securities is
          credit risk, which is the risk that an issuer of a bond will be unable
          to make principal and interest payments when due.
 
          An investment in this Portfolio is not a bank deposit and is not
          insured or guaranteed by the Federal Deposit Insurance Corporation or
          any other government agency.
 
                                                          Risk return summary  5
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Balanced Portfolio by showing how Balanced Portfolio's
          performance has varied over time. The bar chart depicts the change in
          performance from year-to-year during the period indicated, but does
          not include charges and expenses attributable to any insurance product
          which would lower the performance illustrated. The Portfolio does not
          impose any sales or other charges that would affect total return
          computations. Total return figures include the effect of the
          Portfolio's expenses. The table compares the average annual returns
          for the Shares of the Portfolio for the period indicated to a
          broad-based securities market index.
 
           BALANCED PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Balanced Portfolio -
           Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%) 

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
<TABLE>
<CAPTION>
                                                                                               Since Inception
                                                                          1 year    5 years       (9/13/93)
                <S>                                                       <C>       <C>        <C>           
                Balanced Portfolio - Institutional Shares                 34.28%    19.11%         19.53%
                S&P 500 Index*                                            28.74%    24.08%         23.06%
                Lehman Brothers Gov't/Corp Bond Index**                    9.47%     7.30%          6.90%
                                                                      ----------------------------------------
</TABLE>
 
           * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks,
             a widely recognized, unmanaged index of common stock prices.
          ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
             are of investment grade with at least one year until maturity.
 
          Balanced Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
 6 Janus Aspen Series
<PAGE>
 
MONEY MARKET PORTFOLIO
 
          Money Market Portfolio is designed for investors who seek current
          income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF MONEY MARKET PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - MONEY MARKET PORTFOLIO seeks maximum current income to the extent
            consistent with stability of capital.
 
          The Trustees may change this objective without a shareholder vote and
          the Portfolio will notify you of any changes that are material. If
          there is a material change in the Portfolio's objective or policies,
          you should consider whether it remains an appropriate investment for
          you. There is no guarantee that the Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF MONEY MARKET PORTFOLIO?
 
          MONEY MARKET PORTFOLIO will invest only in high-quality, short-term
          money market instruments that present minimal credit risks, as
          determined by Janus Capital. The Portfolio invests primarily in high
          quality debt obligations and obligations of financial institutions.
          Debt obligations may include commercial paper, notes and bonds, and
          variable amount master demand notes. Obligations of financial
          institutions include certificates of deposit and time deposits.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN MONEY MARKET PORTFOLIO?
 
          The Portfolio's yields will vary as the short-term securities in the
          portfolio mature and the proceeds are reinvested in securities with
          different interest rates. Over time, the real value of the Portfolio's
          yield may be eroded by inflation. Although Money Market Portfolio
          invests only in high-quality, short-term money market instruments,
          there is a risk that the value of the securities it holds will fall as
          a result of changes in interest rates, an issuer's actual or perceived
          credit-worthiness or an issuer's ability to meet its obligations.
 
          An investment in Money Market Portfolio is not a deposit of a bank and
          is not insured or guaranteed by the Federal Deposit Insurance
          Corporation or any other government agency. Although the Portfolio
          seeks to preserve the value of your investment at $1.00 per share, it
          is possible to lose money by investing in Money Market Portfolio.
 
                                                          Risk return summary  7
<PAGE>
 
          The following information provides an indication of the risks of
          investing in Money Market Portfolio by showing how Money Market
          Portfolio's performance has varied over time, but does not include
          charges and expenses attributable to any insurance product which would
          lower the performance illustrated. The Portfolio does not impose any
          sales or other charges that would affect total return or yield
          computations. Total return and yield figures include the effect of the
          Portfolio's expenses. The bar chart depicts the change in performance
          from year to year.
 
           MONEY MARKET PORTFOLIO - INSTITUTIONAL SHARES

           A BAR CHART showing Total Annual Returns for Money Market Portfolio -
           Institutional Shares from 1996 through 1998:

           Annual returns for periods ended 12/31

                                                        5.05%    5.17%    5.36%
                                                        1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998   1.35%  Worst Quarter 3rd-1998  1.22%  
 
          The 7-day yield for the Portfolio's Shares on December 31, 1998 was
          4.87%. For the Portfolio's current yield, call the Janus
          XpressLine(TM) at 1-888-979-7737.
 
          Money Market Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
 8 Janus Aspen Series
<PAGE>
 
FEES AND EXPENSES
 
          SHAREHOLDER FEES, such as sales loads, redemption fees or exchange
          fees, are charged directly to an investor's account. All Janus funds
          are no-load investments, so you will not pay any shareholder fees when
          you buy or sell shares of the Portfolios. However, each variable
          insurance contract involves fees and expenses not described in this
          prospectus. See the accompanying contract prospectus for information
          regarding contract fees and expenses and any restrictions on purchases
          or allocations.
 
          ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets
          and include fees for portfolio management, maintenance of shareholder
          accounts, shareholder servicing, accounting and other services. You do
          not pay these fees directly but, as the example on the next page
          shows, these costs are borne indirectly by all shareholders.
 
          This table and example are designed to assist participants in
          qualified plans that invest in the Shares of the Portfolios in
          understanding the fees and expenses that you may pay as an investor in
          the Shares. The information shown is based upon gross expenses
          (without the effect of expense offset arrangements) for the fiscal
          year ended December 31, 1998. OWNERS OF VARIABLE INSURANCE CONTRACTS
          THAT INVEST IN THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE
          CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES AND EXPENSES, AS THE
          TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE SEPARATE ACCOUNT
          LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED UNDER A
          CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%               0.05%                  0.92%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%               0.02%                  0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%                 N/A                  0.74%
    Money Market Portfolio                   0.25%          0.09%          0.34%                 N/A                  0.34%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Capital
     Appreciation, Worldwide Growth and Balanced Portfolios reduce the
     Management Fee to the level of the corresponding Janus retail fund.
     Other waivers, if applicable, are first applied against the Management
     Fee and then against Other Expenses. Janus Capital has agreed to
     continue the waivers and fee reductions until at least the next annual
     renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolios with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in each of the
   Portfolios for the time periods indicated then redeem all of your shares
   at the end of those periods. The example also assumes that your
   investment has a 5% return each year, and that the Portfolios' operating
   expenses remain the same. Although your actual costs may be higher or
   lower, based on these assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                                                  1 Year     3 Years    5 Years    10 Years
                                                                  -----------------------------------------
    <S>                                                           <C>        <C>        <C>        <C>   
    Capital Appreciation Portfolio                                 $ 99       $309      $  536      $1,190
    Worldwide Growth Portfolio                                     $ 76       $237      $  411      $  918
    Balanced Portfolio                                             $ 76       $237      $  411      $  918
    Money Market Portfolio                                         $ 35       $109      $  191      $  431
</TABLE>
 
                                                          Risk return summary  9
<PAGE>
Investment objectives, principal investment
           strategies and risks
 
          Each of the Portfolios has a similar investment objective and similar
          principal investment strategies to a Janus retail fund:
 
<TABLE>
                  <S>                                                 <C>
                  Capital Appreciation Portfolio                           Janus Twenty Fund*
                  Worldwide Growth Portfolio                             Janus Worldwide Fund
                  Balanced Portfolio                                      Janus Balanced Fund
                  Money Market Portfolio                              Janus Money Market Fund
</TABLE>
 
          * Prior to May 1, 1999 Capital Appreciation Portfolio was managed in a
            similar manner to Janus Olympus Fund.
 
          Although it is anticipated that each Portfolio and its corresponding
          retail fund will hold similar securities, differences in asset size,
          cash flow needs and other factors may result in differences in
          investment performance. The expenses of each Portfolio and its
          corresponding retail fund are expected to differ. The variable
          contract owner will also bear various insurance related costs at the
          insurance company level. You should review the accompanying separate
          account prospectus for a summary of fees and expenses.
 
GROWTH PORTFOLIOS
 
          This section takes a closer look at the investment objectives of each
          of the Growth Portfolios, their principal investment strategies and
          certain risks of investing in the Growth Portfolios. Strategies and
          policies that are noted as "fundamental" cannot be changed without a
          shareholder vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 15-16 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
          CAPITAL APPRECIATION PORTFOLIO
          Capital Appreciation Portfolio seeks long-term growth of capital. It
          pursues its objective by investing primarily in common stocks selected
          for their growth potential. The Portfolio may invest in companies of
          any size, from larger, well-established companies to smaller, emerging
          growth companies.
 
          WORLDWIDE GROWTH PORTFOLIO
          Worldwide Growth Portfolio seeks long-term growth of capital in a
          manner consistent with the preservation of capital. It pursues its
          objective by investing primarily in common stocks of companies of any
          size throughout the world. The Portfolio normally invests in issuers
          from at least five different countries, including the United States.
          The Portfolio may at times invest in fewer than five countries or even
          a single country.
 
 10 Janus Aspen Series
<PAGE>
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          Each of the Portfolios may invest substantially all of its assets in
          common stocks if its portfolio manager believes that common stocks
          will appreciate in value. The portfolio managers generally take a
          "bottom up" approach to selecting companies. In other words, they seek
          to identify individual companies with earnings growth potential that
          may not be recognized by the market at large. They make this
          assessment by looking at companies one at a time, regardless of size,
          country of organization, place of principal business activity, or
          other similar selection criteria. Realization of income is not a
          significant consideration when choosing investments for the
          Portfolios. Income realized on the Portfolios' investments will be
          incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio managers seek companies that meet their
          selection criteria, regardless of where a company is located. Foreign
          securities are generally selected on a stock-by-stock basis without
          regard to any defined allocation among countries or geographic
          regions. However, certain factors such as expected levels of
          inflation, government policies influencing business conditions, the
          outlook for currency relationships, and prospects for economic growth
          among countries, regions or geographic areas may warrant greater
          consideration in selecting foreign securities. There are no
          limitations on the countries in which the Portfolios may invest and
          the Portfolios may at times have significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
          Market capitalization is the most commonly used measure of the size
          and value of a company. It is computed by multiplying the current
          market price of a share of the company's stock by the total number of
          its shares outstanding. Although the Growth Portfolios offered by this
          Prospectus do not emphasize companies of any particular size,
          Portfolios with a larger asset base are more likely to invest in
          larger, more established issuers.
 
            Investment objectives, principal investment strategies and risks  11
<PAGE>
 
BALANCED PORTFOLIO
 
          This section takes a closer look at the investment objective of
          Balanced Portfolio, its principal investment strategies and certain
          risks of investing in Balanced Portfolio. Strategies and policies that
          are noted as "fundamental" cannot be changed without a shareholder
          vote.
 
          Please carefully review the "Risks" section of this Prospectus on
          pages 15-16 for a discussion of risks associated with certain
          investment techniques. We've also included a Glossary with
          descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Balanced Portfolio seeks long-term capital growth, consistent with
          preservation of capital and balanced by current income. It pursues its
          objective by normally investing 40-60% of its assets in securities
          selected primarily for their growth potential and 40-60% of its assets
          in securities selected primarily for their income potential. This
          Portfolio normally invests at least 25% of its assets in fixed-income
          securities.
 
The following questions and answers are designed to help you better understand
Balanced Portfolio's principal investment strategies.
 
3. HOW ARE COMMON STOCKS SELECTED FOR BALANCED PORTFOLIO IN COMPARISON TO THE
   GROWTH PORTFOLIOS?
 
          Because income is a part of the investment objective of Balanced
          Portfolio, the portfolio manager may consider dividend-paying
          characteristics to a greater degree in selecting common stocks for
          this Portfolio.
 
4. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S HOLDINGS?
 
          Balanced Portfolio shifts assets between the growth and income
          components of its holdings based on the portfolio manager's analysis
          of relevant market, financial and economic conditions. If the
          portfolio manager believes that growth securities will provide better
          returns than the yields then available or expected on income-producing
          securities, that Portfolio will place a greater emphasis on the growth
          component.
 
5. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED PORTFOLIO'S
   INVESTMENTS?
 
          The growth component of Balanced Portfolio's investments is expected
          to consist primarily of common stocks, but may also include warrants,
          preferred stocks or convertible securities selected primarily for
          their growth potential.
 
6. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   HOLDINGS?
 
          The income component of Balanced Portfolio will consist of securities
          that the portfolio manager believes have income potential. Such
          securities may include equity securities, convertible securities and
          all types of debt securities. Equity securities may be included in the
          income component of the Portfolio if they currently pay dividends or
          the portfolio manager believes they have the potential for either
          increasing their dividends or commencing dividends, if none are
          currently paid.
 
 12 Janus Aspen Series
<PAGE>
 
GENERAL PORTFOLIO POLICIES OF THE PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
 
          Unless otherwise stated, each of the following policies applies to all
          of the Portfolios other than Money Market Portfolio. The percentage
          limitations included in these policies and elsewhere in this
          Prospectus apply at the time of purchase of the security. So, for
          example, if a Portfolio exceeds a limit as a result of market
          fluctuations or the sale of other securities, it will not be required
          to dispose of any securities.
 
          CASH POSITION
          When a portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when he or she is otherwise
          unable to locate attractive investment opportunities, the Portfolios'
          cash or similar investments may increase. In other words, the
          Portfolios do not always stay fully invested in stocks and bonds. Cash
          or similar investments generally are a residual - they represent the
          assets that remain after a portfolio manager has committed available
          assets to desirable investment opportunities. However, a portfolio
          manager may also temporarily increase a Portfolio's cash position to
          protect its assets or maintain liquidity. Partly because the portfolio
          managers act independently of each other, the cash positions of the
          Portfolios may vary significantly.
 
          When a Portfolio's investments in cash or similar investments
          increase, it may not participate in market advances or declines to the
          same extent that it would if the Portfolio remained more fully
          invested in stocks or bonds.
 
          OTHER TYPES OF INVESTMENTS
          The Growth Portfolios invest primarily in domestic and foreign equity
          securities, which may include preferred stocks, common stocks,
          warrants and securities convertible into common or preferred stocks.
          Balanced Portfolio also invests in domestic and foreign equity
          securities with varying degrees of emphasis on income. The Portfolios
          may also invest to a lesser degree in other types of securities. These
          securities (which are described in the Glossary) may include:
 
          - debt securities
 
          - indexed/structured securities
 
          - high-yield/high-risk securities (less than 35% of each Portfolio's
            assets)
 
          - options, futures, forwards and other types of derivatives for
            hedging purposes or for non-hedging purposes such as seeking to
            enhance return
 
          - securities purchased on a when-issued, delayed delivery or forward
            commitment basis
 
          ILLIQUID INVESTMENTS
          Each Portfolio may invest up to 15% of its net assets in illiquid
          investments. An illiquid investment is a security or other position
          that cannot be disposed of quickly in the normal course of business.
          For example, some securities are not registered under U.S. securities
          laws and cannot be sold to the U.S. public because of SEC regulations
          (these are known as "restricted securities"). Under procedures adopted
          by the Portfolios' Trustees, certain restricted securities may be
          deemed liquid, and will not be counted toward this 15% limit.
 
          FOREIGN SECURITIES
          The Portfolios may invest without limit in foreign equity and debt
          securities. The Portfolios may invest directly in foreign securities
          denominated in a foreign currency and not publicly traded in the
          United
 
            Investment objectives, principal investment strategies and risks  13
<PAGE>
 
          States. Other ways of investing in foreign securities include
          depositary receipts or shares, and passive foreign investment
          companies.
 
          SPECIAL SITUATIONS
          Each Portfolio may invest in special situations. A special situation
          arises when, in the opinion of a Portfolio's manager, the securities
          of a particular issuer will be recognized and appreciate in value due
          to a specific development with respect to that issuer. Developments
          creating a special situation might include, among others, a new
          product or process, a technological breakthrough, a management change
          or other extraordinary corporate event, or differences in market
          supply of and demand for the security. A Portfolio's performance could
          suffer if the anticipated development in a "special situation"
          investment does not occur or does not attract the expected attention.
 
          PORTFOLIO TURNOVER
          The Portfolios generally intend to purchase securities for long-term
          investment although, to a limited extent, a Portfolio may purchase
          securities in anticipation of relatively short-term price gains.
          Short-term transactions may also result from liquidity needs,
          securities having reached a price or yield objective, changes in
          interest rates or the credit standing of an issuer, or by reason of
          economic or other developments not foreseen at the time of the
          investment decision. A Portfolio may also sell one security and
          simultaneously purchase the same or a comparable security to take
          advantage of short-term differentials in bond yields or securities
          prices. Changes are made in a Portfolio's holdings whenever its
          portfolio manager believes such changes are desirable. Portfolio
          turnover rates are generally not a factor in making buy and sell
          decisions.
 
          Increased portfolio turnover may result in higher costs for brokerage
          commissions, dealer mark-ups and other transaction costs and may also
          result in taxable capital gains. Higher costs associated with
          increased portfolio turnover may offset gains in a Portfolio's
          performance.
 
 14 Janus Aspen Series
<PAGE>
 
RISKS FOR THE GROWTH AND BALANCED PORTFOLIOS
 
          Because the Portfolios may invest substantially all of their assets in
          common stocks, the main risk is the risk that the value of the stocks
          they hold might decrease in response to the activities of an
          individual company or in response to general market and/or economic
          conditions. If this occurs, a Portfolio's share price may also
          decrease. A Portfolio's performance may also be affected by risks
          specific to certain types of investments, such as foreign securities,
          derivative investments, non-investment grade debt securities or
          companies with relatively small market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth and Balanced Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
          Smaller or newer companies may suffer more significant losses as well
          as realize more substantial growth than larger or more established
          issuers because they may lack depth of management, be unable to
          generate funds necessary for growth or potential development, or be
          developing or marketing new products or services for which markets are
          not yet established and may never become established. In addition,
          such companies may be insignificant factors in their industries and
          may become subject to intense competition from larger or more
          established companies. Securities of smaller or newer companies may
          have more limited trading markets than the markets for securities of
          larger or more established issuers, and may be subject to wide price
          fluctuations. Investments in such companies tend to be more volatile
          and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF CAPITAL APPRECIATION PORTFOLIO AFFECT
   ITS RISK?
 
          Diversification is a way to reduce risk by investing in a broad range
          of stocks or other securities. A "nondiversified" portfolio has the
          ability to take larger positions in a smaller number of issuers.
          Because the appreciation or depreciation of a single stock may have a
          greater impact on the NAV of a nondiversified portfolio, its share
          price can be expected to fluctuate more than a comparable diversified
          portfolio. This fluctuation, if significant, may affect the
          performance of the Portfolio.
 
3. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
          The Portfolios may invest without limit in foreign securities either
          indirectly (e.g., depositary receipts) or directly in foreign markets.
          Investments in foreign securities, including those of foreign
          governments, may involve greater risks than investing in domestic
          securities because the Portfolios' performance may depend on issues
          other than the performance of a particular company. These issues
          include:
 
          - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
            value will be affected by the value of the local currency relative
            to the U.S. dollar. When a Portfolio sells a foreign denominated
            security, its value may be worth less in U.S. dollars even if the
            security increases in value in its home country. U.S. dollar
            denominated securities of foreign issuers may also be affected by
            currency risk.
 
          - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
            heightened political and economic risks, particularly in emerging
            markets which may have relatively unstable governments, immature
            economic structures, national policies restricting investments by
            foreigners, different legal systems, and economies based on only a
            few industries. In some countries, there is the risk that the
            government may take over the assets or operations of a company or
            that the government may impose taxes or limits on the removal of a
            Portfolio's assets from that country.
 
            Investment objectives, principal investment strategies and risks  15
<PAGE>
 
          - REGULATORY RISK. There may be less government supervision of foreign
            markets. As a result, foreign issuers may not be subject to the
            uniform accounting, auditing and financial reporting standards and
            practices applicable to domestic issuers and there may be less
            publicly available information about foreign issuers.
 
          - MARKET RISK. Foreign securities markets, particularly those of
            emerging market countries, may be less liquid and more volatile than
            domestic markets. Certain markets may require payment for securities
            before delivery and delays may be encountered in settling securities
            transactions. In some foreign markets, there may not be protection
            against failure by other parties to complete transactions.
 
          - TRANSACTION COSTS. Costs of buying, selling and holding foreign
            securities, including brokerage, tax and custody costs, may be
            higher than those involved in domestic transactions.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
          High-yield/high-risk securities (or "junk" bonds) are securities rated
          below investment grade by the primary rating agencies such as Standard
          & Poor's and Moody's. The value of lower quality securities generally
          is more dependent on credit risk, or the ability of the issuer to meet
          interest and principal payments, than investment grade debt
          securities. Issuers of high-yield securities may not be as strong
          financially as those issuing bonds with higher credit ratings and are
          more vulnerable to real or perceived economic changes, political
          changes or adverse developments specific to the issuer.
 
5. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
          The Portfolios may use futures, options and other derivative
          instruments to "hedge" or protect their portfolios from adverse
          movements in securities prices and interest rates. The Portfolios may
          also use a variety of currency hedging techniques, including forward
          currency contracts, to manage exchange rate risk. The portfolio
          managers believe the use of these instruments will benefit the
          Portfolios. However, a Portfolio's performance could be worse than if
          the Portfolio had not used such instruments if a portfolio manager's
          judgement proves incorrect. Risks associated with the use of
          derivative instruments are described in the SAI.
 
6. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
          The portfolio managers carefully research each potential investment
          before making an investment decision and, among other things, consider
          Year 2000 readiness when selecting portfolio holdings. However, there
          is no guarantee that the information a portfolio manager receives
          regarding a company's Year 2000 readiness is completely accurate. If a
          company has not satisfactorily addressed Year 2000 issues, the
          Portfolio's performance could suffer.
 
 16 Janus Aspen Series
<PAGE>
 
MONEY MARKET PORTFOLIO
 
          This section takes a closer look at the investment objective of Money
          Market Portfolio, its principal investment strategies and certain
          risks of investing in the Portfolio. Strategies and policies that are
          noted as "fundamental" cannot be changed without a shareholder vote.
 
          Money Market Portfolio is subject to certain specific SEC rule
          requirements. Among other things, the Portfolio is limited to
          investing in U.S. dollar-denominated instruments with a remaining
          maturity of 397 days or less (as calculated pursuant to Rule 2a-7
          under the 1940 Act).
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          Money Market Portfolio seeks maximum current income to the extent
          consistent with stability of capital. It pursues its objective by
          investing primarily in high quality debt obligations and obligations
          of financial institutions. Debt obligations may include commercial
          paper, notes and bonds, and variable amount master demand notes.
          Obligations of financial institutions include certificates of deposit
          and time deposits.
 
          Money Market Portfolio will:
 
          - invest in high quality, short-term money market instruments that
            present minimal credit risks, as determined by Janus Capital
 
          - invest only in U.S. dollar-denominated instruments that have a
            remaining maturity of 397 days or less (as calculated pursuant to
            Rule 2a-7 under the 1940 Act)
 
          - maintain a dollar-weighted average portfolio maturity of 90 days or
            less
 
TYPES OF INVESTMENTS
 
          Money Market Portfolio invests primarily in:
 
          - high quality debt obligations
 
          - obligations of financial institutions
 
          The Portfolio may also invest (to a lesser degree) in:
 
          - U.S. Government Securities (securities issued or guaranteed by the
            U.S. government, its agencies and instrumentalities)
 
          - municipal securities
 
          DEBT OBLIGATIONS
 
          The Portfolio may invest in debt obligations of domestic issuers. Debt
          obligations include:
 
          - commercial paper
 
          - notes and bonds
 
          - variable amount master demand notes (the payment obligations on
            these instruments may be backed by securities, swap agreements or
            other assets, by a guarantee of a third party or solely by the
            unsecured promise of the issuer to make payments when due)
 
          - privately issued commercial paper or other securities that are
            restricted as to disposition under the federal securities laws
 
            Investment objectives, principal investment strategies and risks  17
<PAGE>
 
          OBLIGATIONS OF FINANCIAL INSTITUTIONS
 
          Examples of obligations of financial institutions include:
 
          - negotiable certificates of deposit, bankers' acceptances, time
            deposits and other obligations of U.S. banks (including savings and
            loan associations) having total assets in excess of one billion
            dollars and U.S. branches of foreign banks having total assets in
            excess of ten billion dollars
 
          - Eurodollar and Yankee bank obligations (Eurodollar bank obligations
            are dollar-denominated certificates of deposit or time deposits
            issued outside the U.S. capital markets by foreign branches of U.S.
            banks and by foreign banks. Yankee bank obligations are
            dollar-denominated obligations issued in the U.S. capital markets by
            foreign banks)
 
          - other U.S. dollar-denominated obligations of foreign banks having
            total assets in excess of ten billion dollars that Janus Capital
            believes are of an investment quality comparable to obligations of
            U.S. banks in which the Portfolio may invest
 
          Foreign, Eurodollar (and to a limited extent, Yankee) bank obligations
          are subject to certain sovereign risks. One such risk is the
          possibility that a foreign government might prevent dollar-denominated
          funds from flowing across its borders. Other risks include: adverse
          political and economic developments in a foreign country; the extent
          and quality of government regulation of financial markets and
          institutions; the imposition of foreign withholding taxes; and
          expropriation or nationalization of foreign issuers.
 
INVESTMENT TECHNIQUES
 
          The following is a description of other investment techniques that
          Money Market Portfolio may use:
 
          PARTICIPATION INTERESTS
          A participation interest gives Money Market Portfolio a proportionate,
          undivided interest in underlying debt securities and sometimes carries
          a demand feature.
 
          DEMAND FEATURES
          Demand features give Money Market Portfolio the right to resell
          securities at specified periods prior to their maturity dates. Demand
          features may shorten the life of a variable or floating rate security,
          enhance the instrument's credit quality and provide a source of
          liquidity.
 
          Demand features are often issued by third party financial
          institutions, generally domestic and foreign banks. Accordingly, the
          credit quality and liquidity of Money Market Portfolio's investments
          may be dependent in part on the credit quality of the banks supporting
          Money Market Portfolio's investments. This will result in exposure to
          risks pertaining to the banking industry, including the foreign
          banking industry. Brokerage firms and insurance companies also provide
          certain liquidity and credit support.
 
          VARIABLE AND FLOATING RATE SECURITIES
          Money Market Portfolio may invest in securities which have variable or
          floating rates of interest. These securities pay interest at rates
          that are adjusted periodically according to a specified formula,
          usually with reference to an interest rate index or market interest
          rate. Variable and floating rate securities are subject to changes in
          value based on changes in market interest rates or changes in the
          issuer's or guarantor's creditworthiness.
 
 18 Janus Aspen Series
<PAGE>
 
          MORTGAGE- AND ASSET-BACKED SECURITIES
          Money Market Portfolio may purchase fixed or variable rate
          mortgage-backed securities issued by the Government National Mortgage
          Association, Federal National Mortgage Association, the Federal Home
          Loan Mortgage Corporation, or other governmental or government-related
          entity. The Portfolio may purchase other mortgage- and asset-backed
          securities including securities backed by automobile loans, equipment
          leases or credit card receivables.
 
          Unlike traditional debt instruments, payments on these securities
          include both interest and a partial payment of principal. Prepayments
          of the principal of underlying loans may shorten the effective
          maturities of these securities and may result in the Portfolio having
          to reinvest proceeds at a lower interest rate.
 
          REPURCHASE AGREEMENTS
          Money Market Portfolio may enter into collateralized repurchase
          agreements. Repurchase agreements are transactions in which the
          Portfolio purchases securities and simultaneously commits to resell
          those securities to the seller at an agreed-upon price on an
          agreed-upon future date. The repurchase price reflects a market rate
          of interest and is collateralized by cash or securities.
 
          If the seller of the securities underlying a repurchase agreement
          fails to pay the agreed resale price on the agreed delivery date,
          Money Market Portfolio may incur costs in disposing of the collateral
          and may experience losses if there is any delay in its ability to do
          so.
 
            Investment objectives, principal investment strategies and risks  19
<PAGE>
Management of the portfolios
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of the Portfolios and is responsible
          for the day-to-day management of the investment portfolios and other
          business affairs of the Portfolios.
 
          Janus Capital began serving as investment adviser to Janus Fund in
          1970 and currently serves as investment adviser to all of the Janus
          retail funds, acts as sub-adviser for a number of private-label mutual
          funds and provides separate account advisory services for
          institutional accounts.
 
          Janus Capital furnishes continuous advice and recommendations
          concerning each Portfolio's investments. Janus Capital also furnishes
          certain administrative, compliance and accounting services for the
          Portfolios, and may be reimbursed by the Portfolios for its costs in
          providing those services. In addition, Janus Capital employees serve
          as officers of the Trust and Janus Capital provides office space for
          the Portfolios and pays the salaries, fees and expenses of all
          Portfolio officers and those Trustees who are affiliated with Janus
          Capital.
 
          Participating insurance companies that purchase the Portfolios' shares
          may perform certain administrative services relating to the Portfolios
          and Janus Capital or the Portfolios may pay those companies for such
          services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
          Each Portfolio pays Janus Capital a management fee which is calculated
          daily. The advisory agreement with each Portfolio spells out the
          management fee and other expenses that the Portfolios must pay. Each
          of the Portfolios is subject to the following management fee schedule
          (expressed as an annual rate). In addition, the Shares of each
          Portfolio incur expenses not assumed by Janus Capital, including
          transfer agent and custodian fees and expenses, legal and auditing
          fees, printing and mailing costs of sending reports and other
          information to existing shareholders, and independent Trustees' fees
          and expenses.
 
<TABLE>
<CAPTION>
                                                          Average Daily
                                                           Net Assets         Annual Rate      Expense Limit
     Fee Schedule                                         of Portfolio       Percentage (%)    Percentage (%)
- -------------------------------------------------------------------------------------------------------------
<S>                                                     <C>                  <C>               <C>          
     Capital Appreciation,                              First $300 Million        0.75               N/A(1)
     Worldwide Growth and                               Next $200 Million         0.70
     Balanced Portfolios                                Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
     Money Market Portfolio                             All Asset Levels          0.25              0.50(2)
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Capital Appreciation, Worldwide Growth
    and Balanced Portfolio's management fee to the extent that such fee exceeds
    the effective rate of the Janus retail fund corresponding to such Portfolio.
    Janus Capital has agreed to continue such waivers until at least the next
    annual renewal of the advisory contracts. The effective rate is the
    management fee calculated by the corresponding retail fund as of the last
    day of each calendar quarter (expressed as an annual rate). The effective
    rates of Janus Olympus Fund, Janus Worldwide Fund and Janus Balanced Fund
    were 0.67%, 0.65%, and 0.67%, respectively, for the quarter ended March 31,
    1999.
 
(2) Janus Capital has agreed to limit the Portfolio's expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
 20 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Worldwide
            Growth Portfolio and co-manager of International Growth
            Portfolio, which she has managed or co-managed since inception.
            Ms. Hayes joined Janus Capital in 1987 and has managed or
            co-managed Janus Worldwide Fund and Janus Overseas Fund since
            their inceptions. She holds a Bachelor of Arts in Economics from
            Yale University and is a Chartered Financial Analyst.
 
SHARON S. PICHLER
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Money Market
            Portfolio, which she has managed since inception. She also has
            managed Janus Money Market Fund, Janus Government Money Market
            Fund and Janus Tax-Exempt Money Market Fund since inception. She
            holds a Bachelor of Arts in Economics from Michigan State
            University and a Master of Business Administration from the
            University of Texas at San Antonio. Ms. Pichler is a Chartered
            Financial Analyst.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Balanced
            Portfolio, which he has managed since May 1996 and Equity Income
            Portfolio, which he has managed since inception. He is an
            assistant portfolio manager of Growth Portfolio. Mr. Rollins
            joined Janus Capital in 1990 and has managed Janus Balanced Fund
            since January 1996 and Janus Equity Income Fund since inception.
            He has been an assistant portfolio manager of Janus Fund since
            January 1995. He gained experience as a fixed-income trader and
            equity research analyst prior to managing Balanced Portfolio. He
            holds a Bachelor of Science in Finance from the University of
            Colorado and is a Chartered Financial Analyst.
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
            is Executive Vice President and portfolio manager of Capital
            Appreciation Portfolio, which he has managed since its inception.
            He is portfolio manager of Janus Twenty Fund, which he has
            managed since August 1997. He previously managed Janus Olympus
            Fund from its inception to August 1997. Mr. Schoelzel joined
            Janus Capital in January 1994. He holds a Bachelor of Arts in
            Business from Colorado College.
 
                                                Management of the portfolios  21
<PAGE>
Other information
 
          CLASSES OF SHARES
 
          Each Portfolio currently offers two classes of Shares, one of which,
          the Institutional Shares, are offered pursuant to this prospectus and
          are sold under the name Janus Aspen Series. The Shares offered by this
          Prospectus are available only in connection with investment in and
          payments under variable insurance contracts as well as certain
          qualified retirement plans. Retirement Shares of each Portfolio are
          offered by separate prospectus and are available only to qualified
          plans using plan service providers that are compensated for providing
          distribution and/or recordkeeping and other administrative services.
          Because the expenses of each class may differ, the performance of each
          class is expected to differ. If you would like additional information
          about the Retirement Shares, please call 1-800-525-0020.
 
          CONFLICTS OF INTEREST
 
          The Shares offered by this prospectus are available only to variable
          annuity and variable life separate accounts of insurance companies
          that are unaffiliated with Janus Capital and to certain qualified
          retirement plans. Retirement Shares of the Portfolios (offered through
          a separate prospectus) are available to certain qualified plans.
          Although the Portfolios do not currently anticipate any disadvantages
          to policy owners because each Portfolio offers its shares to such
          entities, there is a possibility that a material conflict may arise.
          The Trustees monitor events in order to identify any disadvantages or
          material irreconcilable conflicts and to determine what action, if
          any, should be taken in response. If a material disadvantage or
          conflict occurs, the Trustees may require one or more insurance
          company separate accounts or qualified plans to withdraw its
          investments in one or more Portfolios or substitute Shares of another
          Portfolio. If this occurs, a Portfolio may be forced to sell its
          securities at disadvantageous prices. In addition, the Trustees may
          refuse to sell Shares of any Portfolio to any separate account or
          qualified plan or may suspend or terminate the offering of a
          Portfolio's Shares if such action is required by law or regulatory
          authority or is in the best interests of that Portfolio's
          shareholders. It is possible that a qualified plan investing in the
          Retirement Shares of the Portfolios could lose its qualified plan
          status under the Internal Revenue Code, which could have adverse tax
          consequences on insurance company separate accounts investing in the
          Shares. Janus Capital intends to monitor such qualified plans and the
          Portfolios may discontinue sales to a qualified plan and require plan
          participants with existing investments in the Retirement Shares to
          redeem those investments if a plan loses (or in the opinion of Janus
          Capital is at risk of losing) its qualified plan status.
 
          YEAR 2000
 
          Preparing for Year 2000 is a high priority for Janus Capital, which
          has established a dedicated group to address this issue. Janus Capital
          has devoted considerable internal resources and has engaged one of the
          foremost experts in the field to help achieve Year 2000 readiness.
          Janus Capital does not anticipate that Year 2000-related issues will
          have a material impact on its ability to continue to provide the
          Portfolios with service at current levels; however, Janus Capital
          cannot make any assurances that the steps it has taken to ensure Year
          2000 readiness will be successful. In addition, there can be no
          assurance that Year 2000 issues will not affect the companies in which
          the Portfolios invest or worldwide markets and economies.
 
 22 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
          To avoid taxation of the Portfolios, the Internal Revenue Code
          requires each Portfolio to distribute net income and any net gains
          realized on its investments annually. A Portfolio's income from
          dividends and interest and any net realized short-term gains are paid
          to shareholders as ordinary income dividends. Net realized long-term
          gains are paid to shareholders as capital gains distributions.
 
PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
 
          Each class of each Portfolio, other than Money Market Portfolio, makes
          semi-annual distributions in June and December of substantially all of
          its investment income and an annual distribution in June of its net
          realized gains, if any. All dividends and capital gains distributions
          from Shares of a Portfolio will automatically be reinvested into
          additional Shares of that Portfolio.
 
          HOW DISTRIBUTIONS AFFECT NAV
 
          Distributions, other than daily income dividends, are paid to
          shareholders as of the record date of the distribution of a Portfolio,
          regardless of how long the shares have been held. Undistributed income
          and realized gains are included in the daily NAV of a Portfolio's
          Shares. The Share price of a Portfolio drops by the amount of the
          distribution, net of any subsequent market fluctuations. For example,
          assume that on December 31, the Shares of Growth Portfolio declared a
          dividend in the amount of $0.25 per share. If the price of Growth
          Portfolio's Shares was $10.00 on December 30, the share price on
          December 31 would be $9.75, barring market fluctuations.
 
MONEY MARKET PORTFOLIO
 
          For the Shares of Money Market Portfolio, dividends representing
          substantially all of the net investment income and any net realized
          gains on sales of securities are declared daily, Saturdays, Sundays
          and holidays included, and distributed on the last business day of
          each month. If a month begins on a Saturday, Sunday or holiday,
          dividends for those days are declared at the end of the preceding
          month and distributed on the first business day of the month. All
          distributions will be automatically reinvested in Shares of the
          Portfolio.
 
TAXES
 
          TAXES ON DISTRIBUTIONS
 
          Because Shares of the Portfolios may be purchased only through
          variable insurance contracts and qualified plans, it is anticipated
          that any income dividends or capital gains distributions made by the
          Shares of a Portfolio will be exempt from current taxation if left to
          accumulate within the variable insurance contract or qualified plan.
          Generally, withdrawals from such contracts may be subject to ordinary
          income tax and, if made before age 59 1/2, a 10% penalty tax. The tax
          status of your investment depends on the features of your qualified
          plan or variable insurance contract. Further information may be found
          in your plan documents or in the prospectus of the separate account
          offering such contract.
 
          TAXATION OF THE PORTFOLIOS
 
          Dividends, interest and some gains received by the Portfolios on
          foreign securities may be subject to withholding of foreign taxes. The
          Portfolios may from year to year make the election permitted under
          Section 853 of the Internal Revenue Code to pass through such taxes to
          shareholders. If such election is not made, any foreign taxes paid or
          accrued will represent an expense to the Portfolios which will reduce
          their investment income.
 
                                                     Distributions and taxes  23
<PAGE>
 
          The Portfolios do not expect to pay any federal income or excise taxes
          because they intend to meet certain requirements of the Internal
          Revenue Code. In addition, each Portfolio intends to qualify under the
          Internal Revenue Code with respect to the diversification requirements
          related to the tax-deferred status of insurance company separate
          accounts.
 
 24 Janus Aspen Series
<PAGE>
                                                             Shareholder's guide
 
          INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS
          DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE
          INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING
          INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN
          PORTFOLIOS MAY NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR
          CONTRACT AND CERTAIN CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE
          PORTFOLIOS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
          COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON
          PURCHASING OR SELLING OF VARIABLE INSURANCE CONTRACTS AND ON HOW TO
          SELECT SPECIFIC PORTFOLIOS AS INVESTMENT OPTIONS FOR A CONTRACT OR A
          QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
          Investments will be processed at the NAV next determined after an
          order is received and accepted by a Portfolio or its agent. In order
          to receive a day's price, your order must be received by the close of
          the regular trading session of the New York Stock Exchange any day
          that the NYSE is open. Securities of the Portfolios other than Money
          Market Portfolio are valued at market value or, if a market quotation
          is not readily available, at their fair value determined in good faith
          under procedures established by and under the supervision of the
          Trustees. Short-term instruments maturing within 60 days are valued at
          amortized cost, which approximates market value. See the SAI for more
          detailed information.
 
          To the extent a Portfolio holds securities that are primarily listed
          on foreign exchanges that trade on weekends or other days when the
          Portfolios do not price their shares, the NAV of a Portfolio's shares
          may change on days when shareholders will not be able to purchase or
          redeem the Portfolio's shares.
 
          Money Market Portfolio's securities are valued at their amortized
          cost. Amortized cost valuation involves valuing an instrument at its
          cost and thereafter assuming a constant amortization to maturity (or
          such other date as permitted by Rule 2a-7) of any discount or premium.
          If fluctuating interest rates cause the market value of the portfolio
          to deviate more than 1/2 of 1% from the value determined on the basis
          of amortized cost, the Trustees will consider whether any action, such
          as adjusting the Share's NAV to reflect current market conditions,
          should be initiated to prevent any material dilutive effect on
          shareholders.
 
PURCHASES
 
          Purchases of Shares may be made only by the separate accounts of
          insurance companies for the purpose of funding variable insurance
          contracts or by qualified plans. Refer to the prospectus of the
          appropriate insurance company separate account or your plan documents
          for information on how to invest in the Shares of each Portfolio.
          Participating insurance companies and certain other designated
          organizations are authorized to receive purchase orders on the
          Portfolios' behalf.
 
          Each Portfolio reserves the right to reject any specific purchase
          order. Purchase orders may be refused if, in Janus Capital's opinion,
          they are of a size that would disrupt the management of a Portfolio.
          Although there is no present intention to do so, the Portfolios may
          discontinue sales of their shares if management and the Trustees
          believe that continued sales may adversely affect a Portfolio's
          ability to achieve its investment objective. If sales of a Portfolio's
          Shares are discontinued, it is expected that existing policy owners
          and plan participants invested in that Portfolio would be permitted to
          continue to authorize investment in that Portfolio and to reinvest any
          dividends or capital gains distributions, absent highly unusual
          circumstances.
 
                                                         Shareholder's guide  25
<PAGE>
 
REDEMPTIONS
 
          Redemptions, like purchases, may be effected only through the separate
          accounts of participating insurance companies or through qualified
          plans. Please refer to the appropriate separate account prospectus or
          plan documents for details.
 
          Shares of any Portfolio may be redeemed on any business day.
          Redemptions are processed at the NAV next calculated after receipt and
          acceptance of the redemption order by the Portfolio or its agent.
          Redemption proceeds will normally be wired to the participating
          insurance company the business day following receipt of the redemption
          order, but in no event later than seven days after receipt of such
          order.
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolios that they have
          authorized for investment. Each report will show the investments owned
          by each Portfolio and the market values thereof, as well as other
          information about the Portfolios and their operations. The Trust's
          fiscal year ends December 31.
 
 26 Janus Aspen Series
<PAGE>
                                                            Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Institutional Shares' financial performance for each of the five
          most recent fiscal years or the life of the Portfolio if less than
          five years. Items 1 through 9 reflect financial results for a single
          Share. Total return in the tables represents the rate that an investor
          would have earned (or lost) on an investment in each of the
          Institutional Shares of the Portfolios (assuming reinvestment of all
          dividends and distributions) but does not include charges and expenses
          attributable to any insurance product. This information has been
          audited by PricewaterhouseCoopers LLP, whose report, along with the
          Portfolios' financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<TABLE>
<CAPTION>
               CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------
                                                                   Periods ending
                                                                    December 31
                                                                  1998      1997(1)
<S>                                                             <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $12.62      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.01        0.05
  3. Net gains or losses on securities (both realized and
     unrealized)                                                   7.32        2.61
  4. Total from investment operations                              7.33        2.66
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.01)      (0.04)
  6. Tax return of capital distributions                             --          --
  7. Distributions (from capital gains)                              --          --
  8. Total distributions                                         (0.01)      (0.04)
  9. NET ASSET VALUE, END OF PERIOD                              $19.94      $12.62
 10. Total return*                                               58.11%      26.60%
 11. Net assets, end of period (in thousands)                   $74,187      $6,833
 12. Average net assets for the period (in thousands)           $25,964      $2,632
 13. Ratio of gross expenses to average net assets**              0.92%(3)    1.26%(2)
 14. Ratio of net expenses to average net assets**                0.91%       1.25%
 15. Ratio of net investment income to average net assets**       0.27%       1.43%
 16. Portfolio turnover rate**                                      91%        101%
- ------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 2.19% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
(3) The ratio was 0.97% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
 
                                                        Financial highlights  27
<PAGE>
 
<TABLE>
<CAPTION>
                                     WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------------------------------------------------
                                                                                 Periods ending December 31
                                                                   1998          1997         1996        1995        1994
<S>                                                             <C>           <C>           <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                           $23.39        $19.44      $15.31      $12.07     $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                            0.16          0.16        0.16        0.11       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                      6.59          4.14        4.27        3.19       0.14
  4. Total from investment operations                                 6.75          4.30        4.43        3.30       0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                         (0.18)        (0.17)      (0.17)      (0.06)         --
  6. Dividends (in excess of net investment income)                     --        (0.02)          --          --         --
  7. Tax return of capital distributions                                --            --          --          --         --
  8. Distributions (from capital gains)                                 --        (0.16)      (0.13)          --         --
  9. Distributions (in excess of realized gains)                    (0.87)            --          --          --         --
 10. Total distributions                                            (1.05)        (0.35)      (0.30)      (0.06)         --
 11. NET ASSET VALUE, END OF PERIOD                                 $29.09        $23.39      $19.44      $15.31     $12.07
 12. Total return                                                   28.92%        22.15%      29.04%      27.37%      1.53%
 13. Net assets, end of period (in thousands)                   $2,890,375    $1,576,548    $582,603    $108,563    $37,728
 14. Average net assets for the period (in thousands)           $2,217,695    $1,148,951    $304,111     $59,440    $22,896
 15. Ratio of gross expenses to average net assets                   0.72%(6)      0.74%(5)    0.80%(4)    0.90%(3)     N/A
 16. Ratio of net expenses to average net assets                     0.72%         0.74%       0.80%       0.87%      1.18%(1)(2)
 17. Ratio of net investment income to average net assets            0.64%         0.67%       0.83%       0.95%      0.50%
 18. Portfolio turnover rate                                           77%           80%         62%        113%       217%
- ----------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
 28 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
                                       BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------------
                                                                               Periods ending December 31
                                                                  1998        1997        1996        1995        1994
<S>                                                             <C>         <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $17.47      $14.77     $13.03      $10.63      $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.39        0.34       0.32        0.17        0.15
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    5.51        2.89       1.81        2.45      (0.06)
  4. Total from investment operations                               5.90        3.23       2.13        2.62        0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.38)      (0.35)     (0.30)      (0.22)      (0.10)
  6. Tax return of capital distributions                              --          --         --          --          --
  7. Distributions (from capital gains)                           (0.45)      (0.18)     (0.09)          --          --
  8. Distributions (in excess of realized gains)                  (0.04)          --         --          --          --
  9. Total distributions                                          (0.87)      (0.53)     (0.39)      (0.22)      (0.10)
 10. NET ASSET VALUE, END OF PERIOD                               $22.50      $17.47     $14.77      $13.03      $10.63
 11. Total return                                                 34.28%      22.10%     16.18%      24.79%       0.84%
 12. Net assets, end of period (in thousands)                   $882,495    $362,409    $85,480     $14,021      $3,153
 13. Average net assets for the period (in thousands)           $555,002    $176,432    $43,414      $5,739      $2,336
 14. Ratio of gross expenses to average net assets                 0.74%(6)    0.83%(5)   0.94%(4)    1.37%(3)      N/A
 15. Ratio of net expenses to average net assets                   0.74%       0.82%      0.92%       1.30%       1.57%(1)(2)
 16. Ratio of net investment income to average net assets          2.41%       2.87%      2.92%       2.41%       1.90%
 17. Portfolio turnover rate                                         70%        139%       103%        149%        158%
- ------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
                                                        Financial highlights  29
<PAGE>
 
<TABLE>
<CAPTION>
                               MONEY MARKET PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------
                                                                         Periods ending December 31
                                                                  1998        1997        1996      1995(1)
<S>                                                             <C>         <C>         <C>         <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $1.00       $1.00       $1.00       $1.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.05        0.05        0.05        0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                     --          --          --          --
  4. Total from investment operations                              0.05        0.05        0.05        0.04
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.05)      (0.05)      (0.05)      (0.04)
  6. Tax return of capital distributions                             --          --          --          --
  7. Distributions (from capital gains)                              --          --          --          --
  8. Total distributions                                         (0.05)      (0.05)      (0.05)      (0.04)
  9. NET ASSET VALUE, END OF PERIOD                               $1.00       $1.00       $1.00       $1.00
 10. Total return*                                                5.36%       5.17%       5.05%       3.63%
 11. Net assets, end of period (in thousands)                   $38,690     $15,374      $6,016      $1,735
 12. Average net assets for the period (in thousands)           $31,665      $8,926      $3,715      $1,543
 13. Ratio of gross expenses to average net assets**              0.34%       0.50%       0.50%       0.50%
 14. Ratio of net expenses to average net assets**                0.34%       0.50%(4)    0.50%(3)    0.50%(2)
 15. Ratio of net investment income to average net assets**       5.21%       5.17%       4.93%       5.30%
- ------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one year.
**  Annualized for periods of less than one full year.
(1) May 1, 1995 (inception) to December 31, 1995.
(2) The ratio was 1.07% before waiver of certain fees incurred by the Portfolio.
(3) The ratio was 0.78% before waiver of certain fees incurred by the Portfolio.
(4) The ratio was 0.55% before waiver of certain fees incurred by the Portfolio.
 
 30 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
          This glossary provides a more detailed description of some of the
          types of securities and other instruments in which the Portfolios may
          invest. The Portfolios may invest in these instruments to the extent
          permitted by their investment objectives and policies. The Portfolios
          are not limited by this discussion and may invest in any other types
          of instruments not precluded by the policies discussed elsewhere in
          this Prospectus. Please refer to the SAI for a more detailed
          discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
          BONDS are debt securities issued by a company, municipality,
          government or government agency. The issuer of a bond is required to
          pay the holder the amount of the loan (or par value of the bond) at a
          specified maturity and to make scheduled interest payments.
 
          COMMERCIAL PAPER is a short-term debt obligation with a maturity
          ranging from 1 to 270 days issued by banks, corporations and other
          borrowers to investors seeking to invest idle cash. The Portfolios may
          purchase commercial paper issued in private placements under Section
          4(2) of the Securities Act of 1933.
 
          COMMON STOCKS are equity securities representing shares of ownership
          in a company and usually carry voting rights and earns dividends.
          Unlike preferred stock, dividends on common stock are not fixed but
          are declared at the discretion of the issuer's board of directors.
 
          CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
          dividend or interest payment and are convertible into common stock at
          a specified price or conversion ratio.
 
          DEBT SECURITIES are securities representing money borrowed that must
          be repaid at a later date. Such securities have specific maturities
          and usually a specific rate of interest or an original purchase
          discount.
 
          DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
          corporation that entitle the holder to dividends and capital gains on
          the underlying security. Receipts include those issued by domestic
          banks (American Depositary Receipts), foreign banks (Global or
          European Depositary Receipts) and broker-dealers (depositary shares).
 
          FIXED-INCOME SECURITIES are securities that pay a specified rate of
          return. The term generally includes short-and long-term government,
          corporate and municipal obligations that pay a specified rate of
          interest or coupons for a specified period of time, and preferred
          stock, which pays fixed dividends. Coupon and dividend rates may be
          fixed for the life of the issue or, in the case of adjustable and
          floating rate securities, for a shorter period.
 
          HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
          investment grade by the primary rating agencies (e.g., BB or lower by
          Standard & Poor's and Ba or lower by Moody's). Other terms commonly
          used to describe such securities include "lower rated bonds,"
          "noninvestment grade bonds" and "junk bonds."
 
          MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
          mortgages or other debt. These securities are generally pass-through
          securities, which means that principal and interest payments on the
          underlying securities (less servicing fees) are passed through to
          shareholders on a pro rata basis. These securities involve prepayment
          risk, which is the risk that the underlying mortgages or other debt
          may be refinanced or paid off prior to their maturities during periods
          of declining interest rates. In that case, a portfolio manager may
          have to reinvest the proceeds from the securities at a lower rate.
          Potential market gains on a security subject to prepayment risk may be
          more limited than potential market gains on a comparable security that
          is not subject to prepayment risk.
 
          PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
          corporations which generate certain amounts of passive income or hold
          certain amounts of assets for the production of passive income.
          Passive
 
                                                Glossary of investment terms  31
<PAGE>
 
          income includes dividends, interest, royalties, rents and annuities.
          To avoid taxes and interest that the Portfolios must pay if these
          investments are profitable, the Portfolios may make various elections
          permitted by the tax laws. These elections could require that the
          Portfolios recognize taxable income, which in turn must be
          distributed, before the securities are sold and before cash is
          received to pay the distributions.
 
          PAY-IN-KIND BONDS are debt securities that normally give the issuer an
          option to pay cash at a coupon payment date or give the holder of the
          security a similar bond with the same coupon rate and a face value
          equal to the amount of the coupon payment that would have been made.
 
          PREFERRED STOCKS are equity securities that generally pay dividends at
          a specified rate and have preference over common stock in the payment
          of dividends and liquidation. Preferred stock generally does not carry
          voting rights.
 
          REPURCHASE AGREEMENTS involve the purchase of a security by a
          Portfolio and a simultaneous agreement by the seller (generally a bank
          or dealer) to repurchase the security from the Portfolio at a
          specified date or upon demand. This technique offers a method of
          earning income on idle cash. These securities involve the risk that
          the seller will fail to repurchase the security, as agreed. In that
          case, a Portfolio will bear the risk of market value fluctuations
          until the security can be sold and may encounter delays and incur
          costs in liquidating the security.
 
          REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
          Portfolio to another party (generally a bank or dealer) in return for
          cash and an agreement by the Portfolio to buy the security back at a
          specified price and time. This technique will be used primarily to
          provide cash to satisfy unusually high redemption requests, or for
          other temporary or emergency purposes.
 
          RULE 144A SECURITIES are securities that are not registered for sale
          to the general public under the Securities Act of 1933, but that may
          be resold to certain institutional investors.
 
          STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
          dealer that give the Portfolio the option to sell a security to the
          dealer at a specified price.
 
          STEP COUPON BONDS are debt securities that trade at a discount from
          their face value and pay coupon interest. The discount from the face
          value depends on the time remaining until cash payments begin,
          prevailing interest rates, liquidity of the security and the perceived
          credit quality of the issuer.
 
          STRIP BONDS are debt securities that are stripped of their interest
          (usually by a financial intermediary) after the securities are issued.
          The market value of these securities generally fluctuates more in
          response to changes in interest rates than interest-paying securities
          of comparable maturity.
 
          TENDER OPTION BONDS are generally long-term securities that are
          coupled with an option to tender the securities to a bank,
          broker-dealer or other financial institution at periodic intervals and
          receive the face value of the bond. This type of security is commonly
          used as a means of enhancing the security's liquidity.
 
          U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
          government that are supported by its full faith and credit. Treasury
          bills have initial maturities of less than one year, Treasury notes
          have initial maturities of one to ten years and Treasury bonds may be
          issued with any maturity but generally have maturities of at least ten
          years. U.S. government securities also include indirect obligations of
          the U.S. government that are issued by federal agencies and government
          sponsored entities. Unlike Treasury securities, agency securities
          generally are not backed by the full faith and credit of the U.S.
          government. Some agency securities are supported by the right of the
          issuer to borrow from the Treasury, others are supported by the
          discretionary authority of the U.S. government to purchase the
          agency's obligations and others are supported only by the credit of
          the sponsoring agency.
 
 32 Janus Aspen Series
<PAGE>
 
          VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates
          of interest and, under certain limited circumstances, may have varying
          principal amounts. These securities pay interest at rates that are
          adjusted periodically according to a specified formula, usually with
          reference to some interest rate index or market interest rate. The
          floating rate tends to decrease the security's price sensitivity to
          changes in interest rates.
 
          WARRANTS are securities, typically issued with preferred stock or
          bonds, that give the holder the right to buy a proportionate amount of
          common stock at a specified price, usually at a price that is higher
          than the market price at the time of issuance of the warrant. The
          right may last for a period of years or indefinitely.
 
          WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
          involve the purchase of a security with payment and delivery at some
          time in the future - i.e., beyond normal settlement. The Portfolios do
          not earn interest on such securities until settlement and bear the
          risk of market value fluctuations in between the purchase and
          settlement dates. New issues of stocks and bonds, private placements
          and U.S. government securities may be sold in this manner.
 
          ZERO COUPON BONDS are debt securities that do not pay regular interest
          at regular intervals, but are issued at a discount from face value.
          The discount approximates the total amount of interest the security
          will accrue from the date of issuance to maturity. The market value of
          these securities generally fluctuates more in response to changes in
          interest rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
          FORWARD CONTRACTS are contracts to purchase or sell a specified amount
          of a financial instrument for an agreed upon price at a specified
          time. Forward contracts are not currently exchange traded and are
          typically negotiated on an individual basis. The Portfolios may enter
          into forward currency contracts to hedge against declines in the value
          of securities denominated in, or whose value is tied to, a currency
          other than the U.S. dollar or to reduce the impact of currency
          appreciation on purchases of such securities. They may also enter into
          forward contracts to purchase or sell securities or other financial
          indices.
 
          FUTURES CONTRACTS are contracts that obligate the buyer to receive and
          the seller to deliver an instrument or money at a specified price on a
          specified date. The Portfolios may buy and sell futures contracts on
          foreign currencies, securities and financial indices including
          interest rates or an index of U.S. government, foreign government,
          equity or fixed-income securities. The Portfolios may also buy options
          on futures contracts. An option on a futures contract gives the buyer
          the right, but not the obligation, to buy or sell a futures contract
          at a specified price on or before a specified date. Futures contracts
          and options on futures are standardized and traded on designated
          exchanges.
 
          INDEXED/STRUCTURED SECURITIES are typically short- to
          intermediate-term debt securities whose value at maturity or interest
          rate is linked to currencies, interest rates, equity securities,
          indices, commodity prices or other financial indicators. Such
          securities may be positively or negatively indexed (i.e. their value
          may increase or decrease if the reference index or instrument
          appreciates). Indexed/structured securities may have return
          characteristics similar to direct investments in the underlying
          instruments and may be more volatile than the underlying instruments.
          A Portfolio bears the market risk of an investment in the underlying
          instruments, as well as the credit risk of the issuer.
 
          INTEREST RATE SWAPS involve the exchange by two parties of their
          respective commitments to pay or receive interest (e.g., an exchange
          of floating rate payments for fixed rate payments).
 
          INVERSE FLOATERS are debt instruments whose interest rate bears an
          inverse relationship to the interest rate on another instrument or
          index. For example, upon reset the interest rate payable on a security
          may go down when the underlying index has risen. Certain inverse
          floaters may have an interest rate reset
 
                                                Glossary of investment terms  33
<PAGE>
 
          mechanism that multiplies the effects of change in the underlying
          index. Such mechanism may increase the volatility of the security's
          market value.
 
          OPTIONS are the right, but not the obligation, to buy or sell a
          specified amount of securities or other assets on or before a fixed
          date at a predetermined price. The Portfolios may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 34 Janus Aspen Series
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolios' Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolios.
The Statement of Additional Information provides detailed
information about the Portfolios and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736
                                      
                                
<PAGE>
       
         


                                                 [JANUS LOGO]
 
                   Janus Aspen Series
 
                                   PROSPECTUS
                                   MAY 1, 1999
                                   Growth Portfolio
                                   Aggressive Growth Portfolio
                                   Capital Appreciation Portfolio
                                   International Growth Portfolio
                                   Worldwide Growth Portfolio
                                   Balanced Portfolio
                                   Equity Income Portfolio
                                   Growth and Income Portfolio
                                   Flexible Income Portfolio
                                   High-Yield Portfolio
                                   Money Market Portfolio
 
                            THE SECURITIES AND EXCHANGE COMMISSION HAS NOT
                            APPROVED OR DISAPPROVED OF THESE SECURITIES OR
                            PASSED ON THE ACCURACY OR ADEQUACY OF THIS
                            PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
                            CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                This prospectus describes eleven mutual funds (the "Portfolios")
                with a variety of investment objectives, including growth of
                capital, current income and a combination of growth and income.
                Each Portfolio of Janus Aspen Series currently offers two
                classes of shares. The Institutional Shares, (the "Shares"), are
                sold under the name of "Janus Aspen Series" and are offered by
                this prospectus in connection with investment in and payments
                under variable annuity contracts and variable life insurance
                contracts, as well as certain qualified retirement plans.
 
                Janus Aspen Series sells and redeems its Shares at net asset
                value without sales charges, commissions or redemption fees.
                Each variable insurance contract involves fees and expenses that
                are not described in this Prospectus. Certain Portfolios may not
                be available in connection with a particular contract and
                certain contracts may limit allocations among the Portfolios.
                See the accompanying contract prospectus for information
                regarding contract fees and expenses and any restrictions on
                purchases or allocations.
 
                This prospectus contains information that a prospective
                purchaser of a variable insurance contract or plan participant
                should consider in conjunction with the accompanying separate
                account prospectus of the specific insurance company product
                before allocating purchase payments or premiums to the
                Portfolios.
<PAGE>

                                                               Table of contents
 
<TABLE>
         <S>                                                          <C>
         RISK/RETURN SUMMARY
            Growth Portfolios.......................................    2
            Combination Portfolios..................................    7
            Fixed-Income Portfolios.................................   11
            Money Market Portfolio..................................   14
            Fees and expenses.......................................   16
         INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
         RISKS
            Growth Portfolios.......................................   18
            Combination Portfolios..................................   21
            Fixed-Income Portfolios.................................   24
            General portfolio policies for Portfolios other than
              Money Market Portfolio................................   26
            Risks for Growth, Global Growth and Combination
              Portfolios............................................   29
            Risks for Fixed-Income Portfolios.......................   30
            Risks Common to All Non-Money Market Portfolios.........   31
            Money Market Portfolio..................................   33
            Investment techniques...................................   35
         MANAGEMENT OF THE PORTFOLIOS
            Investment adviser......................................   37
            Management expenses and expense limits..................   37
            Investment personnel....................................   39
         OTHER INFORMATION..........................................   44
         DISTRIBUTIONS AND TAXES
            Distributions...........................................   46
            Taxes...................................................   46
         SHAREHOLDER'S GUIDE
            Pricing of portfolio shares.............................   48
            Purchases...............................................   49
            Redemptions.............................................   49
            Shareholder communications..............................   49
         FINANCIAL HIGHLIGHTS.......................................   50
         GLOSSARY
            Glossary of investment terms............................   61
         RATING CATEGORIES
            Explanation of rating categories........................   66
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIOS
 
       The Growth Portfolios are designed for long-term investors who seek
       growth of capital and who can tolerate the greater risks associated with
       common stock investments.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE GROWTH PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
       DOMESTIC GROWTH PORTFOLIOS
 
       - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
         consistent with the preservation of capital.
 
       - AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO seek
         long-term growth of capital.
 
       GLOBAL GROWTH PORTFOLIOS
 
       - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
       - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
         manner consistent with the preservation of capital.
 
       The Portfolios' Trustees may change these objectives without a
       shareholder vote and the Portfolios will notify you of any changes
       that are material. If there is a material change to a Portfolio's
       objective or policies, you should consider whether that Portfolio
       remains an appropriate investment for you. There is no guarantee that
       a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE GROWTH PORTFOLIOS?
 
       The portfolio managers apply a "bottom up" approach in choosing
       investments. In other words, they look for companies with earnings growth
       potential one at a time. If a portfolio manager is unable to find
       investments with earnings growth potential, a significant portion of a
       Portfolio's assets may be in cash or similar investments.
 
       GROWTH PORTFOLIO invests primarily in common stocks selected for their
       growth potential. Although the Portfolio can invest in companies of any
       size, it generally invests in larger, more established companies.
 
       AGGRESSIVE GROWTH PORTFOLIO invests primarily in common stocks selected
       for their growth potential, and normally invests at least 50% of its
       equity assets in medium-sized companies.
 
 2 Janus Aspen Series
<PAGE>
 
       CAPITAL APPRECIATION PORTFOLIO invests primarily in common stocks
       selected for their growth potential. The Portfolio may invest in
       companies of any size, from larger, well-established companies to
       smaller, emerging growth companies.
 
       INTERNATIONAL GROWTH PORTFOLIO normally invests at least 65% of its total
       assets in securities of issuers from at least five different countries,
       excluding the United States. Although the Portfolio intends to invest
       substantially all of its assets in issuers located outside the United
       States, it may invest in U.S. issuers and it may at times invest all of
       its assets in fewer than five countries, or even a single country.
 
       WORLDWIDE GROWTH PORTFOLIO invests primarily in common stocks of
       companies of any size throughout the world. The Portfolio normally
       invests in issuers from at least five different countries, including the
       United States. The Portfolio may at times invest in fewer than five
       countries or even a single country.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIOS?
 
       The biggest risk is that the Portfolios' returns may vary, and you could
       lose money. If you are considering investing in any of the Growth
       Portfolios, remember that they are each designed for long-term investors
       who can accept the risks of investing in a portfolio with significant
       common stock holdings. Common stocks tend to be more volatile than other
       investment choices.
 
       The value of a Portfolio may decrease if the value of an individual
       company in the portfolio decreases. The value of a Portfolio could also
       decrease if the stock market goes down. If the value of a Portfolio
       decreases, its net asset value (NAV) will also decrease, which means if
       you sell your shares in a Portfolio you would get back less money.
 
       INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO may have
       significant exposure to foreign markets. As a result, their returns and
       NAV may be affected to a large degree by fluctuations in currency
       exchange rates or political or economic conditions in a particular
       country.
 
       AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL APPRECIATION PORTFOLIO are
       nondiversified. In other words, they may hold larger positions in a
       smaller number of securities than a diversified fund. As a result, a
       single security's increase or decrease in value may have a greater impact
       on a Portfolio's NAV and total return.
 
       An investment in these Portfolios is not a bank deposit and is not
       insured or guaranteed by the Federal Deposit Insurance Corporation or any
       other government agency.
 
                                                          Risk return summary  3
<PAGE>
 
       The following information provides some indication of the risks of
       investing in the Growth Portfolios by showing how each of the Growth
       Portfolios' performance has varied over time. The bar charts depict the
       change in performance from year-to-year during the period indicated, but
       do not include charges and expenses attributable to any insurance product
       which would lower the performance illustrated. The Portfolios do not
       impose any sales or other charges that would affect total return
       computations. Total return figures include the effect of each Portfolio's
       expenses. The tables compare the average annual returns for the Shares of
       each Portfolio for the periods indicated to a broad-based securities
       market index.
 
        GROWTH PORTFOLIO - INSTITUTIONAL SHARES
        
        A BAR CHART showing Total Annual Returns for Growth Portfolio -
        Institutional Shares from 1994 through 1998:

           Annual returns for periods ended 12/31

                                      2.76%   30.17%   18.45%   22.75%   35.66%
                                       1994    1995     1996     1997     1998

          Each percentage is represented by a bar of proportionate size with
          the actual return printed above the bar. 

          Best Quarter 4th-1998  27.71%  Worst Quarter 3rd-1998 (10.92%)
 
                          Average annual total return for periods ended 12/31/98
                         -------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                   Since Inception
                                                1 year   5 years      (9/13/93)
         <S>                                    <C>      <C>       <C> 
         Growth Portfolio - Institutional
           Shares                               35.66%   21.41%        20.91%
         S&P 500 Index*                         28.74%   24.08%        23.06%
                                            --------------------------------------
</TABLE>
 
        * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks, a
          widely recognized, unmanaged index of common stock prices.
 
 4 Janus Aspen Series
<PAGE>
 
        AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Aggressive Growth 
        Portfolio - Institutional Shares from 1994 through 1998:


           Annual returns for periods ended 12/31

                                      16.33%  27.48%    7.95%   12.66%   34.26%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  34.65%  Worst Quarter 3rd-1998 (14.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                   Since Inception
                                                1 year   5 years      (9/13/93)
         <S>                                    <C>      <C>       <C>
         Aggressive Growth Portfolio -
           Institutional Shares                 34.26%   19.35%        21.96%
         S&P 400 Mid Cap Index*                 18.25%   18.67%        18.06%
                                            --------------------------------------
</TABLE>
 
        * The S&P 400 Mid Cap Index is an unmanaged group of 400 domestic stocks
          chosen for their market size, liquidity and industry group
          representation.
 
        CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Capital Appreciation
        Portfolio - Institutional Shares for 1998:

           Annual returns for periods ended 12/31

                                                                         58.11%
                                                                          1998

           The percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  33.98%  Worst Quarter 3rd-1998 (9.98%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                  Since Inception
                                                         1 year      (5/1/97)
         <S>                                             <C>      <C>
         Capital Appreciation Portfolio -
           Institutional Shares                          58.11%       51.65%
         S&P 500 Index*                                  28.74%       31.38%
                                                     ----------------------------
</TABLE>
 
        * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks, a
          widely recognized, unmanaged index of common stock prices.
 
                                                          Risk return summary  5
<PAGE>
 
        INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for International Growth
        Portfolio - Institutional Shares from 1995 through 1998:

           Annual returns for periods ended 12/31

                                              23.15%   34.71%   18.51%   17.23%
                                               1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with 
           the actual return printed above the bar.

           Best Quarter 4th-1998  16.63%  Worst Quarter 3rd-1998 (17.76%)

                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                   Since Inception
                                                          1 year      (5/2/94)
         <S>                                              <C>      <C>
         International Growth
           Portfolio - Institutional Shares               17.23%       18.87%
         Morgan Stanley Capital International EAFE
           Index*                                         20.00%        8.11%
                                                      ----------------------------
</TABLE>
 
        * The Morgan Stanley Capital International EAFE Index is a market
          capitalization weighted index composed of companies representative of
          the market structure of 20 Developed Market countries in Europe,
          Australasia and the Far East.
 
        WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Worldwide Growth 
        Portfolio - Institutional Shares from 1994 through 1998:


           Annual returns for periods ended 12/31

                                      1.53%   27.37%   29.04%   22.15%   28.92%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.87%  Worst Quarter 3rd-1998 (16.03%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                    Since Inception
                                                 1 year   5 years      (9/13/93)
         <S>                                     <C>      <C>       <C>
         Worldwide Growth Portfolio -
           Institutional Shares                  28.92%   21.32%        24.06%
         Morgan Stanley International
           Worldwide Index*                      24.34%   15.68%        14.39%
                                             --------------------------------------
</TABLE>
 
        * The Morgan Stanley International Worldwide Index is a market
          capitalization weighted index composed of countries representative of
          the market structure of 47 Developed and Emerging Markets.
 
       The Growth Portfolios' past performance does not necessarily indicate how
       they will perform in the future.
 
 6 Janus Aspen Series
<PAGE>
 
COMBINATION PORTFOLIOS
 
       The Combination Portfolios are designed for investors who primarily seek
       growth of capital with varying degrees of emphasis on income. They are
       not designed for investors who desire a consistent level of income.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE COMBINATION PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
       - BALANCED PORTFOLIO seeks long-term capital growth, consistent with
         preservation of capital and balanced by current income.
 
       - EQUITY INCOME PORTFOLIO seeks current income and long-term growth of
         capital.
 
       - GROWTH AND INCOME PORTFOLIO seeks long-term capital growth and
         current income.
 
       The Trustees may change these objectives without a shareholder vote and
       the Portfolios will notify you of any changes that are material. If there
       is a material change to a Portfolio's objective or policies, you should
       consider whether that Portfolio remains an appropriate investment for
       you. There is no guarantee that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE COMBINATION PORTFOLIOS?
 
       The portfolio managers apply a "bottom up" approach in choosing
       investments. In other words, they look mostly for equity and
       income-producing securities that meet their investment criteria one at a
       time. If a portfolio manager is unable to find such investments, much of
       a Portfolio's assets may be in cash or similar investments.
 
       BALANCED PORTFOLIO normally invests 40-60% of its assets in securities
       selected primarily for their growth potential and 40-60% of its assets in
       securities selected primarily for their income potential. The Portfolio
       will normally invest at least 25% of its assets in fixed-income
       securities.
 
       EQUITY INCOME PORTFOLIO normally emphasizes investments in common stocks,
       and growth potential is a significant investment consideration. Normally,
       it invests at least 65% of its assets in income-producing equity
       securities.
 
       GROWTH AND INCOME PORTFOLIO normally emphasizes investments in common
       stocks. It will normally invest up to 75% of its assets in equity
       securities selected primarily for their growth potential, and at least
       25% of its assets in securities the portfolio manager believes have
       income potential. Equity securities may make up part of this income
       component if they currently pay dividends or
 
                                                          Risk return summary  7
<PAGE>
 
       the portfolio manager believes they have potential for increasing or
       commencing dividend payments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE COMBINATION PORTFOLIOS?
 
       The biggest risk is that the Portfolios' returns may vary, and you could
       lose money. If you are considering investing in any of the Combination
       Portfolios, remember that they are each designed for long-term investors
       who can accept the risks of investing in a portfolio with significant
       common stock holdings. Common stocks tend to be more volatile than other
       investment choices.
 
       The value of a Portfolio may decrease if the value of an individual
       company in the portfolio decreases. The value of a Portfolio could also
       decrease if the stock market goes down. If the value of a Portfolio
       decreases, its NAV will also decrease, which means if you sell your
       shares in a Portfolio you would get back less money.
 
       The income component of the Portfolios' holdings includes fixed-income
       securities. A fundamental risk to the income component is that the value
       of these securities will fall if interest rates rise. Generally, the
       value of a fixed-income portfolio will decrease when interest rates rise,
       which means the Portfolio's NAV may likewise decrease. Another
       fundamental risk associated with fixed-income securities is credit risk,
       which is the risk that an issuer of a bond will be unable to make
       principal and interest payments when due.
 
       An investment in these Portfolios is not a bank deposit and is not
       insured or guaranteed by the Federal Deposit Insurance Corporation or any
       other government agency.
 
 8 Janus Aspen Series
<PAGE>
 
       The following information provides some indication of the risks of
       investing in the Combination Portfolios by showing how each Combination
       Portfolio's performance has varied over time. The bar chart depicts the
       change in performance from year-to-year during the period indicated, but
       do not include charges and expenses attributable to any insurance product
       which would lower the performance illustrated. The Portfolios do not
       impose any sales or other charges that would affect total return
       computations. Total return figures include the effect of each Portfolio's
       expenses. The tables compare the average annual returns for the Shares of
       each Portfolio for the period indicated to a broad-based securities
       market index.
 
        BALANCED PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Balanced Portfolio -
        Institutional Shares from 1994 through 1998:


           Annual returns for periods ended 12/31

                                      0.84%   24.79%   16.18%   22.10%   34.28%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 4th-1998  20.32%  Worst Quarter 3rd-1998 (4.97%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                   Since Inception
                                                1 year   5 years      (9/13/93)
         <S>                                    <C>      <C>       <C>
         Balanced Portfolio - Institutional
           Shares                               34.28%   19.11%        19.53%
         S&P 500 Index*                         28.74%   24.08%        23.06%
         Lehman Brothers Gov't/Corp Bond
           Index**                               9.47%    7.30%         6.90%
                                            --------------------------------------
</TABLE>
 
        * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks, a
          widely recognized, unmanaged index of common stock prices.
       ** Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
          are of investment grade with at least one year until maturity.
 
                                                          Risk return summary  9
<PAGE>
 
        EQUITY INCOME PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Equity Income Portfolio -
        Institutional Shares for 1998:


           Annual returns for periods ended 12/31

                                                                         46.24%
                                                                          1998

          The percentage is represented by a bar of proportionate size with
          the actual return printed above the bar.
     
          Best Quarter 4th-1998  28.51%  Worst Quarter 3rd-1998 (7.18%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                  Since Inception
                                                         1 year      (5/1/97)
         <S>                                             <C>      <C>
         Equity Income Portfolio - Institutional
           Shares                                        46.24%       50.20%
         S&P 500 Index*                                  28.74%       31.38%
                                                     ----------------------------
</TABLE>
 
       * The S&P 500 is the Standard & Poor's Composite Index of 500 Stocks, a
         widely recognized, unmanaged index of common stock prices.
 
        GROWTH AND INCOME PORTFOLIO - INSTITUTIONAL SHARES
 
       Growth and Income Portfolio does not have a full calendar year return
       because the Portfolio commenced operations on May 1, 1998.
 
       The Combination Portfolios' past performance does not necessarily
       indicate how they will perform in the future.
 
 10 Janus Aspen Series
<PAGE>
 
FIXED-INCOME PORTFOLIOS
 
       The Fixed-Income Portfolios are designed for long-term investors who
       primarily seek current income.
 
1. WHAT ARE THE INVESTMENT OBJECTIVES OF THE FIXED-INCOME PORTFOLIOS?
 
- --------------------------------------------------------------------------------
 
       - FLEXIBLE INCOME PORTFOLIO seeks to obtain maximum total return,
         consistent with preservation of capital.
 
       - HIGH-YIELD PORTFOLIO seeks to obtain high current income. Capital
         appreciation is a secondary objective when consistent with its
         primary objective.
 
       The Trustees may change these objectives without a shareholder vote and
       the Portfolios will notify you of any changes that are material. If there
       is a material change to a Portfolio's objective or policies, you should
       consider whether it remains an appropriate investment for you. There is
       no guarantee that a Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF THE FIXED-INCOME PORTFOLIOS?
 
       In addition to considering economic factors such as the effect of
       interest rates on a Portfolio's investments, the portfolio managers apply
       a "bottom up" approach in choosing investments. In other words, they look
       mostly for income-producing securities that meet their investment
       criteria one at a time. If a portfolio manager is unable to find such
       investments, a Portfolio's assets may be in cash or similar investments.
 
       FLEXIBLE INCOME PORTFOLIO invests primarily in a wide variety of income-
       producing securities such as corporate bonds and notes, government
       securities and preferred stock. As a fundamental policy, the Portfolio
       will invest at least 80% of its assets in income-producing securities.
       The Portfolio may own an unlimited amount of high-yield/high-risk
       fixed-income securities, and these securities may be a big part of the
       portfolio.
 
       HIGH-YIELD PORTFOLIO normally invests at least 65% of its assets in
       high-yield/ high-risk fixed-income securities, and may at times invest
       all of its assets in these securities.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE FIXED-INCOME PORTFOLIOS?
 
       Although the Fixed-Income Portfolios may be less volatile than funds that
       invest most of their assets in common stocks, the Portfolios' returns and
       yields will vary, and you could lose money.
 
                                                         Risk return summary  11
<PAGE>
 
       The Portfolios invest in a variety of fixed-income securities. A
       fundamental risk is that the value of these securities will fall if
       interest rates rise. Generally, the value of a fixed-income portfolio
       will decrease when interest rates rise, which means the Portfolio's NAV
       will likewise decrease. Another fundamental risk associated with
       fixed-income funds is credit risk, which is the risk that an issuer will
       be unable to make principal and interest payments when due.
 
       FLEXIBLE INCOME PORTFOLIO AND HIGH-YIELD PORTFOLIO may invest an
       unlimited amount of their assets in high-yield/high-risk securities, also
       known as "junk" bonds which may be sensitive to economic changes,
       political changes, or adverse developments specific to the company that
       issued the bond. These securities generally have a greater credit risk
       than other types of fixed-income securities. Because of these factors,
       the performance and NAV of the Fixed-Income Portfolios may vary
       significantly, depending upon their holdings of junk bonds.
 
       An investment in these Portfolios is not a bank deposit and is not
       insured or guaranteed by the Federal Deposit Insurance Corporation or any
       other government agency.
 
 12 Janus Aspen Series
<PAGE>
 
       The following information provides some indication of the risks of
       investing in the Fixed-Income Portfolios by showing how each Fixed-Income
       Portfolio's performance has varied over time. The bar charts depict the
       change in performance from year-to-year during the period indicated, but
       do not include charges and expenses attributable to any insurance product
       which would lower the performance illustrated. The Portfolios do not
       impose any sales or other charges that would affect total return
       computations. Total return figures include the effect of each Portfolio's
       expenses. The tables compare the average annual returns for the Shares of
       each Portfolio for the periods indicated to a broad-based securities
       market index.
 
        FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES
 
        A BAR CHART showing Total Annual Returns for Flexible Income Portfolio -
        Institutional Shares from 1994 through 1998:


           Annual returns for periods ended 12/31

                                     (0.91%)  23.86%   9.19%    11.76%   9.11%
                                       1994    1995     1996     1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar. 

           Best Quarter 2nd-1995  27.71%  Worst Quarter 1st-1996 (1.08%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                    Since Inception
                                                 1 year   5 years      (9/13/93)
         <S>                                     <C>      <C>       <C>
         Flexible Income Portfolio -
           Institutional Shares                  9.11%    10.32%         9.87%
         Lehman Brothers Gov't/Corp Bond
           Index*                                9.47%     7.30%         6.90%
                                             --------------------------------------
</TABLE>
 
        * Lehman Brothers Gov't/Corp Bond Index is composed of all bonds that
          are of investment grade with at least one year until maturity.
 
                                                         Risk return summary  13
<PAGE>
 
        HIGH-YIELD PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for High-Yield Portfolio -
        Institutional Shares from 1994 through 1998:


           Annual returns for periods ended 12/31

                                                                15.98%    1.26%
                                                                 1997     1998

           Each percentage is represented by a bar of proportionate size with
           the actual return printed above the bar.

           Best Quarter 1st-1998  5.52%  Worst Quarter 3rd-1998 (6.07%)
 
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                   Since Inception
                                                          1 year      (5/1/96)
         <S>                                              <C>      <C>
         High-Yield Portfolio - Institutional Shares      1.26%        10.97%
         Lehman Brothers High-Yield Bond Index*           1.60%         7.15%
                                                      ---------------------------
</TABLE>
 
        * Lehman Brothers High-Yield Bond Index is composed of fixed-rate,
          publicly issued, noninvestment grade debt.
 
       The Fixed-Income Portfolios' past performance does not necessarily
       indicate how they will perform in the future.
 
MONEY MARKET PORTFOLIO
 
       Money Market Portfolio is designed for investors who seek current income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF MONEY MARKET PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
       - MONEY MARKET PORTFOLIO seeks maximum current income to the extent
         consistent with stability of capital.
 
       The Trustees may change this objective without a shareholder vote and the
       Portfolio will notify you of any changes that are material. If there is a
       material change in the Portfolio's objective or policies, you should
       consider whether it remains an appropriate investment for you. There is
       no guarantee that the Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF MONEY MARKET PORTFOLIO?
 
       MONEY MARKET PORTFOLIO will invest only in high-quality, short-term money
       market instruments that present minimal credit risks, as determined by
       Janus Capital. The Portfolio invests primarily in high quality debt
       obligations and
 
 14 Janus Aspen Series
<PAGE>
 
       obligations of financial institutions. Debt obligations may include
       commercial paper, notes and bonds, and variable amount master demand
       notes. Obligations of financial institutions include certificates of
       deposit and time deposits.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN MONEY MARKET PORTFOLIO?
 
       The Portfolio's yields will vary as the short-term securities in the
       portfolio mature and the proceeds are reinvested in securities with
       different interest rates. Over time, the real value of the Portfolio's
       yield may be eroded by inflation. Although Money Market Portfolio invests
       only in high-quality, short-term money market instruments, there is a
       risk that the value of the securities it holds will fall as a result of
       changes in interest rates, an issuer's actual or perceived credit-
       worthiness or an issuer's ability to meet its obligations.
 
       An investment in Money Market Portfolio is not a deposit of a bank and is
       not insured or guaranteed by the Federal Deposit Insurance Corporation or
       any other government agency. Although the Portfolio seeks to preserve the
       value of your investment at $1.00 per share, it is possible to lose money
       by investing in Money Market Portfolio.
 
                                                         Risk return summary  15
<PAGE>
 
       The following information provides some indication of the risks of
       investing in the Money Market Portfolio by showing how Money Market
       Portfolio's performance has varied over time. The bar chart depicts the
       change in performance from year to year, but does not include charges and
       expenses attributable to any insurance product which would lower the
       performance illustrated. The Portfolio does not impose any sales or other
       charges that would affect total return computations. Total return figures
       include the effect of the Portfolio's expenses.
 
        MONEY MARKET PORTFOLIO - INSTITUTIONAL SHARES

        A BAR CHART showing Total Annual Returns for Money Market Portfolio -
        Institutional Shares from 1996 through 1998:

           Annual returns for periods ended 12/31

                                                        5.05%    5.17%    5.36%
                                                        1996     1997     1998

          Each percentage is represented by a bar of proportionate size with
          the actual return printed above the bar.

           Best Quarter 3rd-1998  1.35%  Worst Quarter 2nd-1996 1.22%
 
       The 7-day yield for the Portfolio's Shares on December 31, 1998 was
       4.87%. For the Portfolio's current yield, call the Janus XpressLine(TM)
       at 1-888-979-7737.
 
       Money Market Portfolio's past performance does not necessarily indicate
       how it will perform in the future.
 
FEES AND EXPENSES
 
       SHAREHOLDER FEES, such as sales loads, redemption fees or exchange fees,
       are charged directly to an investor's account. All Janus funds are
       no-load investments, so you will not pay any shareholder fees when you
       buy or sell shares of the Portfolios. However, each variable insurance
       contract involves fees and expenses not described in this prospectus. See
       the accompanying contract prospectus for information regarding contract
       fees and expenses and any restrictions on purchases or allocations.
 
       ANNUAL FUND OPERATING EXPENSES are paid out of a Portfolio's assets and
       include fees for portfolio management, maintenance of shareholder
       accounts, shareholder servicing, accounting and other services. You do
       not pay these fees directly but, as the example on the next page shows,
       these costs are borne indirectly by all shareholders.
 
 16 Janus Aspen Series
<PAGE>
 
       This table and example are designed to assist participants in qualified
       plans that invest in the Shares of the Portfolios in understanding the
       fees and expenses that you may pay as an investor in the Shares. The
       information shown is based upon gross expenses (without the effect of
       expense offset arrangements) for the fiscal year ended December 31, 1998.
       OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN THE SHARES SHOULD
       REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A DESCRIPTION OF
       FEES AND EXPENSES, AS THE TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT
       THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE
       INCURRED UNDER A CONTRACT.

<TABLE>
<CAPTION>
                                                                      Total Annual                        Total Annual
                                                                    Fund Operating                       Fund Operating
                                                                    Expenses Without        Total        Expenses With
                                           Management      Other      Waivers or           Waivers         Waivers or
                                              Fee         Expenses     Reductions*      and Reductions     Reductions*
    <S>                                      <C>            <C>            <C>            <C>               <C>
    Growth Portfolio                         0.72%          0.03%          0.75%          0.07%              0.68%
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%            N/A              0.75%
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%          0.05%              0.92%
    International Growth Portfolio           0.75%          0.20%          0.95%          0.09%              0.86%
    Worldwide Growth Portfolio               0.67%          0.07%          0.74%          0.02%              0.72%
    Balanced Portfolio                       0.72%          0.02%          0.74%            N/A              0.74%
    Equity Income Portfolio                  0.75%          1.11%          1.86%          0.61%              1.25%
    Growth and Income Portfolio              0.75%          2.31%          3.06%          1.81%              1.25%
    Flexible Income Portfolio                0.65%          0.08%          0.73%            N/A              0.73%
    High-Yield Portfolio                     0.75%          1.36%          2.11%          1.11%              1.00%
    Money Market Portfolio                   0.25%          0.09%          0.34%            N/A              0.34%
</TABLE>
 
- --------------------------------------------------------------------------------
  * All expenses are stated both with and without contractual waivers and fee
    reductions by Janus Capital. Fee reductions for the Growth, Aggressive
    Growth, Capital Appreciation, International Growth, Worldwide Growth,
    Balanced, Equity Income and Growth and Income Portfolios reduce the
    Management Fee to the level of the corresponding Janus retail fund. Other
    waivers, if applicable, are first applied against the Management Fee and
    then against Other Expenses. Janus Capital has agreed to continue the
    waivers and fee reductions until at least the next annual renewal of the
    advisory agreement.
- --------------------------------------------------------------------------------
 
  EXAMPLE:
  THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR REDUCTIONS.
  This example is intended to help you compare the cost of investing in the
  Portfolios with the cost of investing in other mutual funds. The example
  assumes that you invest $10,000 in each of the Portfolios for the time
  periods indicated then redeem all of your shares at the end of those
  periods. The example also assumes that your investment has a 5% return each
  year, and that the Portfolios' operating expenses remain the same. Although
  your actual costs may be higher or lower, based on these assumptions your
  costs would be:
 
<TABLE>
<CAPTION>
                                                            1 Year     3 Years    5 Years    10 Years
                                                            -----------------------------------------
   <S>                                                      <C>        <C>        <C>        <C> 
   Growth Portfolio                                          $ 77       $240      $  417      $  930
   Aggressive Growth Portfolio                               $ 77       $240      $  417      $  930
   Capital Appreciation Portfolio                            $ 99       $309      $  536      $1,190
   International Growth Portfolio                            $ 97       $303      $  526      $1,166
   Worldwide Growth Portfolio                                $ 76       $237      $  411      $  918
   Balanced Portfolio                                        $ 76       $237      $  411      $  918
   Equity Income Portfolio                                   $189       $585      $1,006      $2,180
   Growth and Income Portfolio                               $309       $945      $1,606      $3,374
   Flexible Income Portfolio                                 $ 75       $233      $  406      $  906
   High-Yield Portfolio                                      $214       $661      $1,134      $2,441
   Money Market Portfolio                                    $ 35       $109      $  191      $  431
</TABLE>
 
                                                         Risk return summary  17
<PAGE>
Investment objectives, principal
           investment
           strategies and risks
 
       Each of the Portfolios has a similar investment objective and similar
       principal investment strategies to a Janus retail fund:
 
<TABLE>
      <S>                                      <C>
        Growth Portfolio                                           Janus Fund
        Aggressive Growth Portfolio                     Janus Enterprise Fund
        Capital Appreciation Portfolio                     Janus Twenty Fund*
        International Growth Portfolio                    Janus Overseas Fund
        Worldwide Growth Portfolio                       Janus Worldwide Fund
        Balanced Portfolio                                Janus Balanced Fund
        Equity Income Portfolio                      Janus Equity Income Fund
        Growth and Income Portfolio              Janus Growth and Income Fund
        Flexible Income Portfolio                  Janus Flexible Income Fund
        High-Yield Portfolio                            Janus High-Yield Fund
        Money Market Portfolio                        Janus Money Market Fund
</TABLE>
 
       * Prior to May 1, 1999 Capital Appreciation Portfolio was managed in a
       similar manner to Janus Olympus Fund.
 
       Although it is anticipated that each Portfolio and its corresponding
       retail fund will hold similar securities, differences in asset size, cash
       flow needs and other factors may result in differences in investment
       performance. The expenses of each Portfolio and its corresponding retail
       fund are expected to differ. The variable contract owner will also bear
       various insurance related costs at the insurance company level. You
       should review the accompanying separate account prospectus for a summary
       of fees and expenses.
 
GROWTH PORTFOLIOS
 
       This section takes a closer look at the investment objectives of each of
       the Growth Portfolios, their principal investment strategies and certain
       risks of investing in the Growth Portfolios. Strategies and policies that
       are noted as "fundamental" cannot be changed without a shareholder vote.
 
       Please carefully review the "Risks" section of this Prospectus on pages
       24-27 for a discussion of risks associated with certain investment
       techniques. We've also included a Glossary with descriptions of
       investment terms used throughout this Prospectus.
 
 18 Janus Aspen Series
<PAGE>
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
DOMESTIC GROWTH PORTFOLIOS
 
       GROWTH PORTFOLIO
       Growth Portfolio seeks long-term growth of capital in a manner consistent
       with the preservation of capital. It pursues its objective by investing
       primarily in common stocks selected for their growth potential. Although
       the Portfolio can invest in companies of any size, it generally invests
       in larger, more established companies.
 
       AGGRESSIVE GROWTH PORTFOLIO
       Aggressive Growth Portfolio seeks long-term growth of capital. It pursues
       its objective by investing primarily in common stocks selected for their
       growth potential, and normally invests at least 50% of its equity assets
       in medium-sized companies. Medium-sized companies are those whose market
       capitalizations fall within the range of companies in the S&P MidCap 400
       Index. Market capitalization is a commonly used measure of the size and
       value of a company. The market capitalizations within the Index will
       vary, but as of December 31, 1998, they ranged from approximately $142
       million to $73 billion.
 
       CAPITAL APPRECIATION PORTFOLIO
       Capital Appreciation Portfolio seeks long-term growth of capital. It
       pursues its objective by investing primarily in common stocks selected
       for their growth potential. The Portfolio may invest in companies of any
       size, from larger, well-established companies to smaller, emerging growth
       companies.
 
GLOBAL GROWTH PORTFOLIOS
 
       INTERNATIONAL GROWTH PORTFOLIO
       International Growth Portfolio seeks long-term growth of capital.
       Normally, the Portfolio pursues its objective by investing at least 65%
       of its total assets in securities of issuers from at least five different
       countries, excluding the United States. Although the Portfolio intends to
       invest substantially all of its assets in issuers located outside the
       United States, it may at times invest in U.S. issuers and it may at times
       invest all of its assets in fewer than five countries or even a single
       country.
 
       WORLDWIDE GROWTH PORTFOLIO
       Worldwide Growth Portfolio seeks long-term growth of capital in a manner
       consistent with the preservation of capital. It pursues its objective by
       investing
 
            Investment objectives, principal investment strategies and risks  19
<PAGE>
 
       primarily in common stocks of companies of any size throughout the world.
       The Portfolio normally invests in issuers from at least five different
       countries, including the United States. The Portfolio may at times invest
       in fewer than five countries or even a single country.
 
The following questions and answers are designed to help you better understand
the Growth Portfolios' principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
       Each of the Portfolios may invest substantially all of its assets in
       common stocks if its portfolio manager believes that common stocks will
       appreciate in value. The portfolio managers generally take a "bottom up"
       approach to selecting companies. In other words, they seek to identify
       individual companies with earnings growth potential that may not be
       recognized by the market at large. They make this assessment by looking
       at companies one at a time, regardless of size, country of organization,
       place of principal business activity, or other similar selection
       criteria. Realization of income is not a significant consideration when
       choosing investments for the Portfolios. Income realized on the
       Portfolios' investments will be incidental to their objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
       Generally, yes. The portfolio managers seek companies that meet their
       selection criteria, regardless of where a company is located. Foreign
       securities are generally selected on a stock-by-stock basis without
       regard to any defined allocation among countries or geographic regions.
       However, certain factors such as expected levels of inflation, government
       policies influencing business conditions, the outlook for currency
       relationships, and prospects for economic growth among countries, regions
       or geographic areas may warrant greater consideration in selecting
       foreign securities. There are no limitations on the countries in which
       the Portfolios may invest and the Portfolios may at times have
       significant foreign exposure.
 
3. WHAT DOES "MARKET CAPITALIZATION" MEAN?
 
       Market capitalization is the most commonly used measure of the size and
       value of a company. It is computed by multiplying the current market
       price of a share of the company's stock by the total number of its shares
       outstanding. As noted previously, market capitalization is an important
       investment criteria for Aggressive Growth Portfolio. Although the other
       Growth Portfolios offered by this Prospectus do not emphasize companies
       of any particular size, Portfolios with a larger asset base are more
       likely to invest in larger, more established issuers.
 
 20 Janus Aspen Series
<PAGE>
 
COMBINATION PORTFOLIOS
 
       This section takes a closer look at the investment objectives of each of
       the Combination Portfolios, their principal investment strategies and
       certain risks of investing in the Combination Portfolios. Strategies and
       policies that are noted as "fundamental" cannot be changed without a
       shareholder vote.
 
       Please carefully review the "Risks" section of this Prospectus on pages
       24-27 for a discussion of risks associated with certain investment
       techniques. We've also included a Glossary with descriptions of
       investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
       BALANCED PORTFOLIO
       Balanced Portfolio seeks long-term capital growth, consistent with
       preservation of capital and balanced by current income. It pursues its
       objective by normally investing 40-60% of its assets in securities
       selected primarily for their growth potential and 40-60% of its assets in
       securities selected primarily for their income potential. This Portfolio
       normally invests at least 25% of its assets in fixed-income securities.
 
       EQUITY INCOME PORTFOLIO
       Equity Income Portfolio seeks current income and long-term growth of
       capital. It pursues its objective by normally emphasizing investments in
       common stock, and growth potential is a significant investment
       consideration. The Portfolio tries to provide a lower level of volatility
       than the S&P 500 Index. Normally, it invests at least 65% of its assets
       in income-producing equity securities including common and preferred
       stocks, warrants and securities that are convertible to common or
       preferred stocks.
 
       GROWTH AND INCOME PORTFOLIO
       Growth and Income Portfolio seeks long-term capital growth and current
       income. It normally emphasizes investments in common stocks. It will
       normally invest up to 75% of its assets in equity securities selected
       primarily for their growth potential, and at least 25% of its assets in
       securities the portfolio manager believes have income potential. Because
       of this investment strategy, the Portfolio is not designed for investors
       who need consistent income.
 
            Investment objectives, principal investment strategies and risks  21
<PAGE>
 
The following questions and answers are designed to help you better understand
the Combination Portfolios' principal investment strategies.
 
1. HOW DO THE COMBINATION PORTFOLIOS DIFFER FROM EACH OTHER?
 
       Growth and Income Portfolio places a greater emphasis on aggressive
       growth stocks and may derive a greater portion of its income from
       dividend-paying common stocks. Because of these factors, its NAV can be
       expected to fluctuate more than the other Combination Portfolios.
       Although Equity Income Portfolio invests substantially all of its assets
       in common stocks, it emphasizes investments in dividend-paying common
       stocks and other equity securities characterized by relatively greater
       price stability, and thus may be expected to be less volatile than Growth
       and Income Portfolio, as discussed in more detail below. Balanced
       Portfolio places a greater emphasis on the income component of its
       portfolio and invests to a greater degree in securities selected
       primarily for their income potential. As a result it is expected to be
       the least volatile of the Combination Portfolios.
 
2. HOW DOES EQUITY INCOME PORTFOLIO TRY TO LIMIT PORTFOLIO VOLATILITY?
 
       Equity Income Portfolio seeks to provide a lower level of volatility than
       the stock market at large, as measured by the S&P 500. The lower
       volatility sought by this Portfolio is expected to result primarily from
       investments in dividend-paying common stocks and other equity securities
       characterized by relatively greater price stability. The greater price
       stability sought by Equity Income Portfolio may be characteristic of
       companies that generate above average free cash flows. A company may use
       free cash flows for a number of purposes including commencing or
       increasing dividend payments, repurchasing its own stock or retiring
       outstanding debt. The portfolio manager also considers growth potential
       in selecting this Portfolio's securities and may hold securities selected
       solely for their growth potential.
 
3. HOW ARE COMMON STOCKS SELECTED FOR THE COMBINATION PORTFOLIOS IN COMPARISON
   TO THE GROWTH PORTFOLIOS?
 
       Because income is a part of the investment objective of the Combination
       Portfolios, a portfolio manager may consider dividend-paying
       characteristics to a greater degree in selecting common stocks for these
       Portfolios.
 
4. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF BALANCED
   PORTFOLIO'S AND GROWTH AND INCOME PORTFOLIO'S HOLDINGS?
 
       Balanced Portfolio and Growth and Income Portfolio shift assets between
       the growth and income components of their holdings based on the portfolio
       managers' analysis of relevant market, financial and economic conditions.
       If a portfolio manager believes that growth securities will provide
       better returns than
 
 22 Janus Aspen Series
<PAGE>
 
       the yields then available or expected on income-producing securities,
       that Portfolio will place a greater emphasis on the growth component.
 
5. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF THE COMBINATION
   PORTFOLIOS' INVESTMENTS?
 
       The growth component of the Combination Portfolios' investments is
       expected to consist primarily of common stocks, but may also include
       warrants, preferred stocks or convertible securities selected primarily
       for their growth potential.
 
6. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED PORTFOLIO'S
   AND GROWTH AND INCOME PORTFOLIO'S HOLDINGS?
 
       The income component of Balanced Portfolio and Growth and Income
       Portfolio will consist of securities that the portfolio managers believe
       have income potential. Such securities may include equity securities,
       convertible securities and all types of debt securities. Equity
       securities may be included in the income component of a Portfolio if they
       currently pay dividends or the portfolio manager believes they have the
       potential for either increasing their dividends or commencing dividends,
       if none are currently paid.
 
            Investment objectives, principal investment strategies and risks  23
<PAGE>
 
FIXED-INCOME PORTFOLIOS
 
       This section takes a closer look at the investment objectives of each of
       the Fixed-Income Portfolios, their principal investment strategies and
       certain risks of investing in the Fixed-Income Portfolios. Strategies and
       policies that are noted as "fundamental" cannot be changed without a
       shareholder vote.
 
       Please carefully review the "Risks" section of this Prospectus on pages
       24-27 for a discussion of risks associated with certain investment
       techniques. We've also included a Glossary with descriptions of
       investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVES AND PRINCIPAL INVESTMENT STRATEGIES
 
       In addition to considering economic factors such as the effect of
       interest rates on a Portfolio's investments, the portfolio managers apply
       a "bottom up" approach in choosing investments. In other words, they look
       mostly for income-producing securities that meet their investment
       criteria one at a time. If a portfolio manager is unable to find such
       investments, much of a Portfolio's assets may be in cash or similar
       investments.
 
       FLEXIBLE INCOME PORTFOLIO
       Flexible Income Portfolio seeks to obtain maximum total return,
       consistent with preservation of capital. It pursues its objective by
       primarily investing in a wide variety of income-producing securities such
       as corporate bonds and notes, government securities and preferred stock.
       As a fundamental policy, the Portfolio will invest at least 80% of its
       assets in income-producing securities. The Portfolio may own an unlimited
       amount of high-yield/high-risk securities, and these may be a big part of
       the portfolio. This Portfolio generates total return from a combination
       of current income and capital appreciation, but income is usually the
       dominant portion.
 
       HIGH-YIELD PORTFOLIO
       High-Yield Portfolio seeks to obtain high current income. Capital
       appreciation is a secondary objective when consistent with its primary
       objective. It pursues its objectives by normally investing 65% of its
       assets in high-yield/high-risk fixed-income securities, and may at times
       invest all of its assets in these securities.
 
The following questions and answers are designed to help you better understand
the Fixed-Income Portfolios' principal investment strategies.
 
1. HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
 
       Generally, a fixed-income security will increase in value when interest
       rates fall and decrease in value when interest rates rise. Longer-term
       securities are
 
 24 Janus Aspen Series
<PAGE>
 
       generally more sensitive to interest rate changes than shorter-term
       securities, but they generally offer higher yields to compensate
       investors for the associated risks. High-yield bond prices are generally
       less directly responsive to interest rate changes than investment grade
       issues and may not always follow this pattern. A bond fund's
       average-weighted effective maturity and its duration are measures of how
       the fund may react to interest rate changes.
 
2. HOW DO THE FIXED-INCOME PORTFOLIOS MANAGE INTEREST RATE RISK?
 
       Each Fixed-Income Portfolio may vary the average-weighted effective
       maturity of its assets to reflect its portfolio manager's analysis of
       interest rate trends and other factors. A Portfolio's average-weighted
       effective maturity will tend to be shorter when the portfolio manager
       expects interest rates to rise and longer when its portfolio manager
       expects interest rates to fall. The Portfolios may also use futures,
       options and other derivatives to manage interest rate risks.
 
3. WHAT IS MEANT BY A PORTFOLIO'S "AVERAGE-WEIGHTED EFFECTIVE MATURITY"?
 
       The stated maturity of a bond is the date when the issuer must repay the
       bond's entire principal value to an investor. Some types of bonds may
       also have an "effective maturity" that is shorter than the stated date
       due to prepayment or call provisions. Securities without prepayment or
       call provisions generally have an effective maturity equal to their
       stated maturity. Dollar-weighted effective maturity is calculated by
       averaging the effective maturity of bonds held by a Portfolio with each
       effective maturity "weighted" according to the percentage of net assets
       that it represents.
 
4. WHAT IS MEANT BY A PORTFOLIO'S "DURATION"?
 
       A bond's duration indicates the time it will take an investor to recoup
       his investment. Unlike average maturity, duration reflects both principal
       and interest payments. Generally, the higher the coupon rate on a bond,
       the lower its duration will be. The duration of a bond portfolio is
       calculated by averaging the duration of bonds held by a fund with each
       duration "weighted" according to the percentage of net assets that it
       represents. Because duration accounts for interest payments, a
       Portfolio's duration is usually shorter than its average maturity.
 
5. WHAT IS A HIGH-YIELD/HIGH-RISK SECURITY?
 
       A high-yield/high-risk security (also called a "junk" bond) is a debt
       security rated below investment grade by major rating agencies (i.e., BB
       or lower by Standard & Poor's or Ba or lower by Moody's) or an unrated
       bond of similar quality. It presents greater risk of default (the failure
       to make timely interest and principal payments) than higher quality
       bonds.
 
            Investment objectives, principal investment strategies and risks  25
<PAGE>
 
GENERAL PORTFOLIO POLICIES OF THE PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
 
       Unless otherwise stated, each of the following policies applies to all of
       the Portfolios other than Money Market Portfolio. The percentage
       limitations included in these policies and elsewhere in this Prospectus
       apply at the time of purchase of the security. So, for example, if a
       Portfolio exceeds a limit as a result of market fluctuations or the sale
       of other securities, it will not be required to dispose of any
       securities.
 
       CASH POSITION
       When a portfolio manager believes that market conditions are unfavorable
       for profitable investing, or when he or she is otherwise unable to locate
       attractive investment opportunities, the Portfolios' cash or similar
       investments may increase. In other words, the Portfolios do not always
       stay fully invested in stocks and bonds. Cash or similar investments
       generally are a residual - they represent the assets that remain after a
       portfolio manager has committed available assets to desirable investment
       opportunities. However, a portfolio manager may also temporarily increase
       a Portfolio's cash position to protect its assets or maintain liquidity.
       Partly because the portfolio managers act independently of each other,
       the cash positions of the Portfolios may vary significantly.
 
       When a Portfolio's investments in cash or similar investments increase,
       it may not participate in market advances or declines to the same extent
       that it would if the Portfolio remained more fully invested in stocks or
       bonds.
 
       OTHER TYPES OF INVESTMENTS
       The Growth and Global Growth Portfolios invest primarily in domestic and
       foreign equity securities, which may include preferred stocks, common
       stocks, warrants and securities convertible into common or preferred
       stocks. The Combination Portfolios also invest in domestic and foreign
       equity securities with varying degrees of emphasis on income. The
       Portfolios may also invest to a lesser degree in other types of
       securities. These securities (which are described in the Glossary) may
       include:
 
       - debt securities
 
       - indexed/structured securities
 
       - high-yield/high-risk securities (less than 35% of each Portfolio's
         assets)
 
       - options, futures, forwards and other types of derivatives for hedging
         purposes or for non-hedging purposes such as seeking to enhance return
 
 26 Janus Aspen Series
<PAGE>
 
       - securities purchased on a when-issued, delayed delivery or forward
         commitment basis
 
       The Fixed-Income Portfolios invest primarily in fixed-income securities
       which may include corporate bonds and notes, government securities,
       preferred stock, high-yield/high-risk fixed-income securities and
       municipal obligations. The Portfolios may also invest to a lesser degree
       in other types of securities. These securities (which are described in
       the Glossary) may include:
 
       - common stocks
 
       - mortgage- and asset-backed securities
 
       - zero coupon, pay-in-kind and step coupon securities
 
       - options, futures, forwards and other types of derivatives for hedging
         purposes or for non-hedging purposes such as seeking to enhance return
 
       - securities purchased on a when-issued, delayed delivery or forward
         commitment basis
 
       ILLIQUID INVESTMENTS
       Each Portfolio may invest up to 15% of its net assets in illiquid
       investments. An illiquid investment is a security or other position that
       cannot be disposed of quickly in the normal course of business. For
       example, some securities are not registered under U.S. securities laws
       and cannot be sold to the U.S. public because of SEC regulations (these
       are known as "restricted securities"). Under procedures adopted by the
       Portfolios' Trustees, certain restricted securities may be deemed liquid,
       and will not be counted toward this 15% limit.
 
       FOREIGN SECURITIES
       The Portfolios may invest without limit in foreign equity and debt
       securities. The Portfolios may invest directly in foreign securities
       denominated in a foreign currency and not publicly traded in the United
       States. Other ways of investing in foreign securities include depositary
       receipts or shares, and passive foreign investment companies.
 
       SPECIAL SITUATIONS
       Each Portfolio may invest in special situations. A special situation
       arises when, in the opinion of a Portfolio's manager, the securities of a
       particular issuer will be recognized and appreciate in value due to a
       specific development with respect to that issuer. Developments creating a
       special situation might include, among others, a new product or process,
       a technological breakthrough, a management change or other extraordinary
       corporate event, or differences in
 
            Investment objectives, principal investment strategies and risks  27
<PAGE>
 
       market supply of and demand for the security. A Portfolio's performance
       could suffer if the anticipated development in a "special situation"
       investment does not occur or does not attract the expected attention.
 
       PORTFOLIO TURNOVER
       The Portfolios generally intend to purchase securities for long-term
       investment although, to a limited extent, a Portfolio may purchase
       securities in anticipation of relatively short-term price gains.
       Short-term transactions may also result from liquidity needs, securities
       having reached a price or yield objective, changes in interest rates or
       the credit standing of an issuer, or by reason of economic or other
       developments not foreseen at the time of the investment decision. A
       Portfolio may also sell one security and simultaneously purchase the same
       or a comparable security to take advantage of short-term differentials in
       bond yields or securities prices. Changes are made in a Portfolio's
       holdings whenever its portfolio manager believes such changes are
       desirable. Portfolio turnover rates are generally not a factor in making
       buy and sell decisions.
 
       Increased portfolio turnover may result in higher costs for brokerage
       commissions, dealer mark-ups and other transaction costs and may also
       result in taxable capital gains. Higher costs associated with increased
       portfolio turnover may offset gains in a Portfolio's performance.
 
 28 Janus Aspen Series
<PAGE>
 
RISKS FOR GROWTH, GLOBAL GROWTH AND COMBINATION PORTFOLIOS
 
       Because the Portfolios may invest substantially all of their assets in
       common stocks, the main risk is the risk that the value of the stocks
       they hold might decrease in response to the activities of an individual
       company or in response to general market and/or economic conditions. If
       this occurs, a Portfolio's share price may also decrease. A Portfolio's
       performance may also be affected by risks specific to certain types of
       investments, such as foreign securities, derivative investments,
       non-investment grade debt securities or companies with relatively small
       market capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Growth, Global Growth and Combination
Portfolios.
 
1. THE PORTFOLIOS MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
       Smaller or newer companies may suffer more significant losses as well as
       realize more substantial growth than larger or more established issuers
       because they may lack depth of management, be unable to generate funds
       necessary for growth or potential development, or be developing or
       marketing new products or services for which markets are not yet
       established and may never become established. In addition, such companies
       may be insignificant factors in their industries and may become subject
       to intense competition from larger or more established companies.
       Securities of smaller or newer companies may have more limited trading
       markets than the markets for securities of larger or more established
       issuers, and may be subject to wide price fluctuations. Investments in
       such companies tend to be more volatile and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF AGGRESSIVE GROWTH PORTFOLIO AND CAPITAL
   APPRECIATION PORTFOLIO AFFECT THEIR RISK?
 
       Diversification is a way to reduce risk by investing in a broad range of
       stocks or other securities. A "nondiversified" portfolio has the ability
       to take larger positions in a smaller number of issuers. Because the
       appreciation or depreciation of a single stock may have a greater impact
       on the NAV of a nondiversified portfolio, its share price can be expected
       to fluctuate more than a comparable diversified portfolio. This
       fluctuation, if significant, may affect the performance of a Portfolio.
 
            Investment objectives, principal investment strategies and risks  29
<PAGE>
 
RISKS FOR FIXED-INCOME PORTFOLIOS
 
       Because the Portfolios invest substantially all of their assets in
       fixed-income securities, they are subject to risks such as credit or
       default risks, and decreased value due to interest rate increases. A
       Portfolio's performance may also be affected by risks to certain types of
       investments, such as foreign securities and derivative instruments.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Fixed-Income Portfolios.
 
1. HOW DO THE FIXED-INCOME PORTFOLIOS DIFFER FROM EACH OTHER IN TERMS OF PRIMARY
   INVESTMENT TYPE, CREDIT RISK AND INTEREST RATE RISK?
 
       Flexible Income Portfolio and High-Yield Portfolio invest primarily in
       corporate bonds. High-Yield Portfolio's credit risk is generally higher
       than Flexible Income Portfolio. Flexible Income Portfolio's interest rate
       risk is generally higher than High-Yield Portfolio.
 
2. WHAT IS MEANT BY "CREDIT QUALITY" AND WHAT ARE THE RISKS ASSOCIATED WITH IT?
 
       Credit quality measures the likelihood that the issuer will meet its
       obligations on a bond. One of the fundamental risks associated with all
       fixed-income funds is credit risk, which is the risk that an issuer will
       be unable to make principal and interest payments when due. U.S.
       government securities are generally considered to be the safest type of
       investment in terms of credit risk. Municipal obligations generally rank
       between U.S. government securities and corporate debt securities in terms
       of credit safety. Corporate debt securities, particularly those rated
       below investment grade, present the highest credit risk.
 
3. HOW IS CREDIT QUALITY MEASURED?
 
       Ratings published by nationally recognized statistical rating agencies
       such as Standard & Poor's Ratings Service and Moody's Investors Service,
       Inc. are widely accepted measures of credit risk. The lower a bond issue
       is rated by an agency, the more credit risk it is considered to
       represent. Lower rated bonds generally pay higher yields to compensate
       investors for the associated risk. Please refer to "Explanation of Rating
       Categories" on page 55 for a description of rating categories.
 
 30 Janus Aspen Series
<PAGE>
 
RISKS COMMON TO ALL NON-MONEY MARKET PORTFOLIOS
 
The following questions and answers discuss risks that apply to all Portfolios
other than Money Market Portfolio.
 
1. HOW COULD THE PORTFOLIOS' INVESTMENTS IN FOREIGN SECURITIES AFFECT THEIR
   PERFORMANCE?
 
       The Portfolios may invest without limit in foreign securities either
       indirectly (e.g., depositary receipts) or directly in foreign markets.
       Investments in foreign securities, including those of foreign
       governments, may involve greater risks than investing in domestic
       securities because the Portfolios' performance may depend on issues other
       than the performance of a particular company. These issues include:
 
       - CURRENCY RISK. As long as a Portfolio holds a foreign security, its
         value will be affected by the value of the local currency relative to
         the U.S. dollar. When a Portfolio sells a foreign denominated security,
         its value may be worth less in U.S. dollars even if the security
         increases in value in its home country. U.S. dollar denominated
         securities of foreign issuers may also be affected by currency risk.
 
       - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to
         heightened political and economic risks, particularly in emerging
         markets which may have relatively unstable governments, immature
         economic structures, national policies restricting investments by
         foreigners, different legal systems, and economies based on only a few
         industries. In some countries, there is the risk that the government
         may take over the assets or operations of a company or that the
         government may impose taxes or limits on the removal of a Portfolio's
         assets from that country.
 
       - REGULATORY RISK. There may be less government supervision of foreign
         markets. As a result, foreign issuers may not be subject to the uniform
         accounting, auditing and financial reporting standards and practices
         applicable to domestic issuers and there may be less publicly available
         information about foreign issuers.
 
       - MARKET RISK. Foreign securities markets, particularly those of emerging
         market countries, may be less liquid and more volatile than domestic
         markets. Certain markets may require payment for securities before
         delivery and delays may be encountered in settling securities
         transactions. In some foreign markets, there may not be protection
         against failure by other parties to complete transactions.
 
       - TRANSACTION COSTS. Costs of buying, selling and holding foreign
         securities, including brokerage, tax and custody costs, may be higher
         than those involved in domestic transactions.
 
            Investment objectives, principal investment strategies and risks  31
<PAGE>
 
2. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
       High-yield/high-risk securities (or "junk" bonds) are securities rated
       below investment grade by the primary rating agencies such as Standard &
       Poor's and Moody's. The value of lower quality securities generally is
       more dependent on credit risk, or the ability of the issuer to meet
       interest and principal payments, than investment grade debt securities.
       Issuers of high-yield securities may not be as strong financially as
       those issuing bonds with higher credit ratings and are more vulnerable to
       real or perceived economic changes, political changes or adverse
       developments specific to the issuer.
 
       The junk bond market can experience sudden and sharp price swings.
       Because Flexible Income Portfolio and High-Yield Portfolio may invest a
       significant portion of their assets in high-yield/high-risk securities,
       investors should be willing to tolerate a corresponding increase in the
       risk of significant and sudden changes in NAV.
 
       Please refer to "Explanation of Rating Categories" on page 55 for a
       description of bond rating categories.
 
3. HOW DO THE PORTFOLIOS TRY TO REDUCE RISK?
 
       The Portfolios may use futures, options and other derivative instruments
       to "hedge" or protect their portfolios from adverse movements in
       securities prices and interest rates. The Portfolios may also use a
       variety of currency hedging techniques, including forward currency
       contracts, to manage exchange rate risk. The portfolio managers believe
       the use of these instruments will benefit the Portfolios. However, a
       Portfolio's performance could be worse than if the Portfolio had not used
       such instruments if a portfolio manager's judgement proves incorrect.
       Risks associated with the use of derivative instruments are described in
       the SAI.
 
4. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
       The portfolio managers carefully research each potential investment
       before making an investment decision and, among other things, consider
       Year 2000 readiness when selecting portfolio holdings. However, there is
       no guarantee that the information a portfolio manager receives regarding
       a company's Year 2000 readiness is completely accurate. If a company has
       not satisfactorily addressed Year 2000 issues, the Portfolio's
       performance could suffer.
 
 32 Janus Aspen Series
<PAGE>
 
MONEY MARKET PORTFOLIO
 
       This section takes a closer look at the investment objective of Money
       Market Portfolio, its principal investment strategies and certain risks
       of investing in the Portfolio. Strategies and policies that are noted as
       "fundamental" cannot be changed without a shareholder vote.
 
       Money Market Portfolio is subject to certain specific SEC rule
       requirements. Among other things, the Portfolio is limited to investing
       in U.S. dollar-denominated instruments with a remaining maturity of 397
       days or less (as calculated pursuant to Rule 2a-7 under the 1940 Act).
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
       Money Market Portfolio seeks maximum current income to the extent
       consistent with stability of capital. It pursues its objective by
       investing primarily in high quality debt obligations and obligations of
       financial institutions. Debt obligations may include commercial paper,
       notes and bonds, and variable amount master demand notes. Obligations of
       financial institutions include certificates of deposit and time deposits.
 
       Money Market Portfolio will:
 
       - invest in high quality, short-term money market instruments that
         present minimal credit risks, as determined by Janus Capital
 
       - invest only in U.S. dollar-denominated instruments that have a
         remaining maturity of 397 days or less (as calculated pursuant to Rule
         2a-7 under the 1940 Act)
 
       - maintain a dollar-weighted average portfolio maturity of 90 days or
         less
 
TYPES OF INVESTMENTS
 
       Money Market Portfolio invests primarily in:
 
       - high quality debt obligations
 
       - obligations of financial institutions
 
       The Portfolio may also invest (to a lesser degree) in:
 
       - U.S. Government Securities (securities issued or guaranteed by the U.S.
         government, its agencies and instrumentalities)
 
       - municipal securities
 
            Investment objectives, principal investment strategies and risks  33
<PAGE>
 
       DEBT OBLIGATIONS
 
       The Portfolio may invest in debt obligations of domestic issuers. Debt
       obligations include:
 
       - commercial paper
 
       - notes and bonds
 
       - variable amount master demand notes (the payment obligations on these
         instruments may be backed by securities, swap agreements or other
         assets, by a guarantee of a third party or solely by the unsecured
         promise of the issuer to make payments when due)
 
       - privately issued commercial paper or other securities that are
         restricted as to disposition under the federal securities laws
 
       OBLIGATIONS OF FINANCIAL INSTITUTIONS
 
       Examples of obligations of financial institutions include:
 
       - negotiable certificates of deposit, bankers' acceptances, time deposits
         and other obligations of U.S. banks (including savings and loan
         associations) having total assets in excess of one billion dollars and
         U.S. branches of foreign banks having total assets in excess of ten
         billion dollars
 
       - Eurodollar and Yankee bank obligations (Eurodollar bank obligations are
         dollar-denominated certificates of deposit or time deposits issued
         outside the U.S. capital markets by foreign branches of U.S. banks and
         by foreign banks. Yankee bank obligations are dollar-denominated
         obligations issued in the U.S. capital markets by foreign banks)
 
       - other U.S. dollar-denominated obligations of foreign banks having total
         assets in excess of ten billion dollars that Janus Capital believes are
         of an investment quality comparable to obligations of U.S. banks in
         which the Portfolio may invest
 
       Foreign, Eurodollar (and to a limited extent, Yankee) bank obligations
       are subject to certain sovereign risks. One such risk is the possibility
       that a foreign government might prevent dollar-denominated funds from
       flowing across its borders. Other risks include: adverse political and
       economic developments in a foreign country; the extent and quality of
       government regulation of financial markets and institutions; the
       imposition of foreign withholding taxes; and expropriation or
       nationalization of foreign issuers.
 
 34 Janus Aspen Series
<PAGE>
 
INVESTMENT TECHNIQUES
 
       The following is a description of other investment techniques that Money
       Market Portfolio may use:
 
       PARTICIPATION INTERESTS
       A participation interest gives Money Market Portfolio a proportionate,
       undivided interest in underlying debt securities and sometimes carries a
       demand feature.
 
       DEMAND FEATURES
       Demand features give Money Market Portfolio the right to resell
       securities at specified periods prior to their maturity dates. Demand
       features may shorten the life of a variable or floating rate security,
       enhance the instrument's credit quality and provide a source of
       liquidity.
 
       Demand features are often issued by third party financial institutions,
       generally domestic and foreign banks. Accordingly, the credit quality and
       liquidity of Money Market Portfolio's investments may be dependent in
       part on the credit quality of the banks supporting Money Market
       Portfolio's investments. This will result in exposure to risks pertaining
       to the banking industry, including the foreign banking industry.
       Brokerage firms and insurance companies also provide certain liquidity
       and credit support.
 
       VARIABLE AND FLOATING RATE SECURITIES
       Money Market Portfolio may invest in securities which have variable or
       floating rates of interest. These securities pay interest at rates that
       are adjusted periodically according to a specified formula, usually with
       reference to an interest rate index or market interest rate. Variable and
       floating rate securities are subject to changes in value based on changes
       in market interest rates or changes in the issuer's or guarantor's
       creditworthiness.
 
       MORTGAGE- AND ASSET-BACKED SECURITIES
       Money Market Portfolio may purchase fixed or variable rate
       mortgage-backed securities issued by the Government National Mortgage
       Association, Federal National Mortgage Association, the Federal Home Loan
       Mortgage Corporation, or other governmental or government-related entity.
       The Portfolio may purchase other mortgage- and asset-backed securities
       including securities backed by automobile loans, equipment leases or
       credit card receivables.
 
       Unlike traditional debt instruments, payments on these securities include
       both interest and a partial payment of principal. Prepayments of the
       principal of underlying loans may shorten the effective maturities of
       these securities and may result in the Portfolio having to reinvest
       proceeds at a lower interest rate.
 
            Investment objectives, principal investment strategies and risks  35
<PAGE>
 
       REPURCHASE AGREEMENTS
       Money Market Portfolio may enter into collateralized repurchase
       agreements. Repurchase agreements are transactions in which the Portfolio
       purchases securities and simultaneously commits to resell those
       securities to the seller at an agreed-upon price on an agreed-upon future
       date. The repurchase price reflects a market rate of interest and is
       collateralized by cash or securities.
 
       If the seller of the securities underlying a repurchase agreement fails
       to pay the agreed resale price on the agreed delivery date, Money Market
       Portfolio may incur costs in disposing of the collateral and may
       experience losses if there is any delay in its ability to do so.
 
 36 Janus Aspen Series
<PAGE>
                                                    Management of the portfolios
 
INVESTMENT ADVISER
 
       Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is the
       investment adviser to each of the Portfolios and is responsible for the
       day-to-day management of the investment portfolios and other business
       affairs of the Portfolios.
 
       Janus Capital began serving as investment adviser to Janus Fund in 1970
       and currently serves as investment adviser to all of the Janus retail
       funds, acts as sub-adviser for a number of private-label mutual funds and
       provides separate account advisory services for institutional accounts.
 
       Janus Capital furnishes continuous advice and recommendations concerning
       each Portfolio's investments. Janus Capital also furnishes certain
       administrative, compliance and accounting services for the Portfolios,
       and may be reimbursed by the Portfolios for its costs in providing those
       services. In addition, Janus Capital employees serve as officers of the
       Trust and Janus Capital provides office space for the Portfolios and pays
       the salaries, fees and expenses of all Portfolio officers and those
       Trustees who are affiliated with Janus Capital.
 
       Participating insurance companies that purchase the Portfolios' shares
       may perform certain administrative services relating to the Portfolios
       and Janus Capital or the Portfolios may pay those companies for such
       services.
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
       Each Portfolio pays Janus Capital a management fee which is calculated
       daily. The advisory agreement with each Portfolio spells out the
       management fee and other expenses that the Portfolios must pay. Each of
       the Portfolios is subject to the following management fee schedule
       (expressed as an annual rate). In addition, the Shares of each Portfolio
       incur expenses not assumed by Janus Capital, including transfer agent and
       custodian fees and expenses, legal and auditing fees, printing and
       mailing costs of sending reports and other information to existing
       shareholders, and independent Trustees' fees and expenses.
 
                                                Management of the portfolios  37
<PAGE>
 
<TABLE>
<CAPTION>
                                        Average Daily
                                         Net Assets         Annual Rate      Expense Limit
     Fee Schedule                       of Portfolio       Percentage (%)    Percentage (%)
- ------------------------------------------------------------------------------------------------
<S>                                   <C>                  <C>               <C>
     Growth Portfolio
     Aggressive Growth Portfolio      First $300 Million        0.75               N/A(1)
     Capital Appreciation Portfolio   Next $200 Million         0.70
     International Growth Portfolio   Over $500 Million         0.65
     Worldwide Growth Portfolio
     Balanced Portfolio
- ------------------------------------------------------------------------------------------------
     Equity Income Portfolio          First $300 Million        0.75
     Growth and Income Portfolio      Next $200 Million         0.70              1.25(1)(2)
                                      Over $500 Million         0.65
- ------------------------------------------------------------------------------------------------
     Flexible Income Portfolio        First $300 Million        0.65              1.00(2)
                                      Over $300 Million         0.55
- ------------------------------------------------------------------------------------------------
     High-Yield Portfolio             First $300 Million        0.75              1.00(2)
                                      Over $300 Million         0.65
- ------------------------------------------------------------------------------------------------
     Money Market Portfolio           All Asset Levels          0.25              0.50(2)
- ------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive Growth, Capital
    Appreciation, International Growth, Worldwide Growth, Balanced, Equity
    Income and Growth and Income Portfolio's management fee to the extent that
    such fee exceeds the effective rate of the Janus retail fund corresponding
    to such Portfolio. Janus Capital has agreed to continue such waivers until
    at least the next annual renewal of the advisory contracts. The effective
    rate is the management fee calculated by the corresponding retail fund as of
    the last day of each calendar quarter (expressed as an annual rate). The
    effective rates of Janus Fund, Janus Enterprise Fund, Janus Olympus Fund,
    Janus Overseas Fund, Janus Worldwide Fund, Janus Balanced Fund, Janus Equity
    Income Fund, and Janus Growth and Income Fund were 0.65%, 0.69%, 0.67%,
    0.66%, 0.65%, 0.67%, 0.72%, and 0.66%, respectively, for the quarter ended
    March 31, 1999 (to be filed by amendment).
 
(2) Janus Capital has agreed to limit the Portfolios' expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
 38 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGERS
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
               is Executive Vice President and co-manager of International
               Growth Portfolio and Janus Overseas Fund which he has
               co-managed since May 1998 and April 1998, respectively. He
               served as assistant portfolio manager for these funds since
               1996. He is also assistant portfolio manager for Worldwide
               Growth Portfolio and Janus Worldwide Fund. Mr. Chang joined
               Janus Capital in 1993 after receiving a Masters Degree in
               Political Science from Stanford University. He is a Chartered
               Financial Analyst.
 
DAVID J. CORKINS
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Growth
               and Income Portfolio which he has managed since its inception.
               He is Executive Vice President and portfolio manager of Janus
               Growth and Income Fund which he has managed since August 1997.
               He previously served as an assistant portfolio manager of
               Janus Mercury Fund. He joined Janus in 1995 as a research
               analyst specializing in domestic financial services companies
               and a variety of foreign industries. Prior to joining Janus,
               he was the Chief Financial Officer of Chase U.S. Consumer
               Services, Inc., a Chase Manhattan mortgage business. He holds
               a Bachelor of Arts in English and Russian from Dartmouth and
               received his Master of Business Administration from Columbia
               University in 1993.
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
               is Chief Investment Officer of Janus Capital. He is Executive
               Vice President and portfolio manager of Growth Portfolio,
               which he has managed since inception. He has managed Janus
               Fund since 1986 and has co-managed Janus Venture Fund since
               February 1, 1997. Mr. Craig previously managed Janus Venture
               Fund from its inception, to December 1993, Janus Balanced Fund
               from December 1993 to December 1995 and Balanced Portfolio
               from September 1993 through April 1996. He holds a Bachelor of
               Arts in Business from the University of Alabama and a Master
               of Arts in Finance from the Wharton School of the University
               of Pennsylvania.
 
                                                Management of the portfolios  39
<PAGE>
 
JAMES P. GOFF
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of
               Aggressive Growth Portfolio, which he has managed since
               inception. Mr. Goff joined Janus Capital in 1988 and has
               managed Janus Enterprise Fund since its inception. Mr. Goff
               co-managed or managed Janus Venture Fund from December 1993 to
               February 1, 1997. He holds a Bachelor of Arts in Economics
               from Yale University and is a Chartered Financial Analyst.
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Worldwide
               Growth Portfolio and co-manager of International Growth
               Portfolio, which she has managed or co-managed since
               inception. Ms. Hayes joined Janus Capital in 1987 and has
               managed or co-managed Janus Worldwide Fund and Janus Overseas
               Fund since their inceptions. She holds a Bachelor of Arts in
               Economics from Yale University and is a Chartered Financial
               Analyst.
 
SHARON S. PICHLER
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Money
               Market Portfolio, which she has managed since inception. She
               also has managed Janus Money Market Fund, Janus Government
               Money Market Fund and Janus Tax-Exempt Money Market Fund since
               inception. She holds a Bachelor of Arts in Economics from
               Michigan State University and a Master of Business
               Administration from the University of Texas at San Antonio.
               Ms. Pichler is a Chartered Financial Analyst.
 
 40 Janus Aspen Series
<PAGE>
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Balanced
               Portfolio, which he has managed since May 1996 and Equity
               Income Portfolio, which he has managed since inception. He is
               an assistant portfolio manager of Growth Portfolio. Mr.
               Rollins joined Janus Capital in 1990 and has managed Janus
               Balanced Fund since January 1996 and Janus Equity Income Fund
               since inception. He has been an assistant portfolio manager of
               Janus Fund since January 1995. He gained experience as a
               fixed-income trader and equity research analyst prior to
               managing Balanced Portfolio. He holds a Bachelor of Science in
               Finance from the University of Colorado and is a Chartered
               Financial Analyst.
 
SANDY R. RUFENACHT
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of
               High-Yield Portfolio, which he has managed or co-managed since
               October 1996. He previously co-managed Flexible Income
               Portfolio from January 1997 to May 1998. Mr. Rufenacht joined
               Janus Capital in 1990 and has managed Janus Short-Term Bond
               Fund since January 1996. He is also the portfolio manager of
               Janus High-Yield Fund. He previously co-managed Janus Flexible
               Income Fund from June 1996 to February 1998. He holds a
               Bachelor of Arts in Business from the University of Northern
               Colorado.
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Capital
               Appreciation Portfolio, which he has managed since its
               inception. He is portfolio manager of Janus Twenty Fund, which
               he has managed since August 1997. He previously managed Janus
               Olympus Fund from its inception to August 1997. Mr. Schoelzel
               joined Janus Capital in January 1994. He holds a Bachelor of
               Arts in Business from Colorado College.
 
                                                Management of the portfolios  41
<PAGE>
 
RONALD V. SPEAKER
- --------------------------------------------------------------------------------
               is Executive Vice President and portfolio manager of Flexible
               Income Portfolio which he has managed or co-managed since its
               inception. He previously served as co-manager of High-Yield
               Portfolio, from its inception to May 1998. He managed
               Short-Term Bond Portfolio from its inception through April
               1996. Mr. Speaker joined Janus Capital in 1986. He has managed
               or co-managed Janus Flexible Income Fund since December 1991
               and previously managed both Janus Short-Term Bond Fund and
               Janus Federal Tax-Exempt Fund from inception through December
               1995. He previously managed or co-managed Janus High-Yield
               Fund from its inception to February 1998. He holds a Bachelor
               of Arts in Finance from the University of Colorado and is a
               Chartered Financial Analyst.
 
               In January 1997, Mr. Speaker settled an SEC administrative
               action involving two personal trades made by him in January of
               1993. Without admitting or denying the allegations, Mr.
               Speaker agreed to civil money penalty, disgorgement, and
               interest payments totaling $37,199 and to a 90-day suspension
               which ended on April 25, 1997.
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
               is an assistant portfolio manager of the Growth Portfolio. He
               is also an assistant portfolio manager of Janus Fund. He is
               Executive Vice President and portfolio manager of Janus
               Special Situations Fund. Mr. Decker received a Masters of
               Business Administration in Finance from the Fuqua School of
               Business at Duke University and a Bachelor's Degree in
               Economics and Political Science from Tufts University. He is a
               Chartered Financial Analyst.
 
 42 Janus Aspen Series
<PAGE>
 
RON SACHS
- --------------------------------------------------------------------------------
               is an assistant portfolio manager of Aggressive Growth
               Portfolio. Mr. Sachs joined Janus Capital in 1996 as a
               research analyst. Prior to coming to Janus, he worked as a
               consultant for Bain & Company and as an attorney for Willkie,
               Farr & Gallagher. Mr. Sachs graduated from Princeton cum laude
               with an undergraduate degree in economics. He obtained his law
               degree from the University of Michigan. Mr. Sachs is a
               Chartered Financial Analyst.
 
JOHN H. SCHREIBER
- --------------------------------------------------------------------------------
               is an assistant portfolio manager of Equity Income Portfolio.
               Mr. Schreiber joined Janus Capital in 1997 as an equity
               research analyst. Prior to coming to Janus he was an equity
               analyst with Fidelity Investments. Mr. Schreiber holds a
               Bachelor of Science degree in mechanical engineering from the
               University of Washington and an MBA from Harvard University.
               He is a candidate for the Chartered Financial Analyst
               designation.
 
                                                Management of the portfolios  43
<PAGE>
Other information
 
       CLASSES OF SHARES
 
       Each Portfolio currently offers two classes of Shares, one of which, the
       Institutional Shares, are offered pursuant to this prospectus and are
       sold under the name Janus Aspen Series. The Shares offered by this
       Prospectus are available only in connection with investment in and
       payments under variable insurance contracts as well as certain qualified
       retirement plans. Retirement Shares of each Portfolio are offered by
       separate prospectus and are available only to participant directed
       qualified plans using plan service providers that are compensated for
       providing distribution and/or recordkeeping and other administrative
       services provided to plan participants. Because the expenses of each
       class may differ, the performance of each class is expected to differ. If
       you would like additional information about the Retirement Shares, please
       call 1-800-525-0020.
 
       CONFLICTS OF INTEREST
 
       The Shares offered by this prospectus are available only to variable
       annuity and variable life separate accounts of insurance companies that
       are unaffiliated with Janus Capital and to certain qualified retirement
       plans. Retirement Shares of the Portfolios (offered through a separate
       prospectus) are available to certain participant directed qualified
       plans. Although the Portfolios do not currently anticipate any
       disadvantages to policy owners because each Portfolio offers its shares
       to such entities, there is a possibility that a material conflict may
       arise. The Trustees monitor events in order to identify any disadvantages
       or material irreconcilable conflicts and to determine what action, if
       any, should be taken in response. If a material disadvantage or conflict
       occurs, the Trustees may require one or more insurance company separate
       accounts or qualified plans to withdraw its investments in one or more
       Portfolios or substitute Shares of another Portfolio. If this occurs, a
       Portfolio may be forced to sell its securities at disadvantageous prices.
       In addition, the Trustees may refuse to sell Shares of any Portfolio to
       any separate account or qualified plan or may suspend or terminate the
       offering of a Portfolio's Shares if such action is required by law or
       regulatory authority or is in the best interests of that Portfolio's
       shareholders. It is possible that a qualified plan investing in the
       Retirement Shares of the Portfolios could lose its qualified plan status
       under the Internal Revenue Code, which could have adverse tax
       consequences on insurance company separate accounts investing in the
       Shares. Janus Capital intends to monitor such qualified plans and the
       Portfolios may discontinue sales to a qualified plan and require plan
       participants with existing investments in the Retirement Shares to redeem
       those investments if a plan loses (or in the opinion of Janus Capital is
       at risk of losing) its qualified plan status.
 
 44 Janus Aspen Series
<PAGE>
 
       YEAR 2000
 
       Preparing for Year 2000 is a high priority for Janus Capital, which has
       established a dedicated group to address this issue. Janus Capital has
       devoted considerable internal resources and has engaged one of the
       foremost experts in the field to help achieve Year 2000 readiness. Janus
       Capital does not anticipate that Year 2000-related issues will have a
       material impact on its ability to continue to provide the Portfolios with
       service at current levels; however, Janus Capital cannot make any
       assurances that the steps it has taken to ensure Year 2000 readiness will
       be successful. In addition, there can be no assurance that Year 2000
       issues will not affect the companies in which the Portfolios invest or
       worldwide markets and economies.
 
                                                           Other information  45
<PAGE>
Distributions and taxes
 
DISTRIBUTIONS
 
       To avoid taxation of the Portfolios, the Internal Revenue Code requires
       each Portfolio to distribute net income and any net gains realized on its
       investments annually. A Portfolio's income from dividends and interest
       and any net realized short-term gains are paid to shareholders as
       ordinary income dividends. Net realized long-term gains are paid to
       shareholders as capital gains distributions.
 
PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
 
       Each class of each Portfolio, other than Money Market Portfolio, makes
       semi-annual distributions in June and December of substantially all of
       its investment income and an annual distribution in June of its net
       realized gains, if any. All dividends and capital gains distributions
       from Shares of a Portfolio will automatically be reinvested into
       additional Shares of that Portfolio.
 
       HOW DISTRIBUTIONS AFFECT NAV
 
       Distributions, other than daily income dividends, are paid to
       shareholders as of the record date of the distribution of a Portfolio,
       regardless of how long the shares have been held. Undistributed income
       and realized gains are included in the daily NAV of a Portfolio's Shares.
       The Share price of a Portfolio drops by the amount of the distribution,
       net of any subsequent market fluctuations. For example, assume that on
       December 31, the Shares of Growth Portfolio declared a dividend in the
       amount of $0.25 per share. If the price of Growth Portfolio's Shares was
       $10.00 on December 30, the share price on December 31 would be $9.75,
       barring market fluctuations.
 
MONEY MARKET PORTFOLIO
 
       For the Shares of Money Market Portfolio, dividends representing
       substantially all of the net investment income and any net realized gains
       on sales of securities are declared daily, Saturdays, Sundays and
       holidays included, and distributed on the last business day of each
       month. If a month begins on a Saturday, Sunday or holiday, dividends for
       those days are declared at the end of the preceding month and distributed
       on the first business day of the month. All distributions will be
       automatically reinvested in Shares of the Portfolio.
 
TAXES
 
       TAXES ON DISTRIBUTIONS
 
       Because Shares of the Portfolios may be purchased only through variable
       insurance contracts and qualified plans, it is anticipated that any
       income dividends or capital gains distributions made by the Shares of a
       Portfolio will be
 
 46 Janus Aspen Series
<PAGE>
 
       exempt from current taxation if left to accumulate within the variable
       insurance contract or qualified plan. Generally, withdrawals from such
       contracts may be subject to ordinary income tax and, if made before age
       59 1/2, a 10% penalty tax. The tax status of your investment depends on
       the features of your qualified plan or variable insurance contract.
       Further information may be found in your plan documents or in the
       prospectus of the separate account offering such contract.
 
       TAXATION OF THE PORTFOLIOS
 
       Dividends, interest and some gains received by the Portfolios on foreign
       securities may be subject to withholding of foreign taxes. The Portfolios
       may from year to year make the election permitted under Section 853 of
       the Internal Revenue Code to pass through such taxes to shareholders. If
       such election is not made, any foreign taxes paid or accrued will
       represent an expense to the Portfolios which will reduce their investment
       income.
 
       The Portfolios do not expect to pay any federal income or excise taxes
       because they intend to meet certain requirements of the Internal Revenue
       Code. In addition, each Portfolio intends to qualify under the Internal
       Revenue Code with respect to the diversification requirements related to
       the tax-deferred status of insurance company separate accounts.
 
                                                     Distributions and taxes  47
<PAGE>
Shareholder's guide
 
       INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS DIRECTLY.
       SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE INSURANCE
       CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF PARTICIPATING INSURANCE
       COMPANIES OR THROUGH QUALIFIED RETIREMENT PLANS. CERTAIN PORTFOLIOS MAY
       NOT BE AVAILABLE IN CONNECTION WITH A PARTICULAR CONTRACT AND CERTAIN
       CONTRACTS MAY LIMIT ALLOCATIONS AMONG THE PORTFOLIOS. REFER TO THE
       PROSPECTUS FOR THE PARTICIPATING INSURANCE COMPANY'S SEPARATE ACCOUNT OR
       YOUR PLAN DOCUMENTS FOR INSTRUCTIONS ON PURCHASING OR SELLING OF VARIABLE
       INSURANCE CONTRACTS AND ON HOW TO SELECT SPECIFIC PORTFOLIOS AS
       INVESTMENT OPTIONS FOR A CONTRACT OR A QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
       Investments will be processed at the NAV next determined after an order
       is received and accepted by a Portfolio or its agent. In order to receive
       a day's price, your order must be received by the close of the regular
       trading session of the New York Stock Exchange any day that the NYSE is
       open. Securities of the Portfolios other than Money Market Portfolio are
       valued at market value or, if a market quotation is not readily
       available, at their fair value determined in good faith under procedures
       established by and under the supervision of the Trustees. Short-term
       instruments maturing within 60 days are valued at amortized cost, which
       approximates market value. See the SAI for more detailed information.
 
       To the extent a Portfolio holds securities that are primarily listed on
       foreign exchanges that trade on weekends or other days when the
       Portfolios do not price their shares, the NAV of a Portfolio's shares may
       change on days when shareholders will not be able to purchase or redeem
       the Portfolio's shares.
 
       Money Market Portfolio's securities are valued at their amortized cost.
       Amortized cost valuation involves valuing an instrument at its cost and
       thereafter assuming a constant amortization to maturity (or such other
       date as permitted by Rule 2a-7) of any discount or premium. If
       fluctuating interest rates cause the market value of the portfolio to
       deviate more than 1/2 of 1% from the value determined on the basis of
       amortized cost, the Trustees will consider whether any action, such as
       adjusting the Share's NAV to reflect current market conditions, should be
       initiated to prevent any material dilutive effect on shareholders.
 
 48 Janus Aspen Series
<PAGE>
 
PURCHASES
 
       Purchases of Shares may be made only by the separate accounts of
       insurance companies for the purpose of funding variable insurance
       contracts or by qualified plans. Refer to the prospectus of the
       appropriate insurance company separate account or your plan documents for
       information on how to invest in the Shares of each Portfolio.
       Participating insurance companies and certain other designated
       organizations are authorized to receive purchase orders on the
       Portfolios' behalf.
 
       Each Portfolio reserves the right to reject any specific purchase order.
       Purchase orders may be refused if, in Janus Capital's opinion, they are
       of a size that would disrupt the management of a Portfolio. Although
       there is no present intention to do so, the Portfolios may discontinue
       sales of their shares if management and the Trustees believe that
       continued sales may adversely affect a Portfolio's ability to achieve its
       investment objective. If sales of a Portfolio's Shares are discontinued,
       it is expected that existing policy owners and plan participants invested
       in that Portfolio would be permitted to continue to authorize investment
       in that Portfolio and to reinvest any dividends or capital gains
       distributions, absent highly unusual circumstances.
 
REDEMPTIONS
 
       Redemptions, like purchases, may be effected only through the separate
       accounts of participating insurance companies or through qualified plans.
       Please refer to the appropriate separate account prospectus or plan
       documents for details.
 
       Shares of any Portfolio may be redeemed on any business day. Redemptions
       are processed at the NAV next calculated after receipt and acceptance of
       the redemption order by the Portfolio or its agent. Redemption proceeds
       will normally be wired to the participating insurance company the
       business day following receipt of the redemption order, but in no event
       later than seven days after receipt of such order.
 
SHAREHOLDER COMMUNICATIONS
 
       Shareholders will receive annual and semiannual reports including the
       financial statements of the Shares of the Portfolios that they have
       authorized for investment. Each report will show the investments owned by
       each Portfolio and the market values thereof, as well as other
       information about the Portfolios and their operations. The Trust's fiscal
       year ends December 31.
 
                                                         Shareholder's guide  49
<PAGE>
Financial highlights
 
       The financial highlights tables are intended to help you understand the
       Institutional Shares' financial performance for each of the five most
       recent fiscal years or the life of the Portfolio if less than five years.
       Items 1 through 9 reflect financial results for a single Share. Total
       return in the tables represents the rate that an investor would have
       earned (or lost) on an investment in each of the Institutional Shares of
       the Portfolios (assuming reinvestment of all dividends and distributions)
       but does not include charges and expenses attributable to any insurance
       product. This information has been audited by PricewaterhouseCoopers LLP,
       whose report, along with the Portfolios' financial statements, is
       included in the Annual Report, which is available upon request and
       incorporated by reference into the SAI.
 
<TABLE>
<CAPTION>
GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- --------------------------------------------------------------------------------------------------------------
                                                                      Periods ending December 31
                                                           1998        1997       1996       1995       1994
<S>                                                     <C>          <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                   $18.48     $15.51     $13.45     $10.57    $10.32
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                    0.05       0.15       0.17       0.28      0.09
  3. Net gains or losses on securities (both realized
     and unrealized)                                          6.36       3.34       2.29       2.90      0.20
  4. Total from investment operations                         6.41       3.49       2.46       3.18      0.29
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                 (0.05)     (0.15)     (0.17)     (0.30)    (0.04)
  6. Tax return of capital distributions                        --         --         --         --        --
  7. Distributions (from capital gains)                     (1.30)     (0.37)     (0.23)         --        --
  8. Total distributions                                    (1.35)     (0.52)     (0.40)     (0.30)    (0.04)
  9. NET ASSET VALUE, END OF PERIOD                         $23.54     $18.48     $15.51     $13.45    $10.57
 10. Total return                                           35.66%     22.75%     18.45%     30.17%     2.76%
 11. Net assets, end of period (in thousands)           $1,103,549   $608,281   $325,789   $126,911   $43,549
 12. Average net assets for the period (in thousands)     $789,454   $477,914   $216,125    $77,344   $26,464
 13. Ratio of gross expenses to average net assets           0.68%(6)   0.70%(5)   0.69%(4)   0.78%(3)    N/A
 14. Ratio of net expenses to average net assets             0.68%      0.69%      0.69%      0.76%     0.88%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                  0.26%      0.91%      1.39%      1.24%     1.45%
 16. Portfolio turnover rate                                   73%       122%        87%       185%      169%
- --------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.23% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(3) The ratio was 0.98% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Fund.
 
 50 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
AGGRESSIVE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- --------------------------------------------------------------------------------------------------------------
                                                                       Periods ending December 31
                                                            1998       1997       1996       1995       1994
<S>                                                       <C>        <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                   $20.55     $18.24     $17.08     $13.62    $11.80
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                      --         --         --       0.24      0.11
  3. Net gains or losses on securities (both realized
     and unrealized)                                          7.09       2.31       1.36       3.47      1.82
  4. Total from investment operations                         7.09       2.31       1.36       3.71      1.93
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     --         --         --     (0.25)    (0.11)
  6. Tax return of capital distributions                        --         --     (0.01)         --        --
  7. Distributions (from capital gains)                         --         --     (0.19)         --        --
  8. Total distributions                                        --         --     (0.20)     (0.25)    (0.11)
  9. NET ASSET VALUE, END OF PERIOD                         $27.64     $20.55     $18.24     $17.08    $13.62
 10. Total return                                           34.26%     12.66%      7.95%     27.48%    16.33%
 11. Net assets, end of period (in thousands)             $772,943   $508,198   $383,693   $185,911   $41,289
 12. Average net assets for the period (in thousands)     $576,444   $418,464   $290,629   $107,582   $14,152
 13. Ratio of gross expenses to average net assets           0.75%(6)   0.76%(5)   0.76%(4)   0.86%(3)    N/A
 14. Ratio of net expenses to average net assets             0.75%      0.76%      0.76%      0.84%     1.05%(1)(2)
 15. Ratio of net investment income to average net
     assets                                                (0.36%)    (0.10%)    (0.27%)      0.58%     2.18%
 16. Portfolio turnover rate                                  132%       130%        88%       155%      259%
- --------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.14% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(3) The ratio was 0.93% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(4) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(5) The ratio was 0.78% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
(6) The ratio was 0.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Enterprise Fund.
 
                                                        Financial highlights  51
<PAGE>
 
<TABLE>
<CAPTION>
CAPITAL APPRECIATION PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------
                                                                 Periods ending
                                                                   December 31
                                                                 1998     1997(1)
<S>                                                            <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                       $12.62     $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                        0.01       0.05
  3. Net gains or losses on securities (both realized and
     unrealized)                                                  7.32       2.61
  4. Total from investment operations                             7.33       2.66
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     (0.01)     (0.04)
  6. Tax return of capital distributions                            --         --
  7. Distributions (from capital gains)                             --         --
  8. Total distributions                                        (0.01)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                             $19.94     $12.62
 10. Total return*                                              58.11%     26.60%
 11. Net assets, end of period (in thousands)                  $74,187     $6,833
 12. Average net assets for the period (in thousands)          $25,964     $2,632
 13. Ratio of gross expenses to average net assets**             0.92%(3)   1.26%(2)
 14. Ratio of net expenses to average net assets**               0.91%      1.25%
 15. Ratio of net investment income to average net assets**      0.27%      1.43%
 16. Portfolio turnover rate**                                     91%       101%
- ----------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 2.19% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
(3) The ratio was 0.97% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Olympus Fund.
 
 52 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
INTERNATIONAL GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------------------------------------------------
                                                                           Periods ending December 31
                                                                1998       1997       1996       1995     1994(1)
<S>                                                           <C>        <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                       $18.48     $15.72    $11.95      $9.72     $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                        0.13       0.11      0.05       0.09     (0.09)
  3. Net gains or losses on securities (both realized and
     unrealized)                                                  3.07       2.80      4.06       2.16     (0.19)
  4. Total from investment operations                             3.20       2.91      4.11       2.25     (0.28)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     (0.14)     (0.11)    (0.11)     (0.02)         --
  6. Dividends (in excess of net investment income)                 --         --        --         --         --
  7. Tax return of capital distributions                            --         --        --         --         --
  8. Distributions (from capital gains)                             --     (0.01)    (0.23)         --         --
  9. Distributions (in excess of realized gains)                (0.27)     (0.03)        --         --         --
 10. Total distributions                                        (0.41)     (0.15)    (0.34)     (0.02)         --
 11. NET ASSET VALUE, END OF PERIOD                             $21.27     $18.48    $15.72     $11.95      $9.72
 12. Total return*                                              17.23%     18.51%    34.71%     23.15%    (2.80%)
 13. Net assets, end of period (in thousands)                 $311,110   $161,091   $27,192     $1,608     $1,353
 14. Average net assets for the period (in thousands)         $234,421    $96,164    $7,437     $1,792     $1,421
 15. Ratio of gross expenses to average net assets**             0.86%(6)   0.96%(5)  1.26%(4)   2.69%(3)     N/A
 16. Ratio of net expenses to average net assets**               0.86%      0.96%     1.25%      2.50%      2.50%(2)
 17. Ratio of net investment income to average net assets**      0.73%      0.70%     0.62%    (0.80%)    (1.30%)
 18. Portfolio turnover rate**                                     93%        86%       65%       211%       275%
- ------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1994 (inception) through December 31, 1994.
(2) The ratio was 4.67% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(3) The ratio was 3.57% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(4) The ratio was 2.21% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(5) The ratio was 1.08% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
(6) The ratio was 0.95% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Overseas Fund.
 
                                                        Financial highlights  53
<PAGE>
 
<TABLE>
<CAPTION>
WORLDWIDE GROWTH PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------------------------
                                                                      Periods ending December 31
                                                          1998         1997        1996       1995       1994
<S>                                                    <C>          <C>          <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                  $23.39       $19.44     $15.31     $12.07    $11.89
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                   0.16         0.16       0.16       0.11      0.04
  3. Net gains or losses on securities (both
     realized and unrealized)                                6.59         4.14       4.27       3.19      0.14
  4. Total from investment operations                        6.75         4.30       4.43       3.30      0.18
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                (0.18)       (0.17)     (0.17)     (0.06)        --
  6. Dividends (in excess of net investment income)            --       (0.02)         --         --        --
  7. Tax return of capital distributions                       --           --         --         --        --
  8. Distributions (from capital gains)                        --       (0.16)     (0.13)         --        --
  9. Distributions (in excess of realized gains)           (0.87)           --         --         --        --
 10. Total distributions                                   (1.05)       (0.35)     (0.30)     (0.06)        --
 11. NET ASSET VALUE, END OF PERIOD                        $29.09       $23.39     $19.44     $15.31    $12.07
 12. Total return                                          28.92%       22.15%     29.04%     27.37%     1.53%
 13. Net assets, end of period (in thousands)          $2,890,375   $1,576,548   $582,603   $108,563   $37,728
 14. Average net assets for the period (in
     thousands)                                        $2,217,695   $1,148,951   $304,111    $59,440   $22,896
 15. Ratio of gross expenses to average net assets          0.72%(6)     0.74%(5)   0.80%(4)   0.90%(3)    N/A
 16. Ratio of net expenses to average net assets            0.72%        0.74%      0.80%      0.87%     1.18%(1)(2)
 17. Ratio of net investment income to average net
     assets                                                 0.64%        0.67%      0.83%      0.95%     0.50%
 18. Portfolio turnover rate                                  77%          80%        62%       113%      217%
- ---------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.49% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(3) The ratio was 1.09% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(4) The ratio was 0.91% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(5) The ratio was 0.81% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Worldwide Fund.
 
 54 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
BALANCED PORTFOLIO - INSTITUTIONAL SHARES
- -----------------------------------------------------------------------------------------------------------------
                                                                          Periods ending December 31
                                                               1998       1997       1996       1995       1994
<S>                                                          <C>        <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                      $17.47     $14.77    $13.03     $10.63     $10.64
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                       0.39       0.34      0.32       0.17       0.15
  3. Net gains (or losses) on securities (both realized
     and unrealized)                                             5.51       2.89      1.81       2.45     (0.06)
  4. Total from investment operations                            5.90       3.23      2.13       2.62       0.09
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    (0.38)     (0.35)    (0.30)     (0.22)     (0.10)
  6. Tax return of capital distributions                           --         --        --         --         --
  7. Distributions (from capital gains)                        (0.45)     (0.18)    (0.09)         --         --
  8. Distributions (in excess of realized gains)               (0.04)         --        --         --         --
  9. Total distributions                                       (0.87)     (0.53)    (0.39)     (0.22)     (0.10)
 10. NET ASSET VALUE, END OF PERIOD                            $22.50     $17.47    $14.77     $13.03     $10.63
 11. Total return                                              34.28%     22.10%    16.18%     24.79%      0.84%
 12. Net assets, end of period (in thousands)                $882,495   $362,409   $85,480    $14,021     $3,153
 13. Average net assets for the period (in thousands)        $555,002   $176,432   $43,414     $5,739     $2,336
 14. Ratio of gross expenses to average net assets              0.74%(6)   0.83%(5)  0.94%(4)   1.37%(3)     N/A
 15. Ratio of net expenses to average net assets                0.74%      0.82%     0.92%      1.30%      1.57%(1)(2)
 16. Ratio of net investment income to average net assets       2.41%      2.87%     2.92%      2.41%      1.90%
 17. Portfolio turnover rate                                      70%       139%      103%       149%       158%
- -----------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Commissions payable by the Portfolio for transactions effected by a
    broker-dealer affiliated with Janus Capital were credited against the
    Portfolio's operating expenses. The effect of such directed brokerage
    arrangement was de minimis.
(2) The ratio was 1.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(3) The ratio was 1.55% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(4) The ratio was 1.07% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 0.83% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 0.74% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
                                                        Financial highlights  55
<PAGE>
 
<TABLE>
<CAPTION>
EQUITY INCOME PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------------
                                                                 Periods ending
                                                                   December 31
                                                                 1998     1997(1)
<S>                                                            <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                       $13.46     $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                        0.02       0.01
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                  6.16       3.46
  4. Total from investment operations                             6.18       3.47
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     (0.02)     (0.01)
  6. Tax return of capital distributions                            --         --
  7. Distributions (from capital gains)                         (0.21)         --
  8. Total distributions                                        (0.23)     (0.01)
  9. NET ASSET VALUE, END OF PERIOD                             $19.41     $13.46
 10. Total return*                                              46.24%     34.70%
 11. Net assets, end of period (in thousands)                   $9,017     $3,047
 12. Average net assets for the period (in thousands)           $5,629     $1,101
 13. Ratio of gross expenses to average net assets**             1.25%(3)   1.25%(2)
 14. Ratio of net expenses to average net assets**               1.25%      1.25%
 15. Ratio of net investment income to average net assets**      0.17%      0.35%
 16. Portfolio turnover rate**                                     79%       128%
- ----------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1997 (inception) through December 31, 1997.
(2) The ratio was 5.75% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Equity Income Fund.
(3) The ratio was 1.86% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Equity Income Fund.
 
 56 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
GROWTH AND INCOME PORTFOLIO - INSTITUTIONAL SHARES
- ----------------------------------------------------------------------------
                                                               Period ending
                                                                December 31
                                                                  1998(1)
<S>                                                            <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                         $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                          0.02
  3. Net gains (or losses) on securities (both realized and
     unrealized)                                                    1.96
  4. Total from investment operations                               1.98
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                       (0.02)
  6. Tax return of capital distributions                              --
  7. Distributions (from capital gains)                               --
  8. Total distributions                                          (0.02)
  9. NET ASSET VALUE, END OF PERIOD                               $11.96
 10. Total return*                                                19.80%
 11. Net assets, end of period (in thousands)                     $6,413
 12. Average net assets for the period (in thousands)             $2,883
 13. Ratio of gross expenses to average net assets**               1.25%(2)
 14. Ratio of net expenses to average net assets**                 1.25%
 15. Ratio of net investment income to average net assets**        0.66%
 16. Portfolio turnover rate**                                       62%
- ----------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized.
**  Annualized.
(1) May 1, 1998 (inception) through December 31, 1998.
(2) The ratio was 3.06% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Growth and Income Fund.
 
                                                        Financial highlights  57
<PAGE>
 
<TABLE>
<CAPTION>
FLEXIBLE INCOME PORTFOLIO - INSTITUTIONAL SHARES
- -------------------------------------------------------------------------------------------------------------------
                                                                            Periods ending December 31
                                                                 1998       1997       1996       1995       1994
<S>                                                            <C>        <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $11.78    $11.24     $11.11      $9.48      $9.97
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                         0.64      0.67       0.74       0.53       0.47
  3. Net gains or losses on securities (both realized and
     unrealized)                                                   0.41      0.62       0.24       1.70     (0.56)
  4. Total from investment operations                              1.05      1.29       0.98       2.23     (0.09)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                      (0.67)    (0.64)     (0.72)     (0.60)     (0.40)
  6. Tax return of capital distributions                             --        --         --         --         --
  7. Distributions (from capital gains)                          (0.11)    (0.11)     (0.13)         --         --
  8. Total distributions                                         (0.78)    (0.75)     (0.85)     (0.60)     (0.40)
  9. NET ASSET VALUE, END OF PERIOD                              $12.05    $11.78     $11.24     $11.11      $9.48
 10. Total return                                                 9.11%    11.76%      9.19%     23.86%    (0.91%)
 11. Net assets, end of period (in thousands)                  $129,582   $54,098    $25,315    $10,831     $1,924
 12. Average net assets for the period (in thousands)           $86,627   $36,547    $17,889     $5,556     $1,636
 13. Ratio of gross expenses to average net assets                0.73%     0.75%      0.84%      1.07%        N/A
 14. Ratio of net expenses to average net assets                  0.73%     0.75%      0.83%      1.00%      1.00%(1)
 15. Ratio of net investment income to average net assets         6.36%     6.90%      7.31%      7.46%      5.49%
 16. Portfolio turnover rate                                       145%      119%       250%       236%       234%
- -------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) The ratio was 1.35% before waiver of certain fees incurred by the Portfolio.
 
 58 Janus Aspen Series
<PAGE>
 
<TABLE>
<CAPTION>
HIGH-YIELD PORTFOLIO - INSTITUTIONAL SHARES
- ---------------------------------------------------------------------------------------------
                                                                 Periods ending December 31
                                                                 1998       1997     1996(1)
<S>                                                            <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                       $11.78     $10.83     $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                        0.87       0.70       0.43
  3. Net gains or losses on securities (both realized and
     unrealized)                                                (0.70)       0.99       0.80
  4. Total from investment operations                             0.17       1.69       1.23
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     (0.89)     (0.68)     (0.40)
  6. Tax return of capital distributions                            --         --         --
  7. Distributions (from capital gains)                         (0.05)     (0.06)         --
  8. Distributions (in excess of realized gains)                (0.16)         --         --
  9. Total distributions                                        (1.10)     (0.74)     (0.40)
 10. NET ASSET VALUE, END OF PERIOD                             $10.85     $11.78     $10.83
 11. Total return*                                               1.26%     15.98%     12.40%
 12. Net assets, end of period (in thousands)                   $2,977     $2,914       $783
 13. Average net assets for the period (in thousands)           $3,281     $1,565       $459
 14. Ratio of gross expenses to average net assets**             1.00%(4)   1.00%(3)    1.01(2)
 15. Ratio of net expenses to average net assets**               1.00%      1.00%      1.00%
 16. Ratio of net investment income to average net assets**      7.76%      7.98%      5.74%
 17. Portfolio turnover rate**                                    301%       299%       301%
- ---------------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one full year.
**  Annualized for periods of less than one full year.
(1) May 1, 1996 (inception) through December 31, 1996.
(2) The ratio was 6.29% before waiver of certain fees incurred by the Portfolio.
(3) The ratio was 3.27% before waiver of certain fees incurred by the Portfolio.
(4) The ratio was 2.11% before waiver of certain fees incurred by the Portfolio.
 
                                                        Financial highlights  59
<PAGE>
 
<TABLE>
<CAPTION>
MONEY MARKET PORTFOLIO - INSTITUTIONAL SHARES
- --------------------------------------------------------------------------------------------------------
                                                                      Periods ending December 31
                                                                 1998       1997       1996     1995(1)
<S>                                                            <C>        <C>        <C>        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                        $1.00      $1.00      $1.00      $1.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                        0.05       0.05       0.05       0.04
  3. Net gains or losses on securities (both realized and
     unrealized)                                                    --         --         --         --
  4. Total from investment operations                             0.05       0.05       0.05       0.04
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                     (0.05)     (0.05)     (0.05)     (0.04)
  6. Tax return of capital distributions                            --         --         --         --
  7. Distributions (from capital gains)                             --         --         --         --
  8. Total distributions                                        (0.05)     (0.05)     (0.05)     (0.04)
  9. NET ASSET VALUE, END OF PERIOD                              $1.00      $1.00      $1.00      $1.00
 10. Total return*                                               5.36%      5.17%      5.05%      3.63%
 11. Net assets, end of period (in thousands)                  $38,690    $15,374     $6,016     $1,735
 12. Average net assets for the period (in thousands)          $31,665     $8,926     $3,715     $1,543
 13. Ratio of gross expenses to average net assets**             0.34%      0.50%      0.50%      0.50%
 14. Ratio of net expenses to average net assets**               0.34%      0.50%(4)   0.50%(3)   0.50%(2)
 15. Ratio of net investment income to average net assets**      5.21%      5.17%      4.93%      5.30%
- ------------------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized for periods of less than one year.
**  Annualized for periods of less than one full year.
(1) May 1, 1995 (inception) to December 31, 1995.
(2) The ratio was 1.07% before waiver of certain fees incurred by the Portfolio.
(3) The ratio was 0.78% before waiver of certain fees incurred by the Portfolio.
(4) The ratio was 0.55% before waiver of certain fees incurred by the Portfolio.
 
 60 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
       This glossary provides a more detailed description of some of the types
       of securities and other instruments in which the Portfolios may invest.
       The Portfolios may invest in these instruments to the extent permitted by
       their investment objectives and policies. The Portfolios are not limited
       by this discussion and may invest in any other types of instruments not
       precluded by the policies discussed elsewhere in this Prospectus. Please
       refer to the SAI for a more detailed discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
       BONDS are debt securities issued by a company, municipality, government
       or government agency. The issuer of a bond is required to pay the holder
       the amount of the loan (or par value of the bond) at a specified maturity
       and to make scheduled interest payments.
 
       COMMERCIAL PAPER is a short-term debt obligation with a maturity ranging
       from 1 to 270 days issued by banks, corporations and other borrowers to
       investors seeking to invest idle cash. The Portfolios may purchase
       commercial paper issued in private placements under Section 4(2) of the
       Securities Act of 1933.
 
       COMMON STOCKS are equity securities representing shares of ownership in a
       company and usually carry voting rights and earns dividends. Unlike
       preferred stock, dividends on common stock are not fixed but are declared
       at the discretion of the issuer's board of directors.
 
       CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a fixed
       dividend or interest payment and are convertible into common stock at a
       specified price or conversion ratio.
 
       DEBT SECURITIES are securities representing money borrowed that must be
       repaid at a later date. Such securities have specific maturities and
       usually a specific rate of interest or an original purchase discount.
 
       DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
       corporation that entitle the holder to dividends and capital gains on the
       underlying security. Receipts include those issued by domestic banks
       (American Depositary Receipts), foreign banks (Global or European
       Depositary Receipts) and broker-dealers (depositary shares).
 
       FIXED-INCOME SECURITIES are securities that pay a specified rate of
       return. The term generally includes short- and long-term government,
       corporate and municipal obligations that pay a specified rate of interest
       or coupons for a specified period of time, and preferred stock, which
       pays fixed dividends. Coupon and dividend rates may be fixed for the life
       of the issue or, in the case of adjustable and floating rate securities,
       for a shorter period.
 
                                                Glossary of investment terms  61
<PAGE>
 
       HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated below
       investment grade by the primary rating agencies (e.g., BB or lower by
       Standard & Poor's and Ba or lower by Moody's). Other terms commonly used
       to describe such securities include "lower rated bonds," "noninvestment
       grade bonds" and "junk bonds."
 
       MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of mortgages
       or other debt. These securities are generally pass-through securities,
       which means that principal and interest payments on the underlying
       securities (less servicing fees) are passed through to shareholders on a
       pro rata basis. These securities involve prepayment risk, which is the
       risk that the underlying mortgages or other debt may be refinanced or
       paid off prior to their maturities during periods of declining interest
       rates. In that case, a portfolio manager may have to reinvest the
       proceeds from the securities at a lower rate. Potential market gains on a
       security subject to prepayment risk may be more limited than potential
       market gains on a comparable security that is not subject to prepayment
       risk.
 
       PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign corporations
       which generate certain amounts of passive income or hold certain amounts
       of assets for the production of passive income. Passive income includes
       dividends, interest, royalties, rents and annuities. To avoid taxes and
       interest that the Portfolios must pay if these investments are
       profitable, the Portfolios may make various elections permitted by the
       tax laws. These elections could require that the Portfolios recognize
       taxable income, which in turn must be distributed, before the securities
       are sold and before cash is received to pay the distributions.
 
       PAY-IN-KIND BONDS are debt securities that normally give the issuer an
       option to pay cash at a coupon payment date or give the holder of the
       security a similar bond with the same coupon rate and a face value equal
       to the amount of the coupon payment that would have been made.
 
       PREFERRED STOCKS are equity securities that generally pay dividends at a
       specified rate and have preference over common stock in the payment of
       dividends and liquidation. Preferred stock generally does not carry
       voting rights.
 
       REPURCHASE AGREEMENTS involve the purchase of a security by a Portfolio
       and a simultaneous agreement by the seller (generally a bank or dealer)
       to repurchase the security from the Portfolio at a specified date or upon
       demand. This technique offers a method of earning income on idle cash.
       These securities involve the risk that the seller will fail to repurchase
       the security, as agreed. In that case, a Portfolio will bear the risk of
       market value fluctuations until the security can be sold and may
       encounter delays and incur costs in liquidating the security.
 
 62 Janus Aspen Series
<PAGE>
 
       REVERSE REPURCHASE AGREEMENTS involve the sale of a security by a
       Portfolio to another party (generally a bank or dealer) in return for
       cash and an agreement by the Portfolio to buy the security back at a
       specified price and time. This technique will be used primarily to
       provide cash to satisfy unusually high redemption requests, or for other
       temporary or emergency purposes.
 
       RULE 144A SECURITIES are securities that are not registered for sale to
       the general public under the Securities Act of 1933, but that may be
       resold to certain institutional investors.
 
       STANDBY COMMITMENTS are obligations purchased by a Portfolio from a
       dealer that give the Portfolio the option to sell a security to the
       dealer at a specified price.
 
       STEP COUPON BONDS are debt securities that trade at a discount from their
       face value and pay coupon interest. The discount from the face value
       depends on the time remaining until cash payments begin, prevailing
       interest rates, liquidity of the security and the perceived credit
       quality of the issuer.
 
       STRIP BONDS are debt securities that are stripped of their interest
       (usually by a financial intermediary) after the securities are issued.
       The market value of these securities generally fluctuates more in
       response to changes in interest rates than interest-paying securities of
       comparable maturity.
 
       TENDER OPTION BONDS are generally long-term securities that are coupled
       with an option to tender the securities to a bank, broker-dealer or other
       financial institution at periodic intervals and receive the face value of
       the bond. This type of security is commonly used as a means of enhancing
       the security's liquidity.
 
       U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
       government that are supported by its full faith and credit. Treasury
       bills have initial maturities of less than one year, Treasury notes have
       initial maturities of one to ten years and Treasury bonds may be issued
       with any maturity but generally have maturities of at least ten years.
       U.S. government securities also include indirect obligations of the U.S.
       government that are issued by federal agencies and government sponsored
       entities. Unlike Treasury securities, agency securities generally are not
       backed by the full faith and credit of the U.S. government. Some agency
       securities are supported by the right of the issuer to borrow from the
       Treasury, others are supported by the discretionary authority of the U.S.
       government to purchase the agency's obligations and others are supported
       only by the credit of the sponsoring agency.
 
       VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
       interest and, under certain limited circumstances, may have varying
       principal amounts. These securities pay interest at rates that are
       adjusted periodically according to a
 
                                                Glossary of investment terms  63
<PAGE>
 
       specified formula, usually with reference to some interest rate index or
       market interest rate. The floating rate tends to decrease the security's
       price sensitivity to changes in interest rates.
 
       WARRANTS are securities, typically issued with preferred stock or bonds,
       that give the holder the right to buy a proportionate amount of common
       stock at a specified price, usually at a price that is higher than the
       market price at the time of issuance of the warrant. The right may last
       for a period of years or indefinitely.
 
       WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally involve
       the purchase of a security with payment and delivery at some time in the
       future - i.e., beyond normal settlement. The Portfolios do not earn
       interest on such securities until settlement and bear the risk of market
       value fluctuations in between the purchase and settlement dates. New
       issues of stocks and bonds, private placements and U.S. government
       securities may be sold in this manner.
 
       ZERO COUPON BONDS are debt securities that do not pay regular interest at
       regular intervals, but are issued at a discount from face value. The
       discount approximates the total amount of interest the security will
       accrue from the date of issuance to maturity. The market value of these
       securities generally fluctuates more in response to changes in interest
       rates than interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
       FORWARD CONTRACTS are contracts to purchase or sell a specified amount of
       a financial instrument for an agreed upon price at a specified time.
       Forward contracts are not currently exchange traded and are typically
       negotiated on an individual basis. The Portfolios may enter into forward
       currency contracts to hedge against declines in the value of securities
       denominated in, or whose value is tied to, a currency other than the U.S.
       dollar or to reduce the impact of currency appreciation on purchases of
       such securities. They may also enter into forward contracts to purchase
       or sell securities or other financial indices.
 
       FUTURES CONTRACTS are contracts that obligate the buyer to receive and
       the seller to deliver an instrument or money at a specified price on a
       specified date. The Portfolios may buy and sell futures contracts on
       foreign currencies, securities and financial indices including interest
       rates or an index of U.S. government, foreign government, equity or
       fixed-income securities. The Portfolios may also buy options on futures
       contracts. An option on a futures contract gives the buyer the right, but
       not the obligation, to buy or sell a futures contract at a specified
       price on or before a specified date. Futures contracts and options on
       futures are standardized and traded on designated exchanges.
 
 64 Janus Aspen Series
<PAGE>
 
       INDEXED/STRUCTURED SECURITIES are typically short- to intermediate-term
       debt securities whose value at maturity or interest rate is linked to
       currencies, interest rates, equity securities, indices, commodity prices
       or other financial indicators. Such securities may be positively or
       negatively indexed (i.e. their value may increase or decrease if the
       reference index or instrument appreciates). Indexed/structured securities
       may have return characteristics similar to direct investments in the
       underlying instruments and may be more volatile than the underlying
       instruments. A Portfolio bears the market risk of an investment in the
       underlying instruments, as well as the credit risk of the issuer.
 
       INTEREST RATE SWAPS involve the exchange by two parties of their
       respective commitments to pay or receive interest (e.g., an exchange of
       floating rate payments for fixed rate payments).
 
       INVERSE FLOATERS are debt instruments whose interest rate bears an
       inverse relationship to the interest rate on another instrument or index.
       For example, upon reset the interest rate payable on a security may go
       down when the underlying index has risen. Certain inverse floaters may
       have an interest rate reset mechanism that multiplies the effects of
       change in the underlying index. Such mechanism may increase the
       volatility of the security's market value.
 
       OPTIONS are the right, but not the obligation, to buy or sell a specified
       amount of securities or other assets on or before a fixed date at a
       predetermined price. The Portfolios may purchase and write put and call
       options on securities, securities indices and foreign currencies.
 
                                                Glossary of investment terms  65
<PAGE>
Explanation of rating categories
 
       The following is a description of credit ratings issued by two of the
       major credit ratings agencies. Credit ratings evaluate only the safety of
       principal and interest payments, not the market value risk of lower
       quality securities. Credit rating agencies may fail to change credit
       ratings to reflect subsequent events on a timely basis. Although Janus
       Capital considers security ratings when making investment decisions, it
       also performs its own investment analysis and does not rely solely on the
       ratings assigned by credit agencies.
 
STANDARD & POOR'S
RATINGS SERVICES
 
<TABLE>
         <S>                       <C>
         BOND RATING               EXPLANATION
         ---------------------------------------------------------------------
         Investment Grade
         AAA...................... Highest rating; extremely strong capacity
                                   to pay principal and interest.
         AA....................... High quality; very strong capacity to pay
                                   principal and interest.
         A........................ Strong capacity to pay principal and
                                   interest; somewhat more susceptible to the
                                   adverse effects of changing circumstances
                                   and economic conditions.
         BBB...................... Adequate capacity to pay principal and
                                   interest; normally exhibit adequate
                                   protection parameters, but adverse economic
                                   conditions or changing circumstances more
                                   likely to lead to a weakened capacity to
                                   pay principal and interest than for higher
                                   rated bonds.
         Non-Investment Grade
         BB, B, CCC, CC, C........ Predominantly speculative with respect to
                                   the issuer's capacity to meet required
                                   interest and principal payments.
                                   BB - lowest degree of speculation; C - the
                                   highest degree of speculation. Quality and
                                   protective characteristics outweighed by
                                   large uncertainties or major risk exposure
                                   to adverse conditions.
         D........................ In default.
</TABLE>
 
 66 Janus Aspen Series
<PAGE>
 
MOODY'S INVESTORS SERVICE, INC.
 
<TABLE>
         <S>                       <C>
         BOND RATING               EXPLANATION
         ---------------------------------------------------------------------
         Investment Grade
         Aaa...................... Highest quality, smallest degree of
                                   investment risk.
         Aa....................... High quality; together with Aaa bonds, they
                                   compose the high-grade bond group.
         A........................ Upper-medium grade obligations; many
                                   favorable investment attributes.
         Baa...................... Medium-grade obligations; neither highly
                                   protected nor poorly secured. Interest and
                                   principal appear adequate for the present
                                   but certain protective elements may be
                                   lacking or may be unreliable over any great
                                   length of time.
         Non-Investment Grade
         Ba....................... More uncertain, with speculative elements.
                                   Protection of interest and principal
                                   payments not well safeguarded during good
                                   and bad times.
         B........................ Lack characteristics of desirable
                                   investment; potentially low assurance of
                                   timely interest and principal payments or
                                   maintenance of other contract terms over
                                   time.
         Caa...................... Poor standing, may be in default; elements
                                   of danger with respect to principal or
                                   interest payments.
         Ca....................... Speculative in a high degree; could be in
                                   default or have other marked shortcomings.
         C........................ Lowest-rated; extremely poor prospects of
                                   ever attaining investment standing.
</TABLE>
 
       Unrated securities will be treated as noninvestment grade securities
       unless a portfolio manager determines that such securities are the
       equivalent of investment grade securities. Securities that have received
       ratings from more than one agency are considered investment grade if at
       least one agency has rated the security investment grade.
 
                                            Explanation of rating categories  67
<PAGE>
 
SECURITIES HOLDINGS BY RATING CATEGORY
 
       During the fiscal period ended December 31, 1998, the percentage of
       securities holdings for the following Portfolios by rating category based
       upon a weighted monthly average was:
 
<TABLE>
<CAPTION>
         FLEXIBLE INCOME PORTFOLIO
         ------------------------------------------------------------------
         <S>                                                           <C>
             BONDS-S&P RATING:
          AAA                                                           24%
          AA                                                             4%
          A                                                             13%
          BBB                                                           18%
          BB                                                            13%
          B                                                             15%
          CCC                                                            1%
          CC                                                             0%
          C                                                              0%
          Preferred Stock                                                2%
          Cash and Options                                              10%
          TOTAL                                                        100%
         ------------------------------------------------------------------
</TABLE>
 
<TABLE>
<CAPTION>
         HIGH-YIELD PORTFOLIO
         ------------------------------------------------------------------
         <S>                                                           <C>
             BONDS-S&P RATING:
          AAA                                                            3%
          AA                                                             0%
          A                                                              0%
          BBB                                                            1%
          BB                                                             2%
          B                                                             60%
          CCC                                                            2%
          CC                                                             0%
          C                                                              0%
          Preferred Stock                                                3%
          Cash and Options                                              29%
          TOTAL                                                        100%
         ------------------------------------------------------------------
</TABLE>
 
       No other Portfolio described in this Prospectus held 5% or more of its
       assets in bonds rated below investment grade for the fiscal year ended
       December 31, 1998.
 
 68 Janus Aspen Series
<PAGE>
 
                      (This page intentionally left blank)
 
                                                                              69
<PAGE>
 
You can request other information, including a Statement of Additional
Information, Annual Report or Semiannual Report, free of charge, by contacting
your insurance company or plan sponsor or visiting our Web site at janus.com. In
the Portfolios' Annual Report, you will find a discussion of the market
conditions and investment strategies that significantly affected the Portfolios'
performance during their last fiscal year. Other information is also available
from financial intermediaries that sell shares of the Portfolios.
The Statement of Additional Information provides detailed information about the
Portfolios and is incorporated into this Prospectus by reference. You may review
the Portfolios' Statement of Additional Information at the Public Reference Room
of the SEC or get text only copies for a fee, by writing to or calling the
Public Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You may
obtain the Statement of Additional Information for free from the SEC's Web site
at http://www.sec.gov.
 
                             Investment Company Act File No. 811-7736
 
 70
<PAGE>
 
                      (This page intentionally left blank)
 
                                                                              71
<PAGE>
 
                      (This page intentionally left blank)
 
 72
<PAGE>
 
                      (This page intentionally left blank)
<PAGE>
 
       [JANUS LOGO]
 
                         Please direct all mail to:
                         WESTERN RESERVE LIFE ASSURANCE CO. OF
                         OHIO
 
                         Please complete and return Application
                         to:
 
                         IF MAILED:
                         Janus Retirement Advantage
                         c/o Western Reserve Life Assurance Co.
                         of Ohio
                         Attn: Annuity Dept.
                         P.O. Box 9052
                         Clearwater, FL 33758-9052
 
                         IF OVERNIGHT DELIVERY:
                         Janus Retirement Advantage
                         c/o Western Reserve Life Assurance Co.
                         of Ohio
                         Attn: Annuity Dept.
                         Spectrum Technology Park
                         8550 Ulmerton Rd., Suite 101
                         Largo, FL 33771
 
PCJR0599
<PAGE>
 
                                                     [JANUS LOGO]
 
                         Janus Aspen Series
 
                                       PROSPECTUS
                                       MAY 1, 1999
                                       Growth and Income Portfolio
 
                         THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED
                         OR DISAPPROVED OF THESE SECURITIES OR PASSED ON THE
                         ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
                         REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
<PAGE>
 
    [JANUS LOGO]
 
                         Janus Aspen Series consists of eleven mutual funds (the
                         "Portfolios"), one of which is described in this
                         prospectus. Each Portfolio currently offers two classes
                         of shares. The Institutional Shares, (the "Shares"),
                         are sold under the name of "Janus Aspen Series" and are
                         offered by this prospectus in connection with
                         investment in and payments under variable annuity
                         contracts and variable life insurance contracts, as
                         well as certain qualified retirement plans.
 
                         Janus Aspen Series sells and redeems its Shares at net
                         asset value without sales charges, commissions or
                         redemption fees. Each variable insurance contract
                         involves fees and expenses that are not described in
                         this Prospectus. See the accompanying contract
                         prospectus for information regarding contract fees and
                         expenses and any restrictions on purchases or
                         allocations.
 
                         This prospectus contains information that a prospective
                         purchaser of a variable insurance contract or plan
                         participant should consider in conjunction with the
                         accompanying separate account prospectus of the
                         specific insurance company product before allocating
                         purchase payments or premiums to the Portfolios.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                               <C>
                RISK/RETURN SUMMARY
                   Growth and Income Portfolio..................    2
                   Fees and expenses............................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT
                STRATEGIES AND RISKS
                   Investment objective and principal investment
                   strategies...................................    6
                   General portfolio policies...................    8
                   Risks for Growth and Income Portfolio........   12
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser...........................   16
                   Management expenses and expense limits.......   17
                   Investment personnel.........................   18
                OTHER INFORMATION...............................   19
                DISTRIBUTIONS AND TAXES
                   Distributions................................   21
                   Taxes........................................   21
                SHAREHOLDER'S GUIDE
                   Purchases....................................   23
                   Redemptions..................................   24
                   Shareholder communications...................   24
                FINANCIAL HIGHLIGHTS............................   25
                GLOSSARY
                   Glossary of investment terms.................   27
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH AND INCOME PORTFOLIO
 
               Growth and Income Portfolio is designed for investors who
               primarily seek growth of capital with some emphasis on income. It
               is not designed for investors who desire a consistent level of
               income.
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF GROWTH AND INCOME PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
               - GROWTH AND INCOME PORTFOLIO seeks long-term capital growth
                 and current income.
 
               The Trustees may change this objective without a shareholder vote
               and the Portfolio will notify you of any changes that are
               material. If there is a material change to the Portfolio's
               objective or policies, you should consider whether the Portfolio
               remains an appropriate investment for you. There is no guarantee
               that the Portfolio will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF GROWTH AND INCOME PORTFOLIO?
 
               The portfolio manager applies a "bottom up" approach in choosing
               investments. In other words, he looks mostly for equity and
               income-producing securities that meet his investment criteria one
               at a time. If the portfolio manager is unable to find such
               investments, much of the Portfolio's assets may be in cash or
               similar investments.
 
               Growth and Income Portfolio normally emphasizes investments in
               common stocks. It will normally invest up to 75% of its assets in
               equity securities selected primarily for their growth potential,
               and at least 25% of its assets in securities the portfolio
               manager believes have income potential. Equity securities may
               make up part of this income component if they currently pay
               dividends or the portfolio manager believes they have potential
               for increasing or commencing dividend payments.
 
 2 Janus Aspen Series
<PAGE>
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN GROWTH AND INCOME PORTFOLIO?
 
               The biggest risk is that the Portfolio's returns may vary, and
               you could lose money. If you are considering investing in Growth
               and Income Portfolio, remember that it is designed for long-term
               investors who can accept the risks of investing in a portfolio
               with significant common stock holdings. Common stocks tend to be
               more volatile than other investment choices.
 
               The value of the Portfolio may decrease if the value of an
               individual company in the portfolio decreases. The value of the
               Portfolio could also decrease if the stock market goes down. If
               the value of the Portfolio decreases, its NAV will also decrease,
               which means if you sell your shares in the Portfolio you would
               get back less money.
 
               The income component of the Portfolio's holdings includes fixed-
               income securities. A fundamental risk to the income component is
               that the value of these securities will fall if interest rates
               rise. Generally, the value of a fixed-income portfolio will
               decrease when interest rates rise, which means the Portfolio's
               NAV may likewise decrease. Another fundamental risk associated
               with fixed-income securities is credit risk, which is the risk
               that an issuer of a bond will be unable to make principal and
               interest payments when due.
 
               An investment in this Portfolio is not a bank deposit and is not
               insured or guaranteed by the Federal Deposit Insurance
               Corporation or any other government agency.
 
                                                          Risk return summary  3
<PAGE>
 
FEES AND EXPENSES
 
               SHAREHOLDER FEES, such as sales loads, redemption fees or
               exchange fees, are charged directly to an investor's account. All
               Janus funds are no-load investments, so you will not pay any
               shareholder fees when you buy or sell shares of the Portfolio.
               However, each variable insurance contract involves fees and
               expenses not described in this prospectus. See the accompanying
               contract prospectus for information regarding contract fees and
               expenses and any restrictions on purchases or allocations.
 
               ANNUAL FUND OPERATING EXPENSES are paid out of the Portfolio's
               assets and include fees for portfolio management, maintenance of
               shareholder accounts, shareholder servicing, accounting and other
               services. You do not pay these fees directly but, as the example
               on the next page shows, these costs are borne indirectly by all
               shareholders.
 
 4 Janus Aspen Series
<PAGE>
 
               This table and example are designed to assist participants in
               qualified plans that invest in the Shares of the Portfolio in
               understanding the fees and expenses that you may pay as an
               investor in the Shares. The information shown is based upon gross
               expenses (without the effect of expense offset arrangements) for
               the fiscal year ended December 31, 1998. OWNERS OF VARIABLE
               INSURANCE CONTRACTS THAT INVEST IN THE SHARES SHOULD REFER TO THE
               VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A DESCRIPTION OF FEES
               AND EXPENSES, AS THE TABLE AND EXAMPLE DO NOT REFLECT DEDUCTIONS
               AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES
               THAT MAY BE INCURRED UNDER A CONTRACT.

<TABLE>
<CAPTION>
                                                                    Total Annual Fund                          Total Annual Fund
                                                                    Operating Expenses                         Operating Expenses
                                           Management      Other      Without Waivers          Total              With Waivers
                                              Fee         Expenses     or Reductions*  Waivers and Reductions    or Reductions*
    <S>                                      <C>            <C>            <C>                 <C>                    <C>
    Growth and Income
    Portfolio                                0.75%          2.31%          3.06%               1.81%                  1.25%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth and Income
     Portfolio reduce the Management Fee to the level of Janus Growth and
     Income Fund. Other waivers, if applicable, are first applied against the
     Management Fee and then against Other Expenses. Janus Capital has agreed
     to continue the waivers and fee reductions until at least the next
     annual renewal of the advisory agreement.
- --------------------------------------------------------------------------------
 
   EXAMPLE:
   THE FOLLOWING EXAMPLE IS BASED ON EXPENSES WITHOUT WAIVERS OR
   REDUCTIONS. This example is intended to help you compare the cost of
   investing in the Portfolio with the cost of investing in other mutual
   funds. The example assumes that you invest $10,000 in the Portfolio for
   the time periods indicated then redeem all of your shares at the end of
   those periods. The example also assumes that your investment has a 5%
   return each year, and that the Portfolio's operating expenses remain the
   same. Although your actual costs may be higher or lower, based on these
   assumptions your costs would be:
 
<TABLE>
<CAPTION>
                                   1 Year     3 Years    5 Years    10 Years
                                   -----------------------------------------
    <S>                            <C>        <C>        <C>        <C>    
    Growth and Income Portfolio     $309       $945      $1,606      $3,374
</TABLE>
 
                                                          Risk return summary  5
<PAGE>
Investment objective, principal investment
           strategies and risks
 
               Growth and Income Portfolio has a similar investment objective
               and similar principal investment strategies to Janus Growth and
               Income Fund.
 
               Although it is anticipated that the Portfolio and its
               corresponding retail fund will hold similar securities,
               differences in asset size, cash flow needs and other factors may
               result in differences in investment performance. The expenses of
               Growth and Income Portfolio and Janus Growth and Income Fund are
               expected to differ. The variable contract owner will also bear
               various insurance related costs at the insurance company level.
               You should review the accompanying separate account prospectus
               for a summary of fees and expenses.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
               This section takes a closer look at the investment objective of
               Growth and Income Portfolio, its principal investment strategies
               and certain risks of investing in the Portfolio. Strategies and
               policies that are noted as "fundamental" cannot be changed
               without a shareholder vote.
 
               Please carefully review the "Risks" section of this Prospectus on
               pages 12-15 for a discussion of risks associated with certain
               investment techniques. We've also included a Glossary with
               descriptions of investment terms used throughout this Prospectus.
 
               Growth and Income Portfolio seeks long-term capital growth and
               current income. It normally emphasizes investments in common
               stocks. It will normally invest up to 75% of its assets in equity
               securities selected primarily for their growth potential, and at
               least 25% of its assets in securities the portfolio manager
               believes have income potential. Because of this investment
               strategy, the Portfolio is not designed for investors who need
               consistent income.
 
 6 Janus Aspen Series
<PAGE>
 
The following questions and answers are designed to help you better understand
Growth and Income Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
               The Portfolio may invest substantially all of its assets in
               common stocks if the portfolio manager believes that common
               stocks will appreciate in value. The portfolio manager generally
               takes a "bottom up" approach to selecting companies. In other
               words, he seeks to identify individual companies with earnings
               growth potential that may not be recognized by the market at
               large. He makes this assessment by looking at companies one at a
               time, regardless of size, country of organization, place of
               principal business activity, or other similar selection criteria.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
               Generally, yes. The portfolio manager seeks companies that meet
               his selection criteria, regardless of where a company is located.
               Foreign securities are generally selected on a stock-by-stock
               basis without regard to any defined allocation among countries or
               geographic regions. However, certain factors such as expected
               levels of inflation, government policies influencing business
               conditions, the outlook for currency relationships, and prospects
               for economic growth among countries, regions or geographic areas
               may warrant greater consideration in selecting foreign
               securities. There are no limitations on the countries in which
               the Portfolio may invest.
 
3. HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF GROWTH
   AND INCOME PORTFOLIO'S HOLDINGS?
 
               Growth and Income Portfolio shifts assets between the growth and
               income components of its holdings based on the portfolio
               manager's analysis of relevant market, financial and economic
               conditions. If the portfolio manager believes that growth
               securities will provide better returns than the yields then
               available or expected on income-producing securities, the
               Portfolio will place a greater emphasis on the growth component.
 
              Investment objective, principal investment strategies and risks  7
<PAGE>
 
4. WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF GROWTH AND INCOME
   PORTFOLIO'S INVESTMENTS?
 
               The growth component of the Portfolio's investments is expected
               to consist primarily of common stocks, but may also include
               warrants, preferred stocks or convertible securities selected
               primarily for their growth potential.
 
5. WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF GROWTH AND INCOME
   PORTFOLIO'S HOLDINGS?
 
               The income component of Growth and Income Portfolio will consist
               of securities that the portfolio manager believes have income
               potential. Such securities may include equity securities,
               convertible securities and all types of debt securities. Equity
               securities may be included in the income component of the
               Portfolio if they currently pay dividends or the portfolio
               manager believes they have the potential for either increasing
               their dividends or commencing dividends, if none are currently
               paid.
 
6. HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
 
               Generally, a fixed-income security will increase in value when
               interest rates fall and decrease in value when interest rates
               rise. Longer-term securities are generally more sensitive to
               interest rate changes than shorter-term securities, but they
               generally offer higher yields to compensate investors for the
               associated risks. High-yield bond prices are generally less
               directly responsive to interest rate changes than investment
               grade issues and may not always follow this pattern.
 
GENERAL PORTFOLIO POLICIES
 
               Unless otherwise stated, the percentage limitations included in
               these policies and elsewhere in this Prospectus apply at the time
               of purchase of the security. So, for example, if the Portfolio
               exceeds a limit as a result of market fluctuations or the sale of
               other securities, it will not be required to dispose of any
               securities.
 
 8 Janus Aspen Series
<PAGE>
 
               CASH POSITION
               When the portfolio manager believes that market conditions are
               unfavorable for profitable investing, or when he is otherwise
               unable to locate attractive investment opportunities, the
               Portfolio's cash or similar investments may increase. In other
               words, the Portfolio does not always stay fully invested in
               stocks and bonds. Cash or similar investments generally are a
               residual - they represent the assets that remain after the
               portfolio manager has committed available assets to desirable
               investment opportunities. However, the portfolio manager may also
               temporarily increase the Portfolio's cash position to protect its
               assets or maintain liquidity.
 
               When the Portfolio's investments in cash or similar investments
               increase, it may not participate in market advances or declines
               to the same extent that it would if the Portfolio remained more
               fully invested in stocks or bonds.
 
               OTHER TYPES OF INVESTMENTS
               Growth and Income Portfolio also invests in domestic and foreign
               equity securities with varying degrees of emphasis on income
               which may include preferred stocks, common stocks, warrants and
               securities convertible into common or preferred stocks. The
               Portfolio may also invest to a lesser degree in other types of
               securities. These securities (which are described in the
               Glossary) may include:
 
               - debt securities
 
               - indexed/structured securities
 
               - high-yield/high-risk securities (less than 35% of the
                 Portfolio's assets)
 
               - options, futures, forwards and other types of derivatives for
                 hedging purposes or for non-hedging purposes such as seeking to
                 enhance return
 
               - securities purchased on a when-issued, delayed delivery or
                 forward commitment basis.
 
              Investment objective, principal investment strategies and risks  9
<PAGE>
 
               ILLIQUID INVESTMENTS
               The Portfolio may invest up to 15% of its net assets in illiquid
               investments. An illiquid investment is a security or other
               position that cannot be disposed of quickly in the normal course
               of business. For example, some securities are not registered
               under U.S. securities laws and cannot be sold to the U.S. public
               because of SEC regulations (these are known as "restricted
               securities"). Under procedures adopted by the Portfolio's
               Trustees, certain restricted securities may be deemed liquid, and
               will not be counted toward this 15% limit.
 
               FOREIGN SECURITIES
               The Portfolio may invest without limit in foreign equity and debt
               securities. The Portfolio may invest directly in foreign
               securities denominated in a foreign currency and not publicly
               traded in the United States. Other ways of investing in foreign
               securities include depositary receipts or shares, and passive
               foreign investment companies.
 
               SPECIAL SITUATIONS
               The Portfolio may invest in special situations. A special
               situation arises when, in the opinion of the Portfolio's manager,
               the securities of a particular issuer will be recognized and
               appreciate in value due to a specific development with respect to
               that issuer. Developments creating a special situation might
               include, among others, a new product or process, a technological
               breakthrough, a management change or other extraordinary
               corporate event, or differences in market supply of and demand
               for the security. The Portfolio's performance could suffer if the
               anticipated development in a "special situation" investment does
               not occur or does not attract the expected attention.
 
               PORTFOLIO TURNOVER
               The Portfolio generally intends to purchase securities for
               long-term investment although, to a limited extent, the Portfolio
               may purchase securities in anticipation of relatively short-term
               price gains. Short-term transactions may also result from
               liquidity needs,
 
 10 Janus Aspen Series
<PAGE>
 
               securities having reached a price or yield objective, changes in
               interest rates or the credit standing of an issuer, or by reason
               of economic or other developments not foreseen at the time of the
               investment decision. The Portfolio may also sell one security and
               simultaneously purchase the same or a comparable security to take
               advantage of short-term differentials in bond yields or
               securities prices. Changes are made in the Portfolio's holdings
               whenever the portfolio manager believes such changes are
               desirable. Portfolio turnover rates are generally not a factor in
               making buy and sell decisions.
 
               Increased portfolio turnover may result in higher costs for
               brokerage commissions, dealer mark-ups and other transaction
               costs and may also result in taxable capital gains. Higher costs
               associated with increased portfolio turnover may offset gains in
               the Portfolio's performance.
 
             Investment objective, principal investment strategies and risks  11
<PAGE>
 
RISKS FOR GROWTH AND INCOME PORTFOLIO
 
               Because the Portfolio may invest substantially all of its assets
               in common stocks, the main risk is the risk that the value of the
               stocks it holds might decrease in response to the activities of
               an individual company or in response to general market and/or
               economic conditions. If this occurs, the Portfolio's share price
               may also decrease. The Portfolio's performance may also be
               affected by risks specific to certain types of investments, such
               as foreign securities, derivative investments, non-investment
               grade debt securities or companies with relatively small market
               capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in Growth and Income Portfolio.
 
1. THE PORTFOLIO MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
               Smaller or newer companies may suffer more significant losses as
               well as realize more substantial growth than larger or more
               established issuers because they may lack depth of management, be
               unable to generate funds necessary for growth or potential
               development, or be developing or marketing new products or
               services for which markets are not yet established and may never
               become established. In addition, such companies may be
               insignificant factors in their industries and may become subject
               to intense competition from larger or more established companies.
               Securities of smaller or newer companies may have more limited
               trading markets than the markets for securities of larger or more
               established issuers, and may be subject to wide price
               fluctuations. Investments in such companies tend to be more
               volatile and somewhat more speculative.
 
2. HOW COULD THE PORTFOLIO'S INVESTMENTS IN FOREIGN SECURITIES AFFECT ITS
   PERFORMANCE?
 
               The Portfolio may invest without limit in foreign securities
               either indirectly (e.g., depositary receipts) or directly in
               foreign markets. Investments in foreign securities, including
               those of foreign governments, may involve greater risks than
               investing in domestic
 
 12 Janus Aspen Series
<PAGE>
 
               securities because the Portfolio's performance may depend on
               issues other than the performance of a particular company. These
               issues include:
 
               - CURRENCY RISK. As long as the Portfolio holds a foreign
                 security, its value will be affected by the value of the local
                 currency relative to the U.S. dollar. When the Portfolio sells
                 a foreign denominated security, its value may be worth less in
                 U.S. dollars even if the security increases in value in its
                 home country. U.S. dollar denominated securities of foreign
                 issuers may also be affected by currency risk.
 
               - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject
                 to heightened political and economic risks, particularly in
                 emerging markets which may have relatively unstable
                 governments, immature economic structures, national policies
                 restricting investments by foreigners, different legal systems,
                 and economies based on only a few industries. In some
                 countries, there is the risk that the government may take over
                 the assets or operations of a company or that the government
                 may impose taxes or limits on the removal of the Portfolio's
                 assets from that country.
 
               - REGULATORY RISK. There may be less government supervision of
                 foreign markets. As a result, foreign issuers may not be
                 subject to the uniform accounting, auditing and financial
                 reporting standards and practices applicable to domestic
                 issuers and there may be less publicly available information
                 about foreign issuers.
 
               - MARKET RISK. Foreign securities markets, particularly those of
                 emerging market countries, may be less liquid and more volatile
                 than domestic markets. Certain markets may require payment for
                 securities before delivery and delays may be encountered in
                 settling securities transactions. In some foreign markets,
                 there may not be protection against failure by other parties to
                 complete transactions.
 
               - TRANSACTION COSTS. Costs of buying, selling and holding foreign
                 securities, including brokerage, tax and custody costs, may be
                 higher than those involved in domestic transactions.
 
             Investment objective, principal investment strategies and risks  13
<PAGE>
 
3. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
               High-yield/high-risk securities (or "junk" bonds) are securities
               rated below investment grade by the primary rating agencies such
               as Standard & Poor's and Moody's. The value of lower quality
               securities generally is more dependent on credit risk, or the
               ability of the issuer to meet interest and principal payments,
               than investment grade debt securities. Issuers of high-yield
               securities may not be as strong financially as those issuing
               bonds with higher credit ratings and are more vulnerable to real
               or perceived economic changes, political changes or adverse
               developments specific to the issuer.
 
4. WHAT IS MEANT BY "CREDIT QUALITY" AND WHAT ARE THE RISKS ASSOCIATED WITH IT?
 
               Credit quality measures the likelihood that the issuer will meet
               its obligations on a bond. One of the fundamental risks
               associated with all fixed-income securities is credit risk, which
               is the risk that an issuer will be unable to make principal and
               interest payments when due. U.S. government securities are
               generally considered to be the safest type of investment in terms
               of credit risk. Municipal obligations generally rank between U.S.
               government securities and corporate debt securities in terms of
               credit safety. Corporate debt securities, particularly those
               rated below investment grade, present the highest credit risk.
 
5. HOW DOES THE PORTFOLIO TRY TO REDUCE RISK?
 
               The Portfolio may use futures, options and other derivative
               instruments to "hedge" or protect its portfolio from adverse
               movements in securities prices and interest rates. The Portfolio
               may also use a variety of currency hedging techniques, including
               forward currency contracts, to manage exchange rate risk. The
               portfolio manager believes the use of these instruments will
               benefit the Portfolio. However, the Portfolio's performance could
               be worse than if the Portfolio had not used such instruments if
               the portfolio manager's judgement proves incorrect. Risks
               associated with the use of derivative instruments are described
               in the SAI.
 
 14 Janus Aspen Series
<PAGE>
 
6. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
               The portfolio manager carefully researches each potential
               investment before making an investment decision and, among other
               things, considers Year 2000 readiness when selecting portfolio
               holdings. However, there is no guarantee that the information the
               portfolio manager receives regarding a company's Year 2000
               readiness is completely accurate. If a company has not
               satisfactorily addressed Year 2000 issues, the Portfolio's
               performance could suffer.
 
             Investment objective, principal investment strategies and risks  15
<PAGE>
Management of the portfolio
 
INVESTMENT ADVISER
 
               Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928,
               is the investment adviser to the Portfolio and is responsible for
               the day-to-day management of the investment portfolio and other
               business affairs of the Portfolio.
 
               Janus Capital began serving as investment adviser to Janus Fund
               in 1970 and currently serves as investment adviser to all of the
               Janus retail funds, acts as sub-adviser for a number of
               private-label mutual funds and provides separate account advisory
               services for institutional accounts.
 
               Janus Capital furnishes continuous advice and recommendations
               concerning the Portfolio's investments. Janus Capital also
               furnishes certain administrative, compliance and accounting
               services for the Portfolio, and may be reimbursed by the
               Portfolio for its costs in providing those services. In addition,
               Janus Capital employees serve as officers of the Trust and Janus
               Capital provides office space for the Portfolio and pays the
               salaries, fees and expenses of all Portfolio officers and those
               Trustees who are affiliated with Janus Capital.
 
               Participating insurance companies that purchase the Portfolio's
               shares may perform certain administrative services relating to
               the Portfolio and Janus Capital or the Portfolio may pay those
               companies for such services.
 
 16 Janus Aspen Series
<PAGE>
 
MANAGEMENT EXPENSES AND EXPENSE LIMITS
 
               The Portfolio pays Janus Capital a management fee which is
               calculated daily. The advisory agreement with the Portfolio
               spells out the management fee and other expenses that the
               Portfolio must pay. The Portfolio is subject to the following
               management fee schedule (expressed as an annual rate). In
               addition, the Shares of the Portfolio incur expenses not assumed
               by Janus Capital, including transfer agent and custodian fees and
               expenses, legal and auditing fees, printing and mailing costs of
               sending reports and other information to existing shareholders,
               and independent Trustees' fees and expenses.
 
<TABLE>
<CAPTION>
                        Average Daily
                         Net Assets         Annual Rate      Expense Limit
      Fee Schedule      of Portfolio       Percentage (%)    Percentage (%)
- ----------------------------------------------------------------------------
<S>                    <C>                      <C>            <C>          
     Growth and
     Income Portfolio  First $300 Million       0.75
                       Next $200 Million        0.70           1.25(1)(2)
                       Over $500 Million        0.65
- ----------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth and Income Portfolio's management
    fee to the extent that such fee exceeds the effective rate of Janus Growth
    and Income Fund. Janus Capital has agreed to continue such waivers until at
    least the next annual renewal of the advisory contracts. The effective rate
    is the management fee calculated by the corresponding retail fund as of the
    last day of each calendar quarter (expressed as an annual rate). The
    effective rate of Janus Growth and Income Fund was 0.66% for the quarter
    ended March 31, 1999.
 
(2) Janus Capital has agreed to limit the Portfolio's expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
                                                 Management of the portfolio  17
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
DAVID J. CORKINS
- --------------------------------------------------------------------------------
                   is Executive Vice President and portfolio manager of
                   Growth and Income Portfolio which he has managed since its
                   inception. He is Executive Vice President and portfolio
                   manager of Janus Growth and Income Fund which he has
                   managed since August 1997. He previously served as an
                   assistant portfolio manager of Janus Mercury Fund. He
                   joined Janus in 1995 as a research analyst specializing in
                   domestic financial services companies and a variety of
                   foreign industries. Prior to joining Janus, he was the
                   Chief Financial Officer of Chase U.S. Consumer Services,
                   Inc., a Chase Manhattan mortgage business. He holds a
                   Bachelor of Arts in English and Russian from Dartmouth and
                   received his Master of Business Administration from
                   Columbia University in 1993.
 
 18 Janus Aspen Series
<PAGE>
                                                               Other information
 
               CLASSES OF SHARES
 
               The Portfolio currently offers two classes of Shares, one of
               which, the Institutional Shares, are offered pursuant to this
               prospectus and are sold under the name Janus Aspen Series. The
               Shares offered by this Prospectus are available only in
               connection with investment in and payments under variable
               insurance contracts as well as certain qualified retirement
               plans. Retirement Shares of the Portfolio are offered by separate
               prospectus and are available only to qualified plans using plan
               service providers that are compensated for providing distribution
               and/or recordkeeping and other administrative services provided.
               Because the expenses of each class may differ, the performance of
               each class is expected to differ. If you would like additional
               information about the Retirement Shares, please call
               1-800-525-0020.
 
               CONFLICTS OF INTEREST
 
               The Shares offered by this prospectus are available only to
               variable annuity and variable life separate accounts of insurance
               companies that are unaffiliated with Janus Capital and to certain
               qualified retirement plans. Retirement Shares of the Portfolio
               (offered through a separate prospectus) are available to certain
               qualified plans. Although the Portfolio does not currently
               anticipate any disadvantages to policy owners because the
               Portfolio offers its shares to such entities, there is a
               possibility that a material conflict may arise. The Trustees
               monitor events in order to identify any disadvantages or material
               irreconcilable conflicts and to determine what action, if any,
               should be taken in response. If a material disadvantage or
               conflict occurs, the Trustees may require one or more insurance
               company separate accounts or qualified plans to withdraw its
               investments in the Portfolio or substitute Shares of another
               Portfolio. If this occurs, the Portfolio may be forced to sell
               its securities at disadvantageous prices. In addition, the
               Trustees may refuse to sell Shares of the Portfolio to any
               separate account or qualified plan or may suspend or terminate
               the offering of the Portfolio's Shares if such action is required
               by law or regulatory authority or is in the best interests of the
               Portfolio's shareholders. It is possible that a qualified plan
               investing in the Retirement
 
                                                           Other information  19
<PAGE>
 
               Shares of the Portfolio could lose its qualified plan status
               under the Internal Revenue Code, which could have adverse tax
               consequences on insurance company separate accounts investing in
               the Shares. Janus Capital intends to monitor such qualified plans
               and the Portfolio may discontinue sales to a qualified plan and
               require plan participants with existing investments in the
               Retirement Shares to redeem those investments if a plan loses (or
               in the opinion of Janus Capital is at risk of losing) its
               qualified plan status.
 
               YEAR 2000
 
               Preparing for Year 2000 is a high priority for Janus Capital,
               which has established a dedicated group to address this issue.
               Janus Capital has devoted considerable internal resources and has
               engaged one of the foremost experts in the field to help achieve
               Year 2000 readiness. Janus Capital does not anticipate that Year
               2000-related issues will have a material impact on its ability to
               continue to provide the Portfolio with service at current levels;
               however, Janus Capital cannot make any assurances that the steps
               it has taken to ensure Year 2000 readiness will be successful. In
               addition, there can be no assurance that Year 2000 issues will
               not affect the companies in which the Portfolio invests or
               worldwide markets and economies.
 
 20 Janus Aspen Series
<PAGE>
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
               To avoid taxation of the Portfolio, the Internal Revenue Code
               requires the Portfolio to distribute net income and any net gains
               realized on its investments annually. The Portfolio's income from
               dividends and interest and any net realized short-term gains are
               paid to shareholders as ordinary income dividends. Net realized
               long-term gains are paid to shareholders as capital gains
               distributions.
 
               Each class of the Portfolio makes semi-annual distributions in
               June and December of substantially all of its investment income
               and an annual distribution in June of its net realized gains, if
               any. All dividends and capital gains distributions from Shares of
               the Portfolio will automatically be reinvested into additional
               Shares of the Portfolio.
 
               HOW DISTRIBUTIONS AFFECT NAV
 
               Distributions are paid to shareholders as of the record date of
               the distribution of the Portfolio, regardless of how long the
               shares have been held. Undistributed income and realized gains
               are included in the daily NAV of the Portfolio's Shares. The
               Share price of the Portfolio drops by the amount of the
               distribution, net of any subsequent market fluctuations. For
               example, assume that on December 31, the Shares of Growth and
               Income Portfolio declared a dividend in the amount of $0.25 per
               share. If the price of Growth and Income Portfolio's Shares was
               $10.00 on December 30, the share price on December 31 would be
               $9.75, barring market fluctuations.
 
TAXES
 
               TAXES ON DISTRIBUTIONS
 
               Because Shares of the Portfolio may be purchased only through
               variable insurance contracts and qualified plans, it is
               anticipated that any income dividends or capital gains
               distributions made by the Shares of the Portfolio will be exempt
               from current taxation if left to accumulate within the variable
               insurance contract or qualified plan. Generally, withdrawals from
               such contracts may be
 
                                                     Distributions and taxes  21
<PAGE>
 
               subject to ordinary income tax and, if made before age 59 1/2, a
               10% penalty tax. The tax status of your investment depends on the
               features of your qualified plan or variable insurance contract.
               Further information may be found in your plan documents or in the
               prospectus of the separate account offering such contract.
 
               TAXATION OF THE PORTFOLIO
 
               Dividends, interest and some gains received by the Portfolio on
               foreign securities may be subject to withholding of foreign
               taxes. The Portfolio may from year to year make the election
               permitted under Section 853 of the Internal Revenue Code to pass
               through such taxes to shareholders. If such election is not made,
               any foreign taxes paid or accrued will represent an expense to
               the Portfolio which will reduce their investment income.
 
               The Portfolio does not expect to pay any federal income or excise
               taxes because it intends to meet certain requirements of the
               Internal Revenue Code. In addition, the Portfolio intends to
               qualify under the Internal Revenue Code with respect to the
               diversification requirements related to the tax-deferred status
               of insurance company separate accounts.
 
 22 Janus Aspen Series
<PAGE>
                                                             Shareholder's guide
 
               INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIO
               DIRECTLY. SHARES MAY BE PURCHASED OR REDEEMED ONLY THROUGH
               VARIABLE INSURANCE CONTRACTS OFFERED BY THE SEPARATE ACCOUNTS OF
               PARTICIPATING INSURANCE COMPANIES OR THROUGH QUALIFIED RETIREMENT
               PLANS. REFER TO THE PROSPECTUS FOR THE PARTICIPATING INSURANCE
               COMPANY'S SEPARATE ACCOUNT OR YOUR PLAN DOCUMENTS FOR
               INSTRUCTIONS ON PURCHASING OR SELLING OF VARIABLE INSURANCE
               CONTRACTS AND ON HOW TO SELECT THE PORTFOLIO AS AN INVESTMENT
               OPTION FOR A CONTRACT OR A QUALIFIED PLAN.
 
PRICING OF PORTFOLIO SHARES
 
               Investments will be processed at the NAV next determined after an
               order is received and accepted by the Portfolio or its agent. In
               order to receive a day's price, your order must be received by
               the close of the regular trading session of the New York Stock
               Exchange any day that the NYSE is open. Securities are valued at
               market value or, if a market quotation is not readily available,
               at their fair value determined in good faith under procedures
               established by and under the supervision of the Trustees. Short-
               term instruments maturing within 60 days are valued at amortized
               cost, which approximates market value. See the SAI for more
               detailed information.
 
               To the extent the Portfolio holds securities that are primarily
               listed on foreign exchanges that trade on weekends or other days
               when the Portfolio does not price its shares, the NAV of the
               Portfolio's shares may change on days when shareholders will not
               be able to purchase or redeem the Portfolio's shares.
 
PURCHASES
 
               Purchases of Shares may be made only by the separate accounts of
               insurance companies for the purpose of funding variable insurance
               contracts or by qualified plans. Refer to the prospectus of the
               appropriate insurance company separate account or your plan
               documents for information on how to invest in the Shares of the
               Portfolio. Participating insurance companies and certain other
 
                                                         Shareholder's guide  23
<PAGE>
 
               designated organizations are authorized to receive purchase
               orders on the Portfolio's behalf.
 
               The Portfolio reserves the right to reject any specific purchase
               order. Purchase orders may be refused if, in Janus Capital's
               opinion, they are of a size that would disrupt the management of
               the Portfolio. Although there is no present intention to do so,
               the Portfolio may discontinue sales of its shares if management
               and the Trustees believe that continued sales may adversely
               affect the Portfolio's ability to achieve its investment
               objective. If sales of the Portfolio's Shares are discontinued,
               it is expected that existing policy owners and plan participants
               invested in the Portfolio would be permitted to continue to
               authorize investment in the Portfolio and to reinvest any
               dividends or capital gains distributions, absent highly unusual
               circumstances.
 
REDEMPTIONS
 
               Redemptions, like purchases, may be effected only through the
               separate accounts of participating insurance companies or through
               qualified plans. Please refer to the appropriate separate account
               prospectus or plan documents for details.
 
               Shares of the Portfolio may be redeemed on any business day.
               Redemptions are processed at the NAV next calculated after
               receipt and acceptance of the redemption order by the Portfolio
               or its agent. Redemption proceeds will normally be wired to the
               participating insurance company the business day following
               receipt of the redemption order, but in no event later than seven
               days after receipt of such order.
 
SHAREHOLDER COMMUNICATIONS
 
               Shareholders will receive annual and semiannual reports including
               the financial statements of the Shares of the Portfolio. Each
               report will show the investments owned by the Portfolio and the
               market values thereof, as well as other information about the
               Portfolio and its operations. The Trust's fiscal year ends
               December 31.
 
 24 Janus Aspen Series
<PAGE>
                                                            Financial highlights
 
               The financial highlights tables are intended to help you
               understand the Growth and Income Portfolio - Institutional
               Shares' financial performance for the life of the Portfolio.
               Items 1 through 9 reflect financial results for a single Share.
               Total return in the tables represents the rate that an investor
               would have earned (or lost) on an investment in the Institutional
               Shares of the Portfolio (assuming reinvestment of all dividends
               and distributions) but does not include charges and expenses
               attributable to any insurance product. This information has been
               audited by PricewaterhouseCoopers LLP, whose report, along with
               the Portfolio's financial statements, is included in the Annual
               Report, which is available upon request and incorporated by
               reference into the SAI.
 
                                                        Financial highlights  25
<PAGE>
 
<TABLE>
<CAPTION>
           GROWTH AND INCOME PORTFOLIO - INSTITUTIONAL SHARES
- ------------------------------------------------------------------------
                                                           Period ending
                                                            December 31
                                                              1998(1)
<S>                                                        <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD                      $10.00
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                                       0.02
  3. Net gains (or losses) on securities (both realized
     and unrealized)                                             1.96
  4. Total from investment operations                            1.98
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment income)                    (0.02)
  6. Tax return of capital distributions                           --
  7. Distributions (from capital gains)                            --
  8. Total distributions                                       (0.02)
  9. NET ASSET VALUE, END OF PERIOD                            $11.96
 10. Total return*                                             19.80%
 11. Net assets, end of period (in thousands)                  $6,413
 12. Average net assets for the period (in thousands)          $2,883
 13. Ratio of gross expenses to average net assets**            1.25%(2)
 14. Ratio of net expenses to average net assets**              1.25%
 15. Ratio of net investment income to average net
     assets**                                                   0.66%
 16. Portfolio turnover rate**                                    62%
- ------------------------------------------------------------------------
</TABLE>
 
 *  Total return not annualized.
**  Annualized.
(1) May 1, 1998 (inception) through December 31, 1998.
(2) The ratio was 3.06% before waiver of certain fees and/or reduction of
    adviser's fees to the effective rate of Janus Growth and Income Fund.
 
 26 Janus Aspen Series
<PAGE>
                                                    Glossary of investment terms
 
               This glossary provides a more detailed description of some of the
               types of securities and other instruments in which the Portfolio
               may invest. The Portfolio may invest in these instruments to the
               extent permitted by its investment objective and policies. The
               Portfolio is not limited by this discussion and may invest in any
               other types of instruments not precluded by the policies
               discussed elsewhere in this Prospectus. Please refer to the SAI
               for a more detailed discussion of certain instruments.
 
I. EQUITY AND DEBT SECURITIES
 
               BONDS are debt securities issued by a company, municipality,
               government or government agency. The issuer of a bond is required
               to pay the holder the amount of the loan (or par value of the
               bond) at a specified maturity and to make scheduled interest
               payments.
 
               COMMERCIAL PAPER is a short-term debt obligation with a maturity
               ranging from 1 to 270 days issued by banks, corporations and
               other borrowers to investors seeking to invest idle cash. The
               Portfolio may purchase commercial paper issued in private
               placements under Section 4(2) of the Securities Act of 1933.
 
               COMMON STOCKS are equity securities representing shares of
               ownership in a company and usually carry voting rights and earns
               dividends. Unlike preferred stock, dividends on common stock are
               not fixed but are declared at the discretion of the issuer's
               board of directors.
 
               CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a
               fixed dividend or interest payment and are convertible into
               common stock at a specified price or conversion ratio.
 
               DEBT SECURITIES are securities representing money borrowed that
               must be repaid at a later date. Such securities have specific
               maturities and usually a specific rate of interest or an original
               purchase discount.
 
               DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
               corporation that entitle the holder to dividends and capital
               gains on the underlying security. Receipts include those issued
               by domestic banks (American Depositary Receipts), foreign banks
 
                                                Glossary of investment terms  27
<PAGE>
 
               (Global or European Depositary Receipts) and broker-dealers
               (depositary shares).
 
               FIXED-INCOME SECURITIES are securities that pay a specified rate
               of return. The term generally includes short- and long-term
               government, corporate and municipal obligations that pay a
               specified rate of interest or coupons for a specified period of
               time, and preferred stock, which pays fixed dividends. Coupon and
               dividend rates may be fixed for the life of the issue or, in the
               case of adjustable and floating rate securities, for a shorter
               period.
 
               HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated
               below investment grade by the primary rating agencies (e.g., BB
               or lower by Standard & Poor's and Ba or lower by Moody's). Other
               terms commonly used to describe such securities include "lower
               rated bonds," "noninvestment grade bonds" and "junk bonds."
 
               MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
               mortgages or other debt. These securities are generally pass-
               through securities, which means that principal and interest
               payments on the underlying securities (less servicing fees) are
               passed through to shareholders on a pro rata basis. These
               securities involve prepayment risk, which is the risk that the
               underlying mortgages or other debt may be refinanced or paid off
               prior to their maturities during periods of declining interest
               rates. In that case, the portfolio manager may have to reinvest
               the proceeds from the securities at a lower rate. Potential
               market gains on a security subject to prepayment risk may be more
               limited than potential market gains on a comparable security that
               is not subject to prepayment risk.
 
               PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign
               corporations which generate certain amounts of passive income or
               hold certain amounts of assets for the production of passive
               income. Passive income includes dividends, interest, royalties,
               rents and annuities. To avoid taxes and interest that the
               Portfolio must pay if these investments are profitable, the
               Portfolio may make various elections permitted by the tax laws.
               These elections could require that the Portfolio recognize
               taxable income, which
 
 28 Janus Aspen Series
<PAGE>
 
               in turn must be distributed, before the securities are sold and
               before cash is received to pay the distributions.
 
               PAY-IN-KIND BONDS are debt securities that normally give the
               issuer an option to pay cash at a coupon payment date or give the
               holder of the security a similar bond with the same coupon rate
               and a face value equal to the amount of the coupon payment that
               would have been made.
 
               PREFERRED STOCKS are equity securities that generally pay
               dividends at a specified rate and have preference over common
               stock in the payment of dividends and liquidation. Preferred
               stock generally does not carry voting rights.
 
               REPURCHASE AGREEMENTS involve the purchase of a security by the
               Portfolio and a simultaneous agreement by the seller (generally a
               bank or dealer) to repurchase the security from the Portfolio at
               a specified date or upon demand. This technique offers a method
               of earning income on idle cash. These securities involve the risk
               that the seller will fail to repurchase the security, as agreed.
               In that case, the Portfolio will bear the risk of market value
               fluctuations until the security can be sold and may encounter
               delays and incur costs in liquidating the security.
 
               REVERSE REPURCHASE AGREEMENTS involve the sale of a security by
               the Portfolio to another party (generally a bank or dealer) in
               return for cash and an agreement by the Portfolio to buy the
               security back at a specified price and time. This technique will
               be used primarily to provide cash to satisfy unusually high
               redemption requests, or for other temporary or emergency
               purposes.
 
               RULE 144A SECURITIES are securities that are not registered for
               sale to the general public under the Securities Act of 1933, but
               that may be resold to certain institutional investors.
 
               STANDBY COMMITMENTS are obligations purchased by the Portfolio
               from a dealer that give the Portfolio the option to sell a
               security to the dealer at a specified price.
 
                                                Glossary of investment terms  29
<PAGE>
 
               STEP COUPON BONDS are debt securities that trade at a discount
               from their face value and pay coupon interest. The discount from
               the face value depends on the time remaining until cash payments
               begin, prevailing interest rates, liquidity of the security and
               the perceived credit quality of the issuer.
 
               STRIP BONDS are debt securities that are stripped of their
               interest (usually by a financial intermediary) after the
               securities are issued. The market value of these securities
               generally fluctuates more in response to changes in interest
               rates than interest-paying securities of comparable maturity.
 
               TENDER OPTION BONDS are generally long-term securities that are
               coupled with an option to tender the securities to a bank,
               broker-dealer or other financial institution at periodic
               intervals and receive the face value of the bond. This type of
               security is commonly used as a means of enhancing the security's
               liquidity.
 
               U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
               government that are supported by its full faith and credit.
               Treasury bills have initial maturities of less than one year,
               Treasury notes have initial maturities of one to ten years and
               Treasury bonds may be issued with any maturity but generally have
               maturities of at least ten years. U.S. government securities also
               include indirect obligations of the U.S. government that are
               issued by federal agencies and government sponsored entities.
               Unlike Treasury securities, agency securities generally are not
               backed by the full faith and credit of the U.S. government. Some
               agency securities are supported by the right of the issuer to
               borrow from the Treasury, others are supported by the
               discretionary authority of the U.S. government to purchase the
               agency's obligations and others are supported only by the credit
               of the sponsoring agency.
 
               VARIABLE AND FLOATING RATE SECURITIES have variable or floating
               rates of interest and, under certain limited circumstances, may
               have varying principal amounts. These securities pay interest at
               rates that are adjusted periodically according to a specified
               formula, usually with reference to some interest rate index or
               market
 
 30 Janus Aspen Series
<PAGE>
 
               interest rate. The floating rate tends to decrease the security's
               price sensitivity to changes in interest rates.
 
               WARRANTS are securities, typically issued with preferred stock or
               bonds, that give the holder the right to buy a proportionate
               amount of common stock at a specified price, usually at a price
               that is higher than the market price at the time of issuance of
               the warrant. The right may last for a period of years or
               indefinitely.
 
               WHEN-ISSUED, DELAYED DELIVERY AND FORWARD TRANSACTIONS generally
               involve the purchase of a security with payment and delivery at
               some time in the future - i.e., beyond normal settlement. The
               Portfolio does not earn interest on such securities until
               settlement and bear the risk of market value fluctuations in
               between the purchase and settlement dates. New issues of stocks
               and bonds, private placements and U.S. government securities may
               be sold in this manner.
 
               ZERO COUPON BONDS are debt securities that do not pay regular
               interest at regular intervals, but are issued at a discount from
               face value. The discount approximates the total amount of
               interest the security will accrue from the date of issuance to
               maturity. The market value of these securities generally
               fluctuates more in response to changes in interest rates than
               interest-paying securities.
 
II. FUTURES, OPTIONS AND OTHER DERIVATIVES
 
               FORWARD CONTRACTS are contracts to purchase or sell a specified
               amount of a financial instrument for an agreed upon price at a
               specified time. Forward contracts are not currently exchange
               traded and are typically negotiated on an individual basis. The
               Portfolio may enter into forward currency contracts to hedge
               against declines in the value of securities denominated in, or
               whose value is tied to, a currency other than the U.S. dollar or
               to reduce the impact of currency appreciation on purchases of
               such securities. It may also enter into forward contracts to
               purchase or sell securities or other financial indices.
 
                                                Glossary of investment terms  31
<PAGE>
 
               FUTURES CONTRACTS are contracts that obligate the buyer to
               receive and the seller to deliver an instrument or money at a
               specified price on a specified date. The Portfolio may buy and
               sell futures contracts on foreign currencies, securities and
               financial indices including interest rates or an index of U.S.
               government, foreign government, equity or fixed-income
               securities. The Portfolio may also buy options on futures
               contracts. An option on a futures contract gives the buyer the
               right, but not the obligation, to buy or sell a futures contract
               at a specified price on or before a specified date. Futures
               contracts and options on futures are standardized and traded on
               designated exchanges.
 
               INDEXED/STRUCTURED SECURITIES are typically short- to
               intermediate-term debt securities whose value at maturity or
               interest rate is linked to currencies, interest rates, equity
               securities, indices, commodity prices or other financial
               indicators. Such securities may be positively or negatively
               indexed (i.e. their value may increase or decrease if the
               reference index or instrument appreciates). Indexed/structured
               securities may have return characteristics similar to direct
               investments in the underlying instruments and may be more
               volatile than the underlying instruments. The Portfolio bears the
               market risk of an investment in the underlying instruments, as
               well as the credit risk of the issuer.
 
               INTEREST RATE SWAPS involve the exchange by two parties of their
               respective commitments to pay or receive interest (e.g., an
               exchange of floating rate payments for fixed rate payments).
 
               INVERSE FLOATERS are debt instruments whose interest rate bears
               an inverse relationship to the interest rate on another
               instrument or index. For example, upon reset the interest rate
               payable on a security may go down when the underlying index has
               risen. Certain inverse floaters may have an interest rate reset
               mechanism that multiplies the effects of change in the underlying
               index. Such mechanism may increase the volatility of the
               security's market value.
 
 32 Janus Aspen Series
<PAGE>
 
               OPTIONS are the right, but not the obligation, to buy or sell a
               specified amount of securities or other assets on or before a
               fixed date at a predetermined price. The Portfolio may purchase
               and write put and call options on securities, securities indices
               and foreign currencies.
 
                                                Glossary of investment terms  33
<PAGE>
 
[JANUS LOGO]

        1-800-29JANUS
        100 Fillmore Street
        Denver, Colorado 80206-4928
        janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting your plan sponsor or visiting our Web site at
janus.com. In the Portfolio's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolios' performance during its last
fiscal year. Other information is also available from financial
intermediaries that sell Shares of the Portfolio.
The Statement of Additional Information provides detailed
information about the Portfolio and is incorporated into this
Prospectus by reference. You may review the Portfolios' Statement of
Additional Information at the Public Reference Room of the SEC or
get text only copies for a fee, by writing to or calling the Public
Reference Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You
may obtain the Statement of Additional Information for free from the
SEC's Web site at http://www.sec.gov.
 
                    Investment Company Act File No. 811-7736



© 2022 IncJournal is not affiliated with or endorsed by the U.S. Securities and Exchange Commission