JANUS ASPEN SERIES
497, 1999-05-04
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<PAGE>
 
                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              International 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]
 
                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
                Portfolio.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   International Growth Portfolio...........................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   International Growth Portfolio...........................    5
                   General portfolio policies...............................    6
                   Risks for International Growth Portfolio.................    8
                   Investment Techniques....................................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
INTERNATIONAL GROWTH PORTFOLIO
 
          International Growth Portfolio is 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 INTERNATIONAL GROWTH PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - INTERNATIONAL GROWTH PORTFOLIO seeks long-term growth of capital.
 
          The Portfolio's 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 INTERNATIONAL GROWTH PORTFOLIO?
 
          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 the portfolio managers are unable
          to find investments with earnings growth potential, a significant
          portion of the Portfolio's assets may be in cash or similar
          investments.
 
          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.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN INTERNATIONAL GROWTH PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in International
          Growth 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 net asset value (NAV) will also decrease,
          which means if you sell your shares in the Portfolio you would get
          back less money.
 
          International 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 the Portfolio 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 International Growth Portfolio by showing how
          International Growth 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.
 
           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.
 
          International Growth Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
                                                          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.
 
          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>
    International Growth Portfolio            0.75%         0.20%          0.95%               0.09%                  0.86%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for International Growth
     Portfolio reduce the Management Fee to the level of Janus Overseas 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>    
    International Growth Portfolio                                  $97       $303       $526       $1,166
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          The Portfolio has a similar investment objective and similar principal
          investment strategies to Janus Overseas Fund. Although it is
          anticipated that the Portfolio and Janus Overseas 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 International Growth Portfolio and Janus Overseas
          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
          International Growth Portfolio, its principal investment strategies
          and certain risks of investing in International Growth 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 8-9 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.
 
          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.
 
The following questions and answers are designed to help you better understand
International Growth 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 managers believe 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 Portfolio. Income
          realized on the Portfolio's investments will be incidental to its
          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 Portfolio may invest.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
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 International Growth Portfolio does
          not emphasize companies of any particular size, a Portfolio with a
          larger asset base is more likely to invest in larger, more established
          issuers.
 
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.
 
          CASH POSITION
          When the portfolio managers believe that market conditions are
          unfavorable for profitable investing, or when they are 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 managers have committed
          available assets to desirable investment opportunities. However, the
          portfolio managers 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
          International Growth Portfolio invests primarily in domestic and
          foreign equity securities, 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
 
          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
 
 6 Janus Aspen Series
<PAGE>
 
          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,
          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  7
<PAGE>
 
RISKS FOR INTERNATIONAL GROWTH 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 International Growth 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
          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.
 
 8 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 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 managers
          believe 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 managers'
          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 the portfolio managers receive
          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  9
<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.
 
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>         
     International Growth Portfolio                     First $300 Million        0.75               N/A(1)
                                                        Next $200 Million         0.70
                                                        Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce International Growth Portfolio's
    management fee to the extent that such fee exceeds the effective rate of
    Janus Overseas 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 Overseas Fund, was 0.66%, for the quarter ended
    March 31, 1999.
 
 10 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.
 
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.
 
                                                 Management of the portfolio  11
<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.
          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
          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.
 
 12 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
          International Growth Portfolio declared a dividend in the amount of
          $0.25 per share. If the price of International 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 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 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
          its 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<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 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 financial statements, is included in the Annual Report,
          which is available upon request and incorporated by reference into the
          SAI.
 
<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.
 
 16 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 objectives 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 (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 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  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 Portfolio's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolio's 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 Portfolio's 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
                              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 five 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.......................................    6
                   Fees and expenses........................................    8
                INVESTMENT OBJECTIVES, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Growth Portfolios........................................    9
                   Balanced Portfolio.......................................   11
                   General portfolio policies...............................   12
                   Risks for Growth, Global Growth and Balanced
                   Portfolios...............................................   14
                   Investment Techniques....................................   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
                   Purchases................................................   21
                   Redemptions..............................................   21
                   Shareholder communications...............................   22
                FINANCIAL HIGHLIGHTS........................................   23
                GLOSSARY
                   Glossary of investment terms.............................   28
 
</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 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 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.
 
          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 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 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.
 
           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 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 do 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%
    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%
    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, 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
    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
                  Aggressive Growth Portfolio                           Janus Enterprise 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
 
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  9
<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 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 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 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 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 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 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.
 
 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>          
     Growth Portfolio,
     Aggressive Growth Portfolio,                       First $300 Million        0.75               N/A(1)
     International Growth Portfolio,                    Next $200 Million         0.70
     Worldwide Growth Portfolio and                     Over $500 Million         0.65
     Balanced Portfolio
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth, Aggressive 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 Enterprise Fund,
    Janus Overseas Fund, Janus Worldwide Fund and Janus Balanced Fund were
    0.65%, 0.69%, 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.
 
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.
 
                                                Management of the portfolios  17
<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.
 
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.
 
 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>
                                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.
 
 24 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  25
<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.
 
 26 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  27
<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
 
 28 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  29
<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
 
 30 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  31
<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
 
                   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
                Portfolio.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Growth Portfolio.........................................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   strategies...............................................    5
                   General portfolio policies...............................    6
                   Risks for Growth Portfolio...............................    8
                   Investment Techniques....................................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Pricing of Portfolio Shares..............................   14
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
GROWTH PORTFOLIO
 
          The Growth Portfolio is 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 GROWTH PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - GROWTH PORTFOLIO seeks long-term growth of capital in a manner
            consistent with the preservation of capital.
 
          The Portfolio's 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 GROWTH PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks for companies with earnings
          growth potential one at a time. If the portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of the 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.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE GROWTH PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Growth
          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 net asset value (NAV) will also decrease,
          which means if you sell your shares in the Portfolio you would get
          back less money.
 
          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.
 
 2 Janus Aspen Series
<PAGE>
 
          The following information provides some indication of the risks of
          investing in Growth Portfolio by showing how Growth Portfolio's
          performance has varied over time. The bar charts depict 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.
 
           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.
 
          Growth Portfolio's past performance does not necessarily indicate how
          it will perform in the future.
 
                                                          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.
 
          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 Portfolio                         0.72%          0.03%          0.75%               0.07%                  0.68%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Growth Portfolio
     reduce the Management Fee to the level of Janus 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 Portfolio                                               $ 77       $240      $  417      $  930
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Growth Portfolio has a similar investment objective and similar
          principal investment strategies to Janus Fund. Although it is
          anticipated that the Portfolio and Janus 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 Portfolio and Janus 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
          Portfolio, its principal investment strategies and certain risks of
          investing in Growth 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 8-9 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 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.
 
The following questions and answers are designed to help you better understand
Growth 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. Realization of income is not a significant
          consideration when choosing investments for the Portfolio. Income
          realized on the Portfolio's investments will be incidental to its
          objectives.
 
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. 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
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
          of its shares outstanding. Although Growth Portfolio does not
          emphasize companies of any particular size, a Portfolio with a larger
          asset base is more likely to invest in larger, more established
          issuers.
 
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.
 
          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 Portfolio invests primarily in domestic and foreign equity
          securities, 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
 
          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.
 
 6 Janus Aspen Series
<PAGE>
 
          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,
          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  7
<PAGE>
 
RISKS FOR GROWTH 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 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
          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.
 
 8 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.
 
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 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.
 
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  9
<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.
 
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 Portfolio                                   First $300 Million        0.75               N/A(1)
                                                        Next $200 Million         0.70
                                                        Over $500 Million         0.65
- ------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Growth Portfolio's management fee to the
    extent that such fee exceeds the effective rate of the Janus 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
    Fund was 0.65%, for the quarter ended March 31, 1999.
 
 10 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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.
 
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 portfolio  11
<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.
          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
          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.
 
 12 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 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 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 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
          its 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<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 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.
 
<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.
 
 16 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 objectives 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 Portfolio's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolio's 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 Portfolio's 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
                              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]
 
                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
                Portfolio.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Worldwide Growth Portfolio...............................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   strategies...............................................    5
                   General portfolio policies...............................    6
                   Risks for Worldwide Growth Portfolio.....................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   14
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
WORLDWIDE GROWTH PORTFOLIO
 
          Worldwide Growth Portfolio is 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 WORLDWIDE GROWTH PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - WORLDWIDE GROWTH PORTFOLIO seeks long-term growth of capital in a
            manner consistent with the preservation of capital.
 
          The Portfolio's 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 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 WORLDWIDE GROWTH PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, she looks for companies with earnings
          growth potential one at a time. If the portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of the Portfolio's assets may be in cash or similar investments.
 
          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 WORLDWIDE GROWTH PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in Worldwide Growth
          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 net asset value (NAV) will also decrease,
          which means if you sell your shares in the 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 the Portfolio 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 Worldwide Growth Portfolio by showing how Worldwide
          Growth 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.
 
           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.
 
          Worldwide Growth Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
                                                          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.
 
          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>
    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 Worldwide Growth
     Portfolio reduce the Management Fee to the level of Janus Worldwide
     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>   
    Worldwide Growth Portfolio                                      $76       $237       $411        $918
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Worldwide Growth Portfolio has a similar investment objective and
          similar principal investment strategies to Janus Worldwide 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 Worldwide Growth Portfolio and
          Janus Worldwide 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 objectives of
          Worldwide Growth Portfolio, its principal investment strategies and
          certain risks of investing in Worldwide Growth 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 8-9 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.
 
          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
Worldwide Growth 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, she seeks to
          identify individual companies with earnings growth potential that may
          not be recognized by the market at large. She 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. Realization of income is not a significant
          consideration when choosing investments for the Portfolio. Income
          realized on the Portfolio's investments will be incidental to its
          objectives.
 
2. ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
          Generally, yes. The portfolio manager seeks companies that meet her
          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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
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 Worldwide Growth Portfolio does not
          emphasize companies of any particular size, a Portfolio with a larger
          asset base is more likely to invest in larger, more established
          issuers.
 
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.
 
          CASH POSITION
          When the portfolio manager believes that market conditions are
          unfavorable for profitable investing, or when she 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
          Worldwide Growth Portfolio invests primarily in domestic and foreign
          equity securities, 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
 
          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
 
 6 Janus Aspen Series
<PAGE>
 
          adopted by the Portfolio 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,
          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  7
<PAGE>
 
RISKS FOR WORLDWIDE GROWTH 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 Worldwide Growth 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
          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.
 
 8 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 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.
 
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 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  9
<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.
 
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>        
     Worldwide Growth Portfolio                         First $300 Million        0.75               N/A(1)
                                                        Next $200 Million         0.70
                                                        Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
</TABLE>

(1) Janus Capital has agreed to reduce Worldwide Growth Portfolio's management
    fee to the extent that such fee exceeds the effective rate of the Janus
    Worldwide 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 Worldwide Fund was 0.65% for the quarter ended March
    31, 1999.
 
 10 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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.
 
                                                 Management of the portfolio  11
<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.
          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
          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.
 
 12 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
          Worldwide Growth Portfolio declared a dividend in the amount of $0.25
          per share. If the price of Worldwide 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 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 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
          its 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          the Worldwide Growth Portfolio -- 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 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.
 
<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.
 
 16 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 objectives 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 Portfolio's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolio's 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 Portfolio's 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
 
                   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 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. 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
                Portfolio.
<PAGE>
 
                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Aggressive Growth Portfolio..............................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   objective................................................    5
                   General portfolio policies...............................    7
                   Risks for Aggressive Growth Portfolio....................    9
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   11
                   Management expenses and expense limits...................   11
                   Investment personnel.....................................   12
                OTHER INFORMATION...........................................   13
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   14
                   Taxes....................................................   14
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   15
                   Purchases................................................   15
                   Redemptions..............................................   15
                   Shareholder communications...............................   16
                FINANCIAL HIGHLIGHTS........................................   17
                GLOSSARY
                   Glossary of investment terms.............................   18
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
AGGRESSIVE GROWTH PORTFOLIO
 
          The Portfolio is 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 PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - AGGRESSIVE GROWTH PORTFOLIO seeks long-term growth of capital.
 
          The Portfolio's 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 PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks for companies with earnings
          growth potential one at a time. If the portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of the 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.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN THE PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in the 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 net asset value (NAV) will also decrease,
          which means if you sell your shares in the Portfolio you would get
          back less money.
 
          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 the Portfolio 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 Aggressive Growth Portfolio by showing how the
          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.
 
           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.
 
          Aggressive Growth Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
                                                          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.
 
          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
                                           Management      Other      Operating
                                              Fee         Expenses     Expenses*
    <S>                                      <C>            <C>            <C>
    Aggressive Growth Portfolio              0.72%          0.03%          0.75%
</TABLE>
 
- --------------------------------------------------------------------------------
   * Janus Capital will reduce the Management Fee for Aggressive Growth
     Portfolio to the level of Janus Enterprise Fund. Janus Capital has
     agreed to continue the fee reductions until at least the next annual
     renewal of the advisory agreement. This fee reduction arrangement did
     not affect the Portfolio's fee in 1998.
- --------------------------------------------------------------------------------
 
   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>
    Aggressive Growth Portfolio                                     $77       $240       $417        $930
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Aggressive Growth Portfolio has a similar investment objective and
          similar principal investment strategies to Janus Enterprise 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 the 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.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
          This section takes a closer look at the investment objective of the
          Aggressive Growth Portfolio, its principal investment strategies and
          certain risks of investing in the Aggressive Growth 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 9-10 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.
 
          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.
 
The following questions and answers are designed to help you better understand
Aggressive Growth Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          The Portfolio may invest substantially all of its assets in common
          stocks if its 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. Realization of income is not a significant
          consideration when choosing investments for the Portfolio. Income
          realized on the Portfolio's investments will be incidental to its
          objectives.
 
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 and the
          Portfolio may at times have significant foreign exposure.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
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.
 
 6 Janus Aspen Series
<PAGE>
 
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.
 
          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
          Aggressive Growth Portfolio invests primarily in domestic and foreign
          equity securities, 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
 
          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.
 
              Investment objective, principal investment strategies and risks  7
<PAGE>
 
          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,
          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.
 
 8 Janus Aspen Series
<PAGE>
 
RISKS FOR AGGRESSIVE GROWTH 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 Aggressive Growth 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 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 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
          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.
 
              Investment objective, principal investment strategies and risks  9
<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 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.
 
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.
 
 10 Janus Aspen Series
<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.
 
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> 
     Aggressive Growth Portfolio                        First $300 Million        0.75               N/A(1)
                                                        Next $200 Million         0.70
                                                        Over $500 Million         0.65
- ------------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Aggressive Growth Portfolio's management
    fee to the extent that such fee exceeds the effective rate of the Janus
    Enterprise 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 Enterprise Fund was 0.69% for the quarter ended
    March 31, 1999.
 
                                                 Management of the portfolio  11
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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.
 
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.
 
 12 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.
          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
          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
          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 Portfolio invests or worldwide markets and economies.
 
                                                           Other information  13
<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 a 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
          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 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 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
          its 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.
 
 14 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 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's guide  15
<PAGE>
 
SHAREHOLDER COMMUNICATIONS
 
          Shareholders will receive annual and semiannual reports including the
          financial statements of the Shares of the Portfolio that they have
          authorized for investment. 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.
 
 16 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 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.
 
<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  17
<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 objectives 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 (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
 
 18 Janus Aspen Series
<PAGE>
 
          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 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.
 
          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  19
<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 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.
 
          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
 
 20 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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
                                                Glossary of investment terms  21
<PAGE>
 
                           [This page intentionally left blank]
 
 22 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 Portfolio's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Portfolio's 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 Portfolio's 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
                              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]
 
                Janus Aspen Series consists of eleven mutual funds (the
                "Portfolios"), one of which is described in this prospectus.
                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. 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
                Portfolio.
<PAGE>

                                                               Table of contents
 
<TABLE>
                <S>                                                           <C>
                RISK/RETURN SUMMARY
                   Flexible Income Portfolio................................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Flexible Income Portfolio................................    5
                   General portfolio policies...............................    6
                   Risks for Flexible Income Portfolio......................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Pricing of Portfolio Shares..............................   14
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
                RATING CATEGORIES
                   Explanation of rating categories.........................   21
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
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.
 
 2 Janus Aspen Series
<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 4th-1998  6.71%  Worst Quarter 3rd-1998 (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  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 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 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>
                                           Management      Other    Total Annual Fund
                                              Fee         Expenses  Operating Expenses
    <S>                                      <C>            <C>            <C>
    Flexible Income Portfolio                0.65%          0.08%          0.73%
</TABLE>
 
- --------------------------------------------------------------------------------
   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> 
    Flexible Income Portfolio                                      $ 75       $233      $  406      $  906
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Flexible Income Portfolio has a similar investment objective and
          similar principal investment strategies to Janus Flexible 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 the 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.
 
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 8-9 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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
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.
 
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.
 
          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.
 
 6 Janus Aspen Series
<PAGE>
 
          OTHER TYPES OF INVESTMENTS
          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
          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,
          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 its
 
              Investment objective, principal investment strategies and risks  7
<PAGE>
 
          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.
 
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 20 for a description of
          rating categories.
 
3. 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
          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.
 
 8 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.
 
          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 20 for a
          description of bond rating categories.
 
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.
 
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, consider
          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  9
<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.
 
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>
     Flexible Income Portfolio                          First $300 Million        0.65              1.00(1)
                                                        Over $300 Million         0.55
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to limit the Portfolio's expenses as indicated
    until at least the next annual renewal of the advisory contracts.
 
 10 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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.
 
                                                 Management of the portfolio  11
<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.
          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
          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.
 
 12 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
          Flexible Income Portfolio declared a dividend in the amount of $0.25
          per share. If the price of Flexible 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 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
          its investment income.
 
          The Portfolio does not expect to pay any federal income or excise
          taxes because they intend 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<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 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.
 
<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.
 
 16 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 objectives 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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  21
<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.
 
 22 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>
 
                                            Explanation of rating categories  23
<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 Portfolio's 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 Portfolio's 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
                              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]
 
                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
                   Balanced Portfolio.......................................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   strategies...............................................    5
                   General portfolio policies...............................    6
                   Risks for Balanced Portfolio.............................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   14
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
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 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.
 
 2 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 Portfolio's past performance does not necessarily indicate
          how it will perform in the future.
 
                                                          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.
 
          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>
                            Management           Other          Total Annual Fund
                               Fee              Expenses       Operating Expenses*
    <S>                     <C>                 <C>            <C>
    Balanced Portfolio        0.72%               0.02%               0.74%
</TABLE>
 
- --------------------------------------------------------------------------------
   * Janus Capital will reduce the Management Fee for Balanced Portfolio to
     the level of Janus Balanced Fund. Janus Capital has agreed to continue
     the fee reduction until at least the next annual renewal of the advisory
     agreement. This fee reduction arrangement did not affect the Portfolio's
     fee in 1998.
- --------------------------------------------------------------------------------
 
   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>   
    Balanced Portfolio                                              $76       $237       $411        $918
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Balanced Portfolio has a similar investment objective and similar
          principal investment strategies to Janus Balanced Fund. Although it is
          anticipated that the Portfolio and Janus Balanced 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 the Portfolio and Janus Balanced 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
          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 8-9 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.
 
          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?
 
          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 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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
4. 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.
 
5. 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 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.
 
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. A bond fund's average-weighted effective maturity and its
          duration are measures of how the fund may react to interest rate
          changes.
 
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.
 
          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
          Balanced Portfolio 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
 
 6 Janus Aspen Series
<PAGE>
 
          - 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
 
          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,
          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  7
<PAGE>
 
RISKS FOR BALANCED 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 Balanced 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
          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.
 
 8 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. 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.
 
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  9
<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.
 
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>          
     Balanced Portfolio                               First $300 Million           0.75               N/A(1)
                                                      Next $200 Million            0.70
                                                      Over $500 Million            0.65
- --------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Balanced Portfolio's management fee to
    the extent that such fee exceeds the effective rate of Janus Balanced 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 Balanced Fund was 0.67% for the quarter ended March 31, 1999.
 
 10 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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 portfolio  11
<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.
          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
          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.
 
 12 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
          Balanced Portfolio declared a dividend in the amount of $0.25 per
          share. If the price of Balanced 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 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
          its 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          Balanced Portfolio - 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 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.
 
<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.
 
 16 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 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
<PAGE>

                                         [JANUS LOGO]
 
                   Janus Aspen Series
 
                              PROSPECTUS
                              MAY 1, 1999
                              Capital Appreciation 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
                   Capital Appreciation Portfolio...........................    2
                   Fees and expenses........................................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   strategies...............................................    5
                   General portfolio policies...............................    6
                   Risks for Capital Appreciation Portfolio.................    8
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Pricing of portfolio shares..............................   14
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</TABLE>
 
                                                            Table of contents  1
<PAGE>
Risk return summary
 
CAPITAL APPRECIATION PORTFOLIO
 
          Capital Appreciation Portfolio is 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 CAPITAL APPRECIATION PORTFOLIO?
 
- --------------------------------------------------------------------------------
 
          - CAPITAL APPRECIATION PORTFOLIO seeks long-term growth of capital.
 
          The Portfolio's 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 CAPITAL APPRECIATION PORTFOLIO?
 
          The portfolio manager applies a "bottom up" approach in choosing
          investments. In other words, he looks for companies with earnings
          growth potential one at a time. If the portfolio manager is unable to
          find investments with earnings growth potential, a significant portion
          of the 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.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN CAPITAL APPRECIATION PORTFOLIO?
 
          The biggest risk is that the Portfolio's returns may vary, and you
          could lose money. If you are considering investing in the 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 net asset value (NAV) will also decrease,
          which means if you sell your shares in the Portfolio you would get
          back less money.
 
          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.
 
          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.
 
 2 Janus Aspen Series
<PAGE>
 
          The following information provides some indication of the risks of
          investing in the Capital Appreciation Portfolio by showing how Capital
          Appreciation Portfolio's performance has varied over time. The bar
          chart depicts the performance 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.
 
           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.
 
          Capital Appreciation Portfolio's past performance does not necessarily
          indicate how it will perform in the future.
 
                                                          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.
 
          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>
    Capital Appreciation Portfolio           0.75%          0.22%          0.97%               0.05%                  0.92%
</TABLE>
 
- --------------------------------------------------------------------------------
   * All expenses are stated both with and without contractual waivers and
     fee reductions by Janus Capital. Fee reductions for Capital Appreciation
     Portfolio 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 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> 
    Capital Appreciation Portfolio                                 $ 99       $309      $  536      $1,190
</TABLE>
 
 4 Janus Aspen Series
<PAGE>
                                      Investment objective, principal investment
                                                 strategies and risks
 
          Capital Appreciation Portfolio has a similar investment objective and
          similar principal investment strategies to Janus Twenty Fund. Prior to
          May 1, 1999 Capital Appreciation Portfolio was managed in a similar
          manner to Janus Olympus 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 Capital Appreciation Portfolio
          and Janus Twenty 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
          Capital Appreciation 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 8-9 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.
 
          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.
 
The following questions and answers are designed to help you better understand
Capital Appreciation Portfolio's principal investment strategies.
 
1. HOW ARE COMMON STOCKS SELECTED?
 
          The Portfolio may invest substantially all of its assets in common
          stocks if its 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. Realization of income is not a significant
          consideration when choosing investments for the Portfolio. Income
          realized on the Portfolio's investments will be incidental to its
          objectives.
 
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 and the
          Portfolio may at times have significant foreign exposure.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
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 Capital Appreciation Portfolio does
          not emphasize companies of any particular size, a Portfolio with a
          larger asset base is more likely to invest in larger, more established
          issuers.
 
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.
 
          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
          Capital Appreciation Portfolio invests primarily in domestic and
          foreign equity securities, 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
 
          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
 
 6 Janus Aspen Series
<PAGE>
 
          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,
          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  7
<PAGE>
 
RISKS FOR CAPITAL APPRECIATION 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 Capital Appreciation 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 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 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
          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.
 
 8 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 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.
 
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  9
<PAGE>
Management of the portfolio
 
INVESTMENT ADVISER
 
          Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is
          the investment adviser to each of 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.
 
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>
 
                                                        First $300
     Capital Appreciation Portfolio                     First $300 Million        0.75               N/A(1)
                                                        Next $200 Million         0.70
                                                        Over $500 Million         0.65
- -------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Janus Capital has agreed to reduce Capital Appreciation Portfolio's
    management fee to the extent that such fee exceeds the effective rate of the
    Janus retail fund corresponding to the 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 rate of Janus Olympus Fund was
    0.67% for the quarter ended March 31, 1999.
 
 10 Janus Aspen Series
<PAGE>
 
INVESTMENT PERSONNEL
 
PORTFOLIO MANAGER
 
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 portfolio  11
<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.
          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
          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.
 
 12 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
          Capital Appreciation Portfolio declared a dividend in the amount of
          $0.25 per share. If the price of Capital Appreciation 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 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.
 
                                                     Distributions and taxes  13
<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 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 their 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<PAGE>
Financial highlights
 
          The financial highlights tables are intended to help you understand
          Capital Appreciation 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.
 
<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.
 
 16 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 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
<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........................................    3
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT STRATEGIES AND
                RISKS
                   Investment objective and principal investment
                   strategies...............................................    4
                   General portfolio policies...............................    5
                   Risks for Growth and Income Portfolio....................    7
                MANAGEMENT OF THE PORTFOLIO
                   Investment adviser.......................................   10
                   Management expenses and expense limits...................   10
                   Investment personnel.....................................   11
                OTHER INFORMATION...........................................   12
                DISTRIBUTIONS AND TAXES
                   Distributions............................................   13
                   Taxes....................................................   13
                SHAREHOLDER'S GUIDE
                   Purchases................................................   14
                   Redemptions..............................................   14
                   Shareholder communications...............................   15
                FINANCIAL HIGHLIGHTS........................................   16
                GLOSSARY
                   Glossary of investment terms.............................   17
 
</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.
 
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.
 
 2 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 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.
 
          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  3
<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 7-8 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.
 
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.
 
 4 Janus Aspen Series
<PAGE>
 
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.
 
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.
 
          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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE>
 
          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.
 
          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,
          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
 
 6 Janus Aspen Series
<PAGE>

 
          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  7
<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
          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.
 
 8 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. 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.
 
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  9
<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.
 
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.
 
 10 Janus Aspen Series
<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.
 
                                                 Management of the portfolio  11
<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
          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.
 
 12 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 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.
 
                                                     Distributions and taxes  13
<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 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.
 
 14 Janus Aspen Series
<PAGE>
 
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.
 
                                                         Shareholder's guide  15
<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.
 
<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.
 
 16 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 (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
 
                                                Glossary of investment terms  17
<PAGE>
 
          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 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.
 
          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.
 
 18 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 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.
 
          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
 
                                                Glossary of investment terms  19
<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 Portfolio may purchase and write
          put and call options on securities, securities indices and foreign
          currencies.
 
 20 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 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



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