JANUS ASPEN SERIES
497, 1998-05-05
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<PAGE> 
 
                               JANUS ASPEN SERIES
 
                                   Prospectus
 
                                  MAY 1, 1998
 
This prospectus describes three mutual funds, each of which has its own
investment objective and policies (the "Portfolios"). Each Portfolio is a series
of Janus Aspen Series (the "Trust") and currently offers two classes of shares.
The Institutional Shares are sold under the name "Janus Aspen Series." The Trust
is registered with the Securities and Exchange Commission ("SEC") as an open-
end management investment company. The Institutional Shares of each Portfolio
(collectively, the "Shares") are offered by this prospectus in connection with
investment in and payments under variable annuity contracts and variable life
insurance contracts (collectively "variable insurance contracts"), as well as
certain qualified retirement plans. Janus Capital Corporation ("Janus Capital")
serves as investment adviser to each Portfolio. Janus Capital has been in the
investment advisory business for over 27 years and currently manages
approximately $80 billion in assets.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Certain Portfolios may not be available
in connection with a particular contract and certain contracts may limit
allocations among the Portfolios. 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.
 
This Prospectus contains information about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolio. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                                           <C>
THE PORTFOLIOS AT A GLANCE
Brief description of the Portfolios.........................    2
EXPENSE INFORMATION
Each Portfolio's annual operating expenses..................    3
Financial Highlights - a summary of financial data..........    4
PERFORMANCE TERMS
An explanation of performance terms.........................    6
THE PORTFOLIOS IN DETAIL
Investment Objectives and Policies..........................    7
General Portfolio Policies..................................    9
Additional Risk Factors.....................................   10
MANAGEMENT OF THE PORTFOLIOS
Investment Adviser and Investment Personnel.................   13
Personal Investing..........................................   13
Management Expenses.........................................   14
Portfolio Transactions......................................   14
Other Service Providers.....................................   14
Other Information...........................................   14
DISTRIBUTIONS AND TAXES
Distributions...............................................   16
Taxes.......................................................   16
SHAREHOLDER'S GUIDE
Purchases...................................................   17
Redemptions.................................................   17
Shareholder Communications..................................   17
APPENDIX A
Glossary of Investment Terms................................   18
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIOS AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolios
and their investment emphasis. A more detailed discussion of the Portfolios'
investment objectives and policies begins on page 7.
 
GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital in a
manner consistent with the preservation of capital.
Inception: September 1993
Manager: James P. Craig, III
Assistant Managers: David C. Decker
                    Blaine P. Rollins
 
WORLDWIDE GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign and domestic issuers.
Inception: September 1993
Manager: Helen Young Hayes
Assistant Manager: Laurence J. Chang
 
BALANCED PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital, balanced
by current income. The 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.
Inception: September 1993
Manager: Blaine P. Rollins
 
 2   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolios in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES (applicable to each Portfolio)
 
<TABLE>
         <S>                                                           <C>
         Maximum sales load imposed on purchases                       None
         Maximum sales load imposed on reinvested dividends            None
         Deferred sales charges on redemptions                         None
         Redemption fee                                                None
         Exchange fee                                                  None
</TABLE>
 
ANNUAL OPERATING EXPENSES (after fee reductions)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<CAPTION>
- -----------------------------------------------------------------------------------------------------------------
                                                             MANAGEMENT         OTHER         TOTAL OPERATING
                                                                 FEE           EXPENSES          EXPENSES
- -----------------------------------------------------------------------------------------------------------------
<S>                                                          <C>           <C>                <C>             
Growth Portfolio                                                0.65%           0.05%              0.70%
Worldwide Growth Portfolio                                      0.66%           0.08%              0.74%
Balanced Portfolio                                              0.76%           0.07%              0.83%
 ----------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Management fees for Growth, Worldwide Growth and Balanced Portfolios reflect
    a reduced fee schedule effective July 1, 1997. The management fee for each
    Portfolio reflects the new rate applied to net assets as of December 31,
    1997. Other expenses are based on the gross expenses of the Shares before
    expense offset arrangements for the fiscal year ended December 31, 1997. The
    information for each Portfolio is net of fee reductions from Janus Capital.
    Fee reductions for Growth, Worldwide Growth and Balanced Portfolios reduce
    the management fee to the level of the corresponding Janus retail fund.
    Without such reductions, the Management Fee, Other Expenses and Total
    Operating Expenses for the Shares would have been 0.74%, 0.04%, and 0.78%
    for Growth Portfolio; 0.72%, 0.09%, and 0.81% for Worldwide Growth
    Portfolio; and 0.77%, 0.06%, and 0.83% for Balanced Portfolio, respectively.
    Janus Capital may modify or terminate the fee reductions at any time upon at
    least 90 days' notice to the Trustees.
 
EXAMPLE
 
Assume you invest $1,000, the Shares of the Portfolios return 5% annually and
each Portfolio's expense ratio remains as listed above. The example below shows
the operating expenses that you would indirectly bear as an investor in the
Shares of the Portfolios.
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------
                                                             1 YEAR      3 YEARS      5 YEARS      10 YEARS
- ---------------------------------------------------------------------------------------------------------------
<S>                                                          <C>         <C>          <C>          <C>      
Growth Portfolio                                              $ 7          $22          $39          $ 87
Worldwide Growth Portfolio                                    $ 8          $24          $41          $ 92
Balanced Portfolio                                            $ 8          $26          $46          $103
 --------------------------------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    3
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise noted, the information below is for fiscal periods ending on
December 31st of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolios' financial statements since their inception. Their report
is included in the Portfolios' Annual Report, which is incorporated by reference
into the SAI. A DETAILED EXPLANATION OF THE FINANCIAL HIGHLIGHTS CAN BE FOUND ON
PAGE 5.
<TABLE>
<CAPTION>
                                                                  GROWTH PORTFOLIO
           INSTITUTIONAL SHARES               1997         1996         1995        1994         1993(1)
- --------------------------------------------------------------------------------------------------------
<C>  <S>                                    <C>          <C>          <C>          <C>           <C>
 1.  Net asset value, beginning of period     $15.51       $13.45       $10.57      $10.32        $10.00
- --------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                       .15          .17          .28         .09           .03
 3.  Net gains or (losses) on securities        3.34         2.29         2.90         .20           .32
     (both realized and unrealized)
- --------------------------------------------------------------------------------------------------------
 4.  Total from investment operations           3.49         2.46         3.18         .29           .35
- --------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment            (.15)        (.17)        (.30)       (.04)         (.03)
     income)
 6.  Dividends (in excess of net                  --           --           --          --            --
     investment income)
 7.  Tax return of capital distributions          --           --           --          --            --
 8.  Distributions (from capital gains)        (.37)        (.23)           --          --            --
- --------------------------------------------------------------------------------------------------------
 9.  Total distributions                       (.52)        (.40)        (.30)       (.04)         (.03)
- --------------------------------------------------------------------------------------------------------
10.  Net asset value, end of period           $18.48       $15.51       $13.45      $10.57        $10.32
- --------------------------------------------------------------------------------------------------------
11.  Total return*                            22.75%       18.45%       30.17%       2.76%         3.50%
- --------------------------------------------------------------------------------------------------------
12.  Net assets, end of period (in          $608,281     $325,789     $126,911     $43,549        $7,482
     thousands)
13.  Average net assets for the period (in  $477,914     $216,125      $77,344     $26,464        $3,191
     thousands)
14.  Ratio of gross expenses to average        0.70%(7)     0.69%(6)     0.78%(5)      N/A           N/A
     net assets**
15.  Ratio of net expenses to average net      0.69%        0.69%        0.76%       0.88%(2)(4)   0.25%(3)
     assets**
16.  Ratio of net investment income to         0.91%        1.39%        1.24%       1.45%         2.54%
     average net assets**
17.  Portfolio turnover rate**                  122%          87%         185%        169%          162%
18.  Average commission rate                 $0.0500      $0.0466          N/A         N/A           N/A
- --------------------------------------------------------------------------------------------------------
 
<CAPTION>
                       WORLDWIDE GROWTH PORTFOLIO
        1997          1996         1995        1994         1993(1)
 
<C>  <C>            <C>          <C>          <C>           <C>      
 1.      $19.44       $15.31       $12.07      $11.89       $10.00   
- ---------------------------------------------------------------------
                                                                     
 2.         .16          .16          .11         .04          .02   
 3.        4.14         4.27         3.19         .14         1.89   
                                                                     
- ---------------------------------------------------------------------
 4.        4.30         4.43         3.30         .18         1.91   
- ---------------------------------------------------------------------
                                                                     
 5.       (.17)        (.17)        (.06)          --        (.01)   
                                                                     
 6.       (.02)           --           --          --           --   
                                                                     
 7.          --           --           --          --           --   
 8.       (.16)        (.13)           --          --        (.01)   
- ---------------------------------------------------------------------
 9.       (.35)        (.30)        (.06)          --        (.02)   
- ---------------------------------------------------------------------
10.      $23.39       $19.44       $15.31      $12.07       $11.89   
- ---------------------------------------------------------------------
11.      22.15%       29.04%       27.37%       1.53%       19.10%   
- ---------------------------------------------------------------------
12.  $1,576,548     $582,603     $108,563     $37,728       $4,856   
                                                                     
13.  $1,148,951     $304,111      $59,440     $22,896       $2,200   
                                                                     
14.       0.74%(7)     0.80%(6)     0.90%(5)      N/A          N/A   
                                                                     
15.       0.74%        0.80%        0.87%       1.18%(2)(4)  0.25%(3)
                                                                     
16.       0.67%        0.83%        0.95%       0.50%        0.84%   
                                                                     
17.         80%          62%         113%        217%          57%   
18.     $0.0337      $0.0345          N/A         N/A          N/A   
- ---------------------------------------------------------------------
</TABLE>
 
<TABLE>
<CAPTION>
                                                                                       BALANCED PORTFOLIO
                      INSTITUTIONAL SHARES                           1997        1996        1995        1994         1993(1)
- ----------------------------------------------------------------------------------------------------------------------------------
<C>  <S>                                                           <C>          <C>         <C>         <C>           <C>      
 1.  Net asset value, beginning of period                            $14.77      $13.03      $10.63      $10.64       $10.00
- ----------------------------------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                                              .34         .32         .17         .15          .08
     Net gains or (losses) on securities (both realized and
 3.  unrealized)                                                       2.89        1.81        2.45       (.06)          .64
- ----------------------------------------------------------------------------------------------------------------------------------
 4.  Total from investment operations                                  3.23        2.13        2.62         .09          .72
- ----------------------------------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)                           (.35)       (.30)       (.22)       (.10)        (.08)
 6.  Tax return of capital distributions                                 --          --          --          --           --
 7.  Distributions (from capital gains)                               (.18)       (.09)          --          --           --
- ----------------------------------------------------------------------------------------------------------------------------------
 8.  Total distributions                                              (.53)       (.39)       (.22)       (.10)        (.08)
- ----------------------------------------------------------------------------------------------------------------------------------
 9.  Net asset value, end of period                                  $17.47      $14.77      $13.03      $10.63       $10.64
- ----------------------------------------------------------------------------------------------------------------------------------
10.  Total return*                                                   22.10%      16.18%      24.79%       0.84%        7.20%
- ----------------------------------------------------------------------------------------------------------------------------------
11.  Net assets, end of period (in thousands)                      $362,409     $85,480     $14,021      $3,153         $537
12.  Average net assets for the period (in thousands)               176,432     $43,414      $5,739      $2,336         $521
13.  Ratio of gross expenses to average net assets**                  0.83%(7)    0.94%(6)    1.37%(5)      N/A          N/A
14.  Ratio of net expenses to average net assets**                    0.82%       0.92%       1.30%       1.57%(2)(4)  0.25%(3)
15.  Ratio of net investment income to average net assets**           2.87%       2.92%       2.41%       1.90%        2.69%
16.  Portfolio turnover rate**                                         139%        103%        149%        158%         126%
17.  Average commission rate                                        $0.0456     $0.0426         N/A         N/A          N/A
- ----------------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) 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.
(3) The ratio was 2.16%, 2.71% and 7.92%, respectively, for Growth, Worldwide
    Growth and Balanced Portfolios before waiver of certain fees and/or
    voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(4) The ratio was 1.23%, 1.49% and 1.74%, respectively, for Growth, Worldwide
    Growth and Balanced Portfolios before voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(5) The ratio was 0.98%, 1.09% and 1.55%, respectively, for Growth, Worldwide
    Growth and Balanced Portfolios before voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(6) The ratio was 0.83%, 0.91% and 1.07%, respectively, for Growth, Worldwide
    Growth and Balanced Portfolios before voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(7) The ratio was 0.78%, 0.81% and 0.83%, respectively, for Growth, Worldwide
    Growth and Balanced Portfolios before voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
 
 4   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights table. The table contains important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolios'
Annual Report contains additional information about each Portfolio's
performance, including a comparison to an appropriate securities index. To
request a copy of the Annual Report, please call or write your insurance
company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of a Portfolio. It is
computed by adding the value of all of a Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between NET ASSET VALUE, BEGINNING OF PERIOD,
and NET ASSET VALUE, END OF PERIOD in the Financial Highlights table represents
the change in value of the Share of a Portfolio over the fiscal period, but not
its total return.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by a Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that a Portfolio paid from
net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities a Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that a Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of a Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolios were not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in a Portfolios's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is a Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of a Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of a Portfolio's securities.
 
AVERAGE COMMISSION RATE is the total of a Portfolio's agency commission paid on
equity securities trades divided by the number of shares purchased and sold.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    5
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe a Portfolio's performance. You may see references to these terms in our
newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of a Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. Growth
Portfolio, Worldwide Growth Portfolio and Balanced Portfolio generally measure
performance in terms of total return.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights table shows total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of a Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in a Portfolio's return and are not
the same as actual annual results.
 
THE PORTFOLIOS IMPOSE NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
COMPUTATIONS. TOTAL RETURN FIGURES INCLUDE THE EFFECT OF DEDUCTING EACH
PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES ATTRIBUTABLE TO
ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES ARE BASED UPON
HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE PERFORMANCE.
INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT SHARES, WHEN
REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
 6   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                            THE PORTFOLIOS IN DETAIL
 
This section takes a closer look at the Portfolios' investment objectives,
policies and the securities in which they invest. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed description of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before choosing a Portfolio.
 
Each Portfolio has an investment objective and policies that are similar to
those of a Janus retail fund, as illustrated in the chart below. Although it is
anticipated that each Portfolio and its corresponding Janus retail fund will
hold similar securities, differences in asset size and cash flow needs as well
as the relative weightings of securities selections may result in differences in
investment performance. Expenses of each Portfolio and its corresponding Janus
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 contract
fees and expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including each Portfolios' investment objective, are
not fundamental and may be changed by the Portfolios' Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in a Portfolio's objective or policies, you should
consider whether that Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
  EACH OF THE PORTFOLIOS HAS A SIMILAR INVESTMENT
  OBJECTIVE AND SIMILAR INVESTMENT POLICIES TO AN EXISTING JANUS RETAIL FUND.
 
Growth Portfolio....................................................Janus Fund
Worldwide Growth Portfolio................................Janus Worldwide Fund
Balanced Portfolio.........................................Janus Balanced Fund
 
GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO ARE DESIGNED FOR LONG-TERM
INVESTORS WHO SEEK GROWTH OF CAPITAL ONLY AND WHO CAN TOLERATE THE GREATER RISKS
ASSOCIATED WITH COMMON STOCK INVESTMENTS.
 
GROWTH PORTFOLIOS
Investment Objective:..........................................Growth of Capital
Primary Holdings:..................................................Common Stocks
Shareholder's Investment Horizon:......................................Long-Term
 
GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its investment objective by investing in primarily common
stocks of issuers of any size. The Portfolio generally invests in larger, more
established issuers.
 
WORLDWIDE GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective primarily through investments in common
stocks of foreign and domestic issuers. The Portfolio has the flexibility to
invest on a worldwide basis in companies and other organizations of any size,
regardless of country of organization or place of principal business activity.
Worldwide Growth 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.
 
TYPES OF INVESTMENTS
Each of these Portfolios invests primarily in common stocks of foreign and
domestic companies. However, the percentage of each Portfolio's assets invested
in common stocks will vary and each Portfolio may at times hold substantial
positions in cash equivalents or interest bearing securities. See "General
Portfolio Policies" on page 9. The Portfolios may invest to a lesser degree in
other types of securities including preferred stocks, warrants, convertible
securities and debt securities when its portfolio manager perceives an
opportunity for capital growth from such securities or to receive a return on
idle cash. Some securities that the Portfolios purchase may be on a when-issued,
delayed delivery or forward commitment basis. The Portfolios may invest up to
25% of their assets in mortgage- and asset-backed securities, up to 10% of their
assets in zero coupon, pay-in-kind and step coupon securities, and without limit
in indexed/structured securities. The Portfolios will not invest 35% or more of
their assets in high-yield/high-risk securities.
 
Although Worldwide Growth Portfolio is committed to foreign investing, Growth
Portfolio may also invest without limit in foreign equity and debt securities.
The Portfolios may invest directly in foreign securities denominated in a
foreign currency and not publicly traded in the United States. Other ways of
investing in foreign securities include depositary receipts or shares, and
passive foreign investment companies ("PFICs"). The Portfolios may use futures,
options and other derivatives for hedging purposes or for non-hedging purposes
such as seeking to enhance return. See "Additional Risk Factors" on page 10 for
a discussion of the risks associated with foreign investing and derivatives.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    7
<PAGE> 
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN GROWTH PORTFOLIO OR WORLDWIDE GROWTH PORTFOLIO.
 
Q:    HOW ARE COMMON STOCKS SELECTED?

A:    The Portfolios invest substantially all of their assets in common stocks
      to the extent their portfolio managers believe that the relevant market
environment favors profitable investing in those securities. Portfolio managers
generally take a "bottom up" approach to building their portfolios. In other
words, they seek to identify individual companies with earnings growth potential
that may not be recognized by the market at large. Although themes may emerge in
a Portfolio, securities are generally selected without regard to any defined
industry sector or other similarly defined selection procedure. Realization of
income is not a significant investment consideration. Any income realized on a
Portfolio's investments will be incidental to their objectives.
 
Q:    ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?

A:    Generally, yes. Portfolio managers seek companies that meet their
      selection criteria, regardless of country of organization or place of
principal business activity. 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. See "Additional Risk Factors" on page 10.
 
Q:    WHAT IS THE MAIN RISK OF INVESTING IN THE GROWTH PORTFOLIOS?

A:    Since the Growth Portfolios usually invest heavily in common stocks, the
      fundamental risk is that the value of the stocks a portfolio holds might
decrease. Stock values may fluctuate in response to the activities of an
individual company or in response to general market and economic conditions.
Historically, common stocks have provided greater long-term returns and have
entailed greater short-term risks than other investment choices. Smaller or
newer issuers are more likely to realize more substantial growth as well as
suffer more significant losses than larger or more established issuers.
Investments in such companies can be both more volatile and more speculative.
See "Additional Risk Factors" on page 10.
 
Q:    WHAT IS MEANT BY "MARKET CAPITALIZATION"?

A:    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 Growth Portfolio and Worldwide Growth Portfolio do not emphasize
companies of any particular size, Portfolios with a larger asset base are more
likely to invest in larger, more-established issuers.
 
Q:    HOW DO THESE PORTFOLIOS TRY TO REDUCE RISK?

A:    Diversification of a Portfolio's assets reduces the effect of any single
      holding on its overall portfolio value. A Portfolio may also use futures,
options and other derivative instruments to protect its portfolio from movements
in securities' prices and interest rates. The Portfolios may use a variety of
currency hedging techniques, including forward currency contracts, to manage
exchange rate risk when investing directly in foreign markets. See "Additional
Risk Factors" on page 10. In addition, to the extent that a Portfolio holds a
larger cash position, it may not participate in market declines to the same
extent as if the Portfolio remained more fully invested in common stocks.
 
BALANCED PORTFOLIO IS DESIGNED FOR INVESTORS WHO PRIMARILY SEEK GROWTH OF
CAPITAL WITH A DEGREE OF EMPHASIS ON INCOME. IT IS NOT DESIGNED FOR INVESTORS
WHO DESIRE A CONSISTENT LEVEL OF INCOME.
 
BALANCED PORTFOLIO
Investment Objective:..........................................Growth of Capital
                                                 Including an Emphasis on Income
Primary Holdings:..............................................Common Stocks and
                                                     Income-Producing Securities
Shareholder's Investment Horizon:......................................Long-Term
 
BALANCED PORTFOLIO
The investment objective of this Portfolio is long-term capital growth,
consistent with preservation of capital and balanced by current income. It is a
diversified portfolio that, under normal circumstances, pursues its objective by
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. The Portfolio normally invests at least 25% of its assets in
fixed-income senior securities, which include debt securities and preferred
stocks.
 
TYPES OF INVESTMENTS
Balanced Portfolio may invest in the types of investments previously described
under "Growth Portfolios" on page 7. The Portfolio may also invest in a wide
variety of income-producing securities including corporate bonds and notes,
government securities and indexed/structured securities. Balanced Portfolio's
investments in high-yield/high-risk securities will not exceed 35% of net assets
and investments in mortgage-and asset-backed securities will not exceed 25% of
assets.
 
 8   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN BALANCED PORTFOLIO.
 
Q:    HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF
      BALANCED PORTFOLIO?                                                 

A:    Balanced Portfolio may shift assets between the growth and income
      components of its portfolio 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, then the Portfolio will
place a greater emphasis on the growth component.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    The growth component of Balanced Portfolio is expected to consist
      primarily of common stocks. The selection criteria for common stocks are
described on page 8. 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 equity securities. Balanced
Portfolio may also find opportunities for capital growth from debt securities
because of anticipated changes in interest rates, credit standing, currency
relationships or other factors.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    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
Balanced 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.
 
GENERAL PORTFOLIO POLICIES
In investing their assets, the Portfolios will follow the general policies
listed below. The percentage limitations included in these policies and
elsewhere in this Prospectus apply only at the time of purchase of the security.
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's manager believes that market conditions are not favorable for
profitable investing or when the portfolio manager is otherwise unable to locate
favorable investment opportunities, a Portfolio's investments may be hedged to a
greater degree and/or its 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 are a residual - they represent the assets that
remain after the portfolio manager has committed available assets to desirable
investment opportunities. Partly because the portfolio managers act
independently of each other, the cash positions of the Portfolios may vary
significantly. A larger hedged position and/or larger cash position may serve as
a means of preserving capital in unfavorable market conditions.
 
Securities that a Portfolio may invest in as means of receiving a return on idle
cash include high-grade commercial paper, certificates of deposit, repurchase
agreements or other short-term debt obligations. The Portfolios may also invest
in money market funds (including funds managed by Janus Capital). When a
Portfolio is hedged or its investments in cash or similar investments increase,
it may not participate in stock or bond market advances or declines to the same
extent that it would if a Portfolio was not hedged or remained more fully
invested in stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. All of the Portfolios qualify
as diversified funds under the 1940 Act and are subject to the following
diversification requirements:
 
- - As a fundamental policy, no Portfolio may not own more than 10% of the
  outstanding voting shares of any issuer.
 
- - As a fundamental policy, with respect to 75% of its total assets, no Portfolio
  will purchase a security of any issuer (other than cash items and U.S.
  government securities, as defined in the 1940 Act) if such purchase would
  cause a Portfolio's holdings of that issuer to amount to more than 5% of that
  Portfolio's total assets.
 
- - No Portfolio will invest more than 25% of its total assets in a single issuer
  (other than U.S. government securities).
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, each Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, no Portfolio will invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
Each Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may 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 initial investment
decision. Changes are made in a Portfolio
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    9
<PAGE> 
 
whenever its portfolio manager believes such changes are desirable. Portfolio
turnover rates are generally not a factor in making buy and sell decisions.
 
To a limited extent, a Portfolio may purchase securities in anticipation of
relatively short-term price gains. A Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
Each Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of a Portfolio's net
assets after the time of purchase, the Portfolio will take steps to reduce in an
orderly fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public because of
their terms or because of SEC regulations. Janus Capital will follow guidelines
established by the Trustees of the Trust ("Trustees") in making liquidity
determinations for Rule 144A securities and other securities, including
privately placed commercial paper and municipal lease obligations.
 
BORROWING AND LENDING
Each Portfolio may borrow money and lend securities or other assets, as follows:
 
- - Each Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - Each Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, each Portfolio may lend securities or other assets
  if, as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, each Portfolio may
borrow money from or lend money to other funds that permit such transactions and
for which Janus Capital serves as investment adviser. All such borrowing and
lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. A Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though 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 underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolios may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk, because their values depend
on the performance of a foreign security denominated in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or
 
 10   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
potential development, or they may 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 be 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 wider price fluctuations. Investments
in such companies tend to be more volatile and somewhat more speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
Each Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolios intend to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolios may also use derivative instruments for non-hedging
purposes such as seeking to increase a Portfolio's income or otherwise seeking
to enhance return. Please refer to Appendix A to this Prospectus and the SAI for
a more detailed discussion of these instruments.
 
The use of derivative instruments exposes the Portfolios to additional
investment risks and transaction costs. Risks inherent in the use of derivative
instruments include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that a portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than a Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately-negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave a
  Portfolio worse off than if it had not entered into the position.
 
Although the Portfolios' managers believe the use of derivative instruments will
benefit the Portfolios, a Portfolio's performance could be worse than if the
Portfolio had not used such instruments if the portfolio manager's judgment
proves incorrect.
 
When a Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for higher quality securities. Conversely, the value of higher
quality securities may be more sensitive to interest rate movements than lower
rated securities. Issuers of high-yield/high-risk securities may not be as
strong financially as those issuing bonds with higher credit ratings.
Investments in such companies are considered to be more speculative than higher
quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, the Portfolio
would experience a reduction of its income and could expect a decline in the
market value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as higher
quality securities. Sovereign debt of foreign governments is generally rated by
country. Because these ratings do not take into account individual factors
relevant to each issue and may not be updated regularly, Janus Capital may treat
such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    11
<PAGE> 
 
addition, zero coupon, pay-in-kind and delayed interest bonds often do not pay
interest until maturity. However, a Portfolio must recognize imputed interest
income and pay dividends to shareholders even though it has received no cash. In
some instances, the Portfolio may have to sell securities to have sufficient
cash to pay the dividends.
 
SHORT SALES
Each Portfolio may engage in "short sales against the box." This technique
involves selling either a security that a Portfolio owns, or a security
equivalent in kind and amount that a Portfolio has the right to obtain, for
delivery at a specified date in the future. A Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
SPECIAL SITUATIONS
Each Portfolio may invest in "special situations" from time to time. 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.
Investment in special situations may carry an additional risk of loss in the
event that the anticipated development does not occur or does not attract the
expected attention.
 
See Appendix A for risks associated with certain other investments.
 
 12   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                                 MANAGEMENT OF
                                 THE PORTFOLIOS
 
TRUSTEES
 
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to each Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolios to the
officers of the Trust and meet at least quarterly to review the Portfolios'
investment policies, performance, expenses and other business affairs.
 
INVESTMENT ADVISER
 
Janus Capital, 100 Fillmore Street, Denver, Colorado 80206-4928, is the
investment adviser to the Portfolios and is responsible for the day-to-day
management of the investment portfolios and other business affairs of the
Portfolios.
 
Janus Capital has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
INVESTMENT PERSONNEL
PORTFOLIO MANAGERS
JAMES P. CRAIG III is Chief Investment Officer of Janus Capital. He is Executive
Vice President and portfolio manager of Growth Portfolio, which he has managed
since inception. Mr. Craig previously managed Balanced Portfolio from September
1993 through April 1996 and has co-managed Janus Venture Fund since February 1,
1997. Mr. Craig previously managed Janus Venture Fund from its inception
December 1993 and Janus Balanced Fund from December 1993 to December 1995. He
holds a Bachelor of Arts in Business from the University of Alabama and a Master
of Arts in Finance from the Wharton School of the University of Pennsylvania.
 
HELEN YOUNG HAYES is Executive Vice President and portfolio manager of Worldwide
Growth Portfolio and co-manager of International Growth Portfolio which she has
managed or co-managed since inception. Ms. Hayes joined Janus Capital in 1987
and has managed or co-managed Janus Worldwide Fund and Janus Overseas Fund since
their inceptions. She holds a Bachelor of Arts in Economics from Yale University
and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS is Executive Vice President and portfolio manager of Balanced
Portfolio, which he has managed since May 1996 and Equity Income Portfolio,
which he has managed since inception. He is an assistant portfolio manager of
Growth Portfolio. Mr. Rollins joined Janus Capital in 1990 and has managed Janus
Balanced Fund since January 1996 and Janus Equity Income Fund since June 1996.
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
LAURENCE J. CHANG is assistant portfolio manager of Worldwide Growth Portfolio,
and co-manager of International Growth Portfolio. Mr. Chang also co-manages
Janus Overseas Fund and is assistant portfolio manager of Janus Worldwide Fund.
He received an undergraduate degree with honors in religion and philosophy from
Dartmouth College and a Master's Degree in Political Science from Stanford
University. He is a Chartered Financial Analyst.
 
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.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    13
<PAGE> 
 
activities in a manner that Janus Capital believes is not detrimental to the
Portfolio or Janus Capital's other advisory clients. See the SAI for more
detailed information.
 
MANAGEMENT EXPENSES
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):
 
<TABLE>
<CAPTION>
                                  AVERAGE DAILY
                                   NET ASSETS        ANNUAL RATE
        FEE SCHEDULE              OF PORTFOLIO      PERCENTAGE(%)
- ------------------------------------------------------------------
<S>                            <C>                  <C>
Growth                         First $300 Million        0.75
Worldwide Growth, and          Next $200 Million         0.70
Balanced Portfolios            Over $500 Million         0.65
- ------------------------------------------------------------------
</TABLE>
 
  Janus Capital has agreed to reduce each Portfolio's advisory fee to the
  extent that such fee exceeds the effective rate of the Janus retail fund
  corresponding to such Portfolio. Janus Capital may terminate this fee
  reduction at any time upon at least 90 days' notice to the Trustees. The
  effective rate is the advisory 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, Janus Worldwide Fund and Janus
  Balanced Fund was .65%, .65% and .73%, respectively, for the quarter ended
  March 31, 1998.
 
Differences in the actual management fees incurred by the Portfolios are due
primarily to variances in the asset sizes of the corresponding retail funds. As
asset size increases, the annual rate of the management fee declines in
accordance with the above schedule. 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. The fee schedule for the Portfolios was
effective July 1, 1997.
 
During the most recent fiscal year the Portfolios paid the following management
fees expressed as a percentage of average net assets: Growth Portfolio .65%;
Worldwide Growth Portfolio .66%, and Balanced Portfolio .76%.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of the Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for the Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for the Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolios' Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for the Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses serving to reduce those expenses. The SAI
further explains the selection of broker-dealers.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolios with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust consists of twelve separate series, three of which are offered by this
Prospectus.
 
Each Portfolio currently offers two classes of Shares, one of which, the
Institutional Shares, are offered pursuant to this prospectus and are sold under
the name Janus Aspen Series. The Shares offered by this Prospectus are available
only in connection with investment in and payments under variable insurance
contracts as well as certain qualified retirement plans. Retirement Shares of
each Portfolio are offered by separate prospectus and are available only to
participant directed qualified plans using plan service providers that are
compensated for providing distribution and/or recordkeeping and other
administrative services provided to plan participants. Because the expenses of
each class may differ, the performance of each class is expected to differ. If
you would like additional information about the Retirement Shares, please call
1-800-525-0020.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific class or Portfolio or for the Trust as a
whole for purposes such as electing or removing Trustees, terminating or
reorganizing the Trust, changing fundamental policies, or for any other purpose
requiring a shareholder vote under the 1940 Act. Separate votes are taken by
each class or Portfolio only if a matter affects or requires the vote of only
that class or Portfolio or the interest of a class or Portfolio in the matter
differs from the interest of
 
 14   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
the other class or Portfolios of the Trust. As a shareholder, you are entitled
to one vote for each share that you own.
 
An insurance company issuing a variable contract invested in Shares of a
Portfolio will request voting instructions from variable contract holders. Under
current law, the insurance company must vote all Shares held by the separate
account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Each Portfolio's Shares are available only to variable annuity and variable life
separate accounts of insurance companies that are unaffiliated with Janus
Capital and to certain qualified retirement plans. Retirement Shares of the
Portfolios (offered through a separate prospectus) are available to certain
participant directed qualified plans. Although the Portfolios do not currently
anticipate any disadvantages to policy owners arising out of the fact that 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 anticipated disadvantages or material irreconcilable conflicts to determine
what action, if any, should be taken in response to such conflicts. If a
material disadvantage or conflict occurs, the Trustees may require one or more
insurance company separate accounts or plans to withdraw its investment in one
or more Portfolios or substitute shares of another Portfolio. If this occurs, a
Portfolio may be forced to sell securities at disadvantageous prices. In
addition, the Trustees may refuse to sell shares of any Portfolio to any
separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve a Portfolio's investment objective
by investing all of that Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to that Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolios with
service at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of a Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per Share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of Shares outstanding.
 
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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    15
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. EACH CLASS OF EACH PORTFOLIO MAKES SEMIANNUAL 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 THE SHARES OF THE PORTFOLIOS WILL BE
AUTOMATICALLY REINVESTED INTO ADDITIONAL SHARES OF THE PORTFOLIOS.
 
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 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 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 in the Shares depends on the
features of the variable insurance contracts purchased from a participating
insurance company. Further information may be found 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 as a foreign tax credit.
If such an 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
it intends 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.
 
 16   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<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. 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
PORTFOLIOS AS AN INVESTMENT OPTION FOR A CONTRACT OR A QUALIFIED PLAN.
 
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's
separate account or your plan documents for information on how to invest in the
Shares of each Portfolio.
 
All investments in the Portfolios are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by a Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by a Portfolio.
 
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. Any Portfolio may
discontinue sales of its shares if management believes that a substantial
further increase may adversely affect that Portfolio's ability to achieve its
investment objective. In such event, however, it is anticipated that existing
policy owners and plan participants invested in that Portfolio would be
permitted to continue to authorize investment in the Portfolio and to reinvest
any dividends or capital gains distributions.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolios that they have authorized for
investment. Each report will show the investments owned by each Portfolio and
the market values thereof, as well as other information about the Portfolios and
their operations. The Trust's fiscal year ends December 31.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    17
<PAGE> 
 
                                   APPENDIX A
 
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 its
investment objective 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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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 (BB or lower by Standard & Poor's and Ba or
lower by Moody's). Other terms commonly used to describe such securities include
"lower rated bonds," "noninvestment grade bonds" and "junk bonds."
 
MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of mortgages or other
debt. These securities are generally pass-through securities, which means that
principal and interest payments on the underlying securities (less servicing
fees) are passed through to shareholders on a pro rata basis. These securities
involve prepayment risk, which is the risk that the underlying mortgages or
other debt may be refinanced or paid off prior to their maturities during
periods of declining interest rates. In that case, the portfolio manager may
have to reinvest the proceeds from the securities at a lower rate. Potential
market gains on a security subject to prepayment risk may be more limited than
potential market gains on a comparable security that is not subject to
prepayment risk.
 
PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign corporations which
generate certain amounts of passive income or hold certain amounts of assets for
the production of passive income. Passive income includes dividends, interest,
royalties, rents and annuities. To avoid taxes and interest that the 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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 to provide cash to satisfy unusually high redemption
requests or for other temporary or emergency purposes.
 
 18   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
RULE 144A SECURITIES are securities that are not registered for sale to the
general public under the Securities Act of 1933, but that may be resold to
certain institutional investors.
 
STANDBY COMMITMENTS are obligations purchased by a Portfolio from a dealer that
give the Portfolio the option to sell a security to the dealer at a specified
price.
 
STEP COUPON BONDS are debt securities that trade at a discount from their face
value and pay coupon interest. The discount from the face value depends on the
time remaining until cash payments begin, prevailing interest rates, liquidity
of the security and the perceived credit quality of the issuer.
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a 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 bears the risk of market value fluctuations
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 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 of comparable 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 non-dollar denominated securities or to
reduce the impact of currency appreciation on purchases of non-dollar
denominated securities. The Portfolios 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    19
<PAGE> 
 
instrument or index. For example, upon reset the interest rate payable on a
security may go down when the underlying index has risen. Certain inverse
floaters may have an interest rate reset mechanism that multiplies the effects
of change in the underlying index. Such mechanism may increase the volatility of
the security's market value.
 
OPTIONS are the right, but not the obligation, to buy or sell a specified amount
of securities or other assets on or before a fixed date at a predetermined
price. The Portfolios may purchase and write put and call options on securities,
securities indices and foreign currencies.
 
 20   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 

                               JANUS ASPEN SERIES
                                   PROSPECTUS
                               Growth Portfolio
                          Worldwide Growth Portfolio
                               Balanced Portfolio


                                 [JANUS LOGO]



            P.O. Box 173375 o Denver, CO 80217-3375 o 1-800-525-1068
                                     (5/98)
<PAGE> 
 
                               JANUS ASPEN SERIES
 
                                   Prospectus
 
                                  MAY 1, 1998
 
This prospectus describes five mutual funds each of which has its own investment
objective and policies (the "Portfolios"). Each Portfolio is a series of Janus
Aspen Series (the "Trust") and currently offers two classes of shares. The
Institutional Shares are sold under the name "Janus Aspen Series." The Trust is
registered with the Securities and Exchange Commission ("SEC") as an open-end
management investment company. The Institutional Shares of each Portfolio
(collectively, the "Shares") are offered by this prospectus in connection with
investment in and payments under variable annuity contracts and variable life
insurance contracts (collectively "variable insurance contracts"), as well as
certain qualified retirement plans. Janus Capital Corporation ("Janus Capital")
serves as investment adviser to each Portfolio. Janus Capital has been in the
investment advisory business for over 27 years and currently manages
approximately $80 billion in assets.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Each variable insurance contract
involves fees and expenses 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 about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolios. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                                           <C>
PORTFOLIOS AT A GLANCE
Brief description of the Portfolios.........................    2
EXPENSE INFORMATION
Each Portfolio's annual operating expenses..................    3
Financial Highlights - a summary of financial data..........    4
PERFORMANCE TERMS
An Explanation of Performance Terms.........................    7
THE PORTFOLIOS IN DETAIL
Investment Objectives and Policies..........................    8
General Portfolio Policies..................................   10
Additional Risk Factors.....................................   11
MANAGEMENT OF THE PORTFOLIOS
Investment Adviser and Investment Personnel.................   14
Personal Investing..........................................   15
Management Expenses.........................................   15
Portfolio Transactions......................................   15
Other Service Providers.....................................   15
Other Information...........................................   15
DISTRIBUTIONS AND TAXES
Distributions...............................................   17
Taxes.......................................................   17
SHAREHOLDER'S GUIDE
Purchases...................................................   18
Redemptions.................................................   18
Shareholder Communications..................................   18
APPENDIX A
Glossary of Investment Terms................................   19
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIOS AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolios
and their investment emphasis. A more detailed discussion of the Portfolios'
investment objectives and policies begins on page 8.
 
GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on companies with larger
market capitalizations.
Inception: September 1993
Manager: James P. Craig, III
Assistant Managers: David C. Decker
                    Blaine P. Rollins
 
AGGRESSIVE GROWTH PORTFOLIO
Focus: A nondiversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks with an emphasis on securities issued by
medium-sized companies.
Inception: September 1993
Manager: James P. Goff
 
INTERNATIONAL GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign issuers.
Inception: May 1994
Managers: Laurence J. Chang
          Helen Young Hayes
 
WORLDWIDE GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign and domestic issuers.
Inception: September 1993
Manager: Helen Young Hayes
Assistant Manager: Laurence J. Chang
 
BALANCED PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital balanced
by current income. The Portfolio normally invests 40-60% of its assets in
securities selected primarily for their growth potential and 40-60% in
securities selected primarily for their income potential.
Inception: September 1993
Manager: Blaine P. Rollins
 
 2   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolios in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES (applicable to each Portfolio)
 
<TABLE>
         <S>                                                           <C>
         Maximum sales load imposed on purchases                       None
         Maximum sales load imposed on reinvested dividends            None
         Deferred sales charges on redemptions                         None
         Redemption fee                                                None
         Exchange fee                                                  None
</TABLE>
 
ANNUAL OPERATING EXPENSES (after fee reductions)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<CAPTION>
- -----------------------------------------------------------------------------------------------------------------
                                                             MANAGEMENT         OTHER         TOTAL OPERATING
                                                                 FEE           EXPENSES          EXPENSES
- -----------------------------------------------------------------------------------------------------------------
<S>                                                          <C>           <C>                <C>             
Growth Portfolio                                                0.65%           0.05%              0.70%
Aggressive Growth Portfolio                                     0.73%           0.03%              0.76%
International Growth Portfolio                                  0.67%           0.29%              0.96%
Worldwide Growth Portfolio                                      0.66%           0.08%              0.74%
Balanced Portfolio                                              0.76%           0.07%              0.83%
 ----------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Management fees for Growth, Aggressive Growth, International Growth,
    Worldwide Growth and Balanced Portfolios reflect a reduced fee schedule
    effective July 1, 1997. The management fee for each Portfolio reflects the
    new rate applied to net assets as of December 31, 1997. Other expenses are
    based on the gross expenses of the Shares before expense offset arrangements
    for the fiscal year ended December 31, 1997. The information for each
    Portfolio is net of fee reductions from 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. Without such reductions, the Management
    Fee, Other Expenses and Total Operating Expenses for the Shares would have
    been 0.74%, 0.04%, and 0.78% for Growth Portfolio; 0.74%, 0.04% and 0.78%
    for Aggressive Growth Portfolio; 0.79%, 0.29% and 1.08% for International
    Growth Portfolio; 0.72%, 0.09%, and 0.81% for Worldwide Growth Portfolio;
    and 0.77%, 0.06%, and 0.83% for Balanced Portfolio, respectively. Janus
    Capital may modify or terminate the fee reductions at any time upon at least
    90 days' notice to the Trustees.
 
EXAMPLE
 
Assume you invest $1,000, the Shares of the Portfolios return 5% annually and
each Portfolio's expense ratio remains as listed above. The example below shows
the operating expenses that you would indirectly bear as an investor in the
Shares of the Portfolios.
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------
                                                             1 YEAR      3 YEARS      5 YEARS      10 YEARS
- ---------------------------------------------------------------------------------------------------------------
<S>                                                          <C>         <C>          <C>          <C>      
Growth Portfolio                                              $ 7          $22          $39          $ 87
Aggressive Growth Portfolio                                   $ 8          $24          $42          $ 94
International Growth Portfolio                                $10          $31          $53          $118
Worldwide Growth Portfolio                                    $ 8          $24          $41          $ 92
Balanced Portfolio                                            $ 8          $26          $46          $103
 --------------------------------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    3
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise noted, the information below is for fiscal periods ending on
December 31st of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolios' financial statements since their inception. Their report
is included in the Portfolios' Annual Report, which is incorporated by reference
into the SAI. A DETAILED EXPLANATION OF THE FINANCIAL HIGHLIGHTS CAN BE FOUND ON
PAGE 6.
<TABLE>
<CAPTION>
                                                                      GROWTH PORTFOLIO
             INSTITUTIONAL SHARES                 1997         1996         1995        1994         1993(1)
- ------------------------------------------------------------------------------------------------------------
<C>  <S>                                        <C>          <C>          <C>          <C>           <C>
 1.  Net asset value, beginning of period         $15.51       $13.45       $10.57      $10.32       $10.00
- ------------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                           .15          .17          .28         .09          .03
 3.  Net gains or (losses) on securities (both
     realized and unrealized)                       3.34         2.29         2.90         .20          .32
- ------------------------------------------------------------------------------------------------------------
 4.  Total from investment operations               3.49         2.46         3.18         .29          .35
- ------------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)        (.15)        (.17)        (.30)       (.04)        (.03)
 6.  Tax return of capital distributions              --           --           --          --        (.19)
 7.  Distributions (from capital gains)            (.37)        (.23)           --          --           --
- ------------------------------------------------------------------------------------------------------------
 8.  Total distributions                           (.52)        (.40)        (.30)       (.04)        (.03)
- ------------------------------------------------------------------------------------------------------------
 9.  Net asset value, end of period               $18.48       $15.51       $13.45      $10.57       $10.32
- ------------------------------------------------------------------------------------------------------------
10.  Total return*                                22.75%       18.45%       30.17%       2.76%        3.50%
- ------------------------------------------------------------------------------------------------------------
11.  Net assets, end of period (in thousands)   $608,281     $325,789     $126,911     $43,549       $7,482
12.  Average net assets for the period (in
     thousands)                                 $477,914     $216,125       77,344     $26,464       $3,191
13.  Ratio of gross expenses to average net
     assets**                                      0.70%(8)     0.69%(7)     0.78%(6)      N/A          N/A
14.  Ratio of net expenses to average net
     assets**                                      0.69%        0.69%        0.76%       0.88%(3)(5)  0.25%(4)
15.  Ratio of net investment income to average
     net assets**                                  0.91%        1.39%        1.24%       1.45%        2.54%
16.  Portfolio turnover rate**                      122%          87%         185%        169%         162%
17.  Average commission rate                     $0.0500      $0.0466          N/A         N/A          N/A
- ------------------------------------------------------------------------------------------------------------
 
<CAPTION>
                     AGGRESSIVE GROWTH PORTFOLIO
       1997         1996         1995        1994         1993(1)
 
<C>  <C>          <C>          <C>          <C>           <C>      
 1.    $18.24       $17.08       $13.62      $11.80       $10.00   
- -------------------------------------------------------------------
                                                                   
 2.        --           --          .24         .11          .01   
 3.                                                                
         2.31         1.36         3.47        1.82         1.80   
- -------------------------------------------------------------------
 4.      2.31         1.36         3.71        1.93         1.81   
- -------------------------------------------------------------------
                                                                   
 5.        --           --        (.25)       (.11)        (.01)   
 6.        --        (.01)           --          --           --   
 7.        --        (.19)           --          --           --   
- -------------------------------------------------------------------
 8.        --        (.20)        (.25)       (.11)        (.01)   
- -------------------------------------------------------------------
 9.    $20.55       $18.24       $17.08      $13.62       $11.80   
- -------------------------------------------------------------------
10.    12.66%        7.95%       27.48%      16.33%       18.05%   
- -------------------------------------------------------------------
11.  $505,198     $383,693     $185,911     $41,289       $1,985   
12.                                                                
     $418,464     $290,629     $107,582     $14,152       $1,091   
13.                                                                
        0.76%(8)     0.76%(7)     0.86%(6)      N/A          N/A   
14.                                                                
        0.76%        0.76%        0.84%       1.05%(3)(5)  0.25%(4)
15.                                                                
      (0.10%)      (0.27%)        0.58%       2.18%        0.34%   
16.      130%          88%         155%        259%          31%   
17.   $0.0367      $0.0347          N/A         N/A          N/A   
- -------------------------------------------------------------------
</TABLE>
<TABLE>
<CAPTION>
                                                               INTERNATIONAL GROWTH PORTFOLIO
                 INSTITUTIONAL SHARES                     1997        1996        1995       1994(2)
- ----------------------------------------------------------------------------------------------------
<C>  <S>                                                <C>          <C>         <C>         <C>
 1.  Net asset value, beginning of period                 $15.72      $11.95       $9.72     $10.00
- ----------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                                   .11         .05         .09      (.09)
 3.  Net gains or (losses) on securities (both
     realized and unrealized)                               2.80        4.06        2.16      (.19)
- ----------------------------------------------------------------------------------------------------
 4.  Total from investment operations                       2.91        4.11        2.25      (.28)
- ----------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)                (.11)       (.11)       (.02)         --
 6.  Dividends (in excess of net investment income)           --          --          --         --
 7.  Tax return of capital distributions                      --          --          --         --
 8.  Distributions (from capital gains)                    (.01)       (.23)          --         --
 9.  Distributions (in excess of realized gains)           (.03)          --          --         --
- ----------------------------------------------------------------------------------------------------
10.  Total distributions                                   (.15)       (.34)       (.02)         --
- ----------------------------------------------------------------------------------------------------
11.  Net asset value, end of period                       $18.48      $15.72      $11.95      $9.72
- ----------------------------------------------------------------------------------------------------
12.  Total return*                                        18.51%      34.71%      23.15%     (2.80%)
- ----------------------------------------------------------------------------------------------------
13.  Net assets, end of period (in thousands)           $161,091     $27,192      $1,608     $1,353
14.  Average net assets for the period (in thousands)    $96,164      $7,437      $1,792     $1,421
15.  Ratio of gross expenses to average net assets**       0.96%(8)    1.26%(7)    2.69%(6)     N/A
16.  Ratio of net expenses to average net assets**         0.96%       1.25%       2.50%      2.50%(5)
17.  Ratio of net investment income to average net
     assets**                                              0.70%       0.62%     (0.80%)     (1.30%)
18.  Portfolio turnover rate**                               86%         65%        211%       275%
19.  Average commission rate                             $0.0241     $0.0305         N/A        N/A
- ----------------------------------------------------------------------------------------------------
 
<CAPTION>
                       WORLDWIDE GROWTH PORTFOLIO
        1997          1996         1995        1994         1993(1)
- ----------------------------------------------------------------------------------------------------
<C>  <C>            <C>          <C>          <C>           <C>      
 1.      $19.44       $15.31       $12.07      $11.89       $10.00
- ----------------------------------------------------------------------------------------------------
 
 2.         .16          .16          .11         .04          .02
 3.
           4.14         4.27         3.19         .14         1.89
- ----------------------------------------------------------------------------------------------------
 4.        4.30         4.43         3.30         .18         1.91
- ----------------------------------------------------------------------------------------------------
 
 5.       (.17)        (.17)        (.06)          --        (.01)
 6.       (.02)           --           --          --           --
 7.          --           --           --          --           --
 8.       (.16)        (.13)           --          --        (.01)
 9.          --           --           --          --           --
- ----------------------------------------------------------------------------------------------------
10.       (.35)        (.30)        (.06)          --        (.02)
- ----------------------------------------------------------------------------------------------------
11.      $23.39       $19.44       $15.31      $12.07       $11.89
- ----------------------------------------------------------------------------------------------------
12.      22.15%       29.04%       27.37%       1.53%       19.10%
- ----------------------------------------------------------------------------------------------------
13.  $1,576,548     $582,603     $108,563     $37,728       $4,856
14.  $1,148,951     $304,111      $59,440     $22,896       $2,200
15.       0.74%(8)     0.80%(7)     0.90%(6)      N/A          N/A
16.       0.74%        0.80%        0.87%       1.18%(3)(5)  0.25%(4)
17.
          0.67%        0.83%        0.95%       0.50%        0.84%
18.         80%          62%         113%        217%          57%
19.     $0.0337      $0.0345          N/A         N/A          N/A
- ----------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) May 2, 1994 (inception) to December 31, 1994.
(3) 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.
(4) The ratio was 2.16%, 5.79% and 2.71%, respectively, for Growth, Aggressive
    Growth and Worldwide Growth Portfolios, before waiver of certain fees and/or
    voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(5) The ratio was 1.23%, 1.14%, 4.67% and 1.49%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(6) The ratio was 0.98%, 0.93%, 3.57% and 1.09%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(7) The ratio was 0.83%, 0.83%, 2.21% and 0.91%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(8) The ratio was 0.78%, 0.78%, 1.08% and 0.81%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios
    before voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
 4   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
<TABLE>
<CAPTION>
                                                                                       BALANCED PORTFOLIO
                      INSTITUTIONAL SHARES                           1997        1996        1995        1994         1993(1)
- ----------------------------------------------------------------------------------------------------------------------------------
<C>  <S>                                                           <C>          <C>         <C>         <C>           <C>       
 1.  Net asset value, beginning of period                            $14.77      $13.03      $10.63      $10.64       $10.00
- ----------------------------------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                                              .34         .32         .17         .15          .08
 3.  Net gains or (losses) on securities (both realized and
     unrealized)                                                       2.89        1.81        2.45       (.06)          .64
- ----------------------------------------------------------------------------------------------------------------------------------
 4.  Total from investment operations                                  3.23        2.13        2.62         .09          .72
- ----------------------------------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)                           (.35)       (.30)       (.22)       (.10)        (.08)
 6.  Tax return of capital distributions                                 --          --          --          --           --
 7.  Distributions (from capital gains)                               (.18)       (.09)          --          --           --
- ----------------------------------------------------------------------------------------------------------------------------------
 8.  Total distributions                                              (.53)       (.39)       (.22)       (.10)        (.08)
- ----------------------------------------------------------------------------------------------------------------------------------
 9.  Net asset value, end of period                                  $17.47      $14.77      $13.03      $10.63       $10.64
- ----------------------------------------------------------------------------------------------------------------------------------
10.  Total return*                                                   22.10%      16.18%      24.79%       0.84%        7.20%
- ----------------------------------------------------------------------------------------------------------------------------------
11.  Net assets, end of period (in thousands)                      $362,409     $85,480     $14,021      $3,153         $537
12.  Average net assets for the period (in thousands)              $176,432     $43,414      $5,739      $2,336         $521
13.  Ratio of gross expenses to average net assets**                  0.83%(7)    0.94%(6)    1.37%(5)      N/A          N/A
14.  Ratio of net expenses to average net assets**                    0.82%       0.92%       1.30%       1.57%(2)(4)  0.25%(3)
15.  Ratio of net investment income to average net assets**           2.87%       2.92%       2.41%       1.90%        2.69%
16.  Portfolio turnover rate**                                         139%        103%        149%        158%         126%
17.  Average commission rate                                        $0.0456     $0.0426         N/A         N/A          N/A
- ----------------------------------------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) 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.
(3) The ratio was 7.92% for the Balanced Portfolio, before waiver of certain
    fees and/or voluntary reduction of adviser's fees to the effective rate of
    the Janus Balanced Fund.
(4) The ratio was 1.74% for Balanced Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(5) The ratio was 1.55% for Balanced Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(6) The ratio was 1.07% for Balanced Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
(7) The ratio was 0.83% for Balanced Portfolio before voluntary reduction of
    adviser's fees to the effective rate of Janus Balanced Fund.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    5
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights tables. The tables contain important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolios'
Annual Report contains additional information about each Portfolio's
performance, including a comparison to an appropriate securities index. To
request a copy of the Annual Report, please call or write your insurance
company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of a Portfolio. It is
computed by adding the value of all of a Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between NET ASSET VALUE, BEGINNING OF PERIOD,
and NET ASSET VALUE, END OF PERIOD in the Financial Highlights tables represents
the change in value of a Share of a Portfolio over the fiscal period, but not
its total return.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by a Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that a Portfolio paid from
net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities a Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that a Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of a Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolios were not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in a Portfolio's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is a Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of a Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of a Portfolio's securities.
 
AVERAGE COMMISSION RATE is the total of a Portfolio's agency commission paid on
equity securities trades divided by the number of shares purchased and sold.
 
 6   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe a Portfolio's performance. You may see references to these terms in our
newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of a Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. Growth
Portfolio, Aggressive Growth Portfolio, International Growth Portfolio,
Worldwide Growth Portfolio and Balanced Portfolio generally measure performance
in terms of total return.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights tables show total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of a Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in a Portfolio's return and are not
the same as actual annual results.
 
THE PORTFOLIOS IMPOSE NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
OR YIELD COMPUTATIONS. TOTAL RETURN FIGURES INCLUDE THE EFFECT OF DEDUCTING EACH
PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES ATTRIBUTABLE TO
ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES ARE BASED UPON
HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE PERFORMANCE.
INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT SHARES, WHEN
REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    7
<PAGE> 
 
                            THE PORTFOLIOS IN DETAIL
 
This section takes a closer look at the Portfolios' investment objectives,
policies and the securities in which they invest. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed description of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before choosing a Portfolio.
 
Each Portfolio has an investment objective and policies that are similar to
those of a Janus retail fund, as illustrated in the chart below. Although it is
anticipated that each Portfolio and its corresponding retail fund will hold
similar securities, differences in asset size and cash flow needs as well as the
relative weightings of securities selections may result in differences in
investment performance. 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 contract fees and
expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including each Portfolio's investment objective, are
not fundamental and may be changed by the Portfolios' Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in a Portfolio's objective or policies, you should
consider whether that Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
  EACH OF THE PORTFOLIOS HAS A SIMILAR INVESTMENT
  OBJECTIVE AND SIMILAR INVESTMENT POLICIES TO AN EXISTING JANUS RETAIL FUND.
 
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
 
GROWTH PORTFOLIO, AGGRESSIVE GROWTH PORTFOLIO, INTERNATIONAL GROWTH PORTFOLIO
AND WORLDWIDE GROWTH PORTFOLIO ARE DESIGNED FOR LONG-TERM INVESTORS WHO SEEK
GROWTH OF CAPITAL ONLY AND WHO CAN TOLERATE THE GREATER RISKS ASSOCIATED WITH
COMMON STOCK INVESTMENTS.
 
GROWTH PORTFOLIOS
Investment Objective:..........................................Growth of Capital
Primary Holdings:..................................................Common Stocks
Shareholder's Investment Horizon:......................................Long-Term
 
GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective by investing primarily in common stocks of
issuers of any size. This Portfolio generally invests in larger, more
established issuers.
 
AGGRESSIVE GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a nondiversified portfolio that pursues its investment objective by normally
investing at least 50% of its equity assets in securities issued by medium-sized
companies. Medium-sized companies are those whose market capitalizations fall
within the range of companies in the S&P MidCap 400 Index (the "MidCap Index").
Companies whose capitalization falls outside this range after the Portfolio's
initial purchase continue to be considered medium-sized Companies for the
purpose of this policy. As of December 31, 1997, the MidCap Index included
companies with capitalizations between approximately $213 million to $13.7
billion. The range of the MidCap Index is expected to change on a regular basis.
Subject to the above policy, the Portfolio may also invest in smaller or larger
issuers.
 
INTERNATIONAL GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a diversified portfolio that pursues its objective primarily through investments
in common stocks of issuers located outside the United States. The Portfolio has
the flexibility to invest on a worldwide basis in companies and other
organizations of any size, regardless of country of organization or place of
principal business activity. The 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 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
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective primarily through investments in common
stocks of foreign and domestic issuers. The Portfolio has the flexibility to
invest on a worldwide basis in companies and other organizations of any size,
regardless of country of organization or place of principal business activity.
Worldwide Growth 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.
 
TYPES OF INVESTMENTS
Each of these Portfolios invests primarily in common stocks of foreign and
domestic companies. However, the percentage of
 
 8   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
each Portfolio's assets invested in common stocks will vary and each Portfolio
may at times hold substantial positions in cash equivalents or interest bearing
securities. See "General Portfolio Policies" on page 10. Each Portfolio may
invest to a lesser degree in other types of securities including preferred
stocks, warrants, convertible securities and debt securities when its portfolio
manager perceives an opportunity for capital growth from such securities or to
receive a return on idle cash. Some securities that the Portfolios purchase may
be on a when-issued, delayed delivery or forward commitment basis. The
Portfolios may invest up to 25% of their assets in mortgage- and asset-backed
securities, up to 10% of their assets in zero coupon, pay-in-kind and step
coupon securities, and without limit in indexed/structured securities. No Growth
Portfolio will invest 35% or more of its assets in high-yield/high-risk
securities.
 
Although Worldwide Growth Portfolio and International Growth Portfolio are
committed to foreign investing, Growth Portfolio and Aggressive Growth Portfolio
may also invest without limit in foreign equity and debt securities. The
Portfolios may invest directly in foreign securities denominated in a foreign
currency and not publicly traded in the United States. Other ways of investing
in foreign securities include depositary receipts or shares, and passive foreign
investment companies ("PFICs"). These Portfolios may use futures, options and
other derivatives for hedging purposes or for non-hedging purposes such as
seeking to enhance return. See "Additional Risk Factors" on page 11 for a
discussion of the risks associated with foreign investing and derivatives.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE GROWTH PORTFOLIOS.
 
Q:    HOW ARE COMMON STOCKS SELECTED?

A:    Each of these Portfolios invests substantially all of its assets in common
      stocks to the extent its portfolio manager believes that the relevant
market environment favors profitable investing in those securities. Portfolio
managers generally take a "bottom up" approach to building their portfolios. In
other words, they seek to identify individual companies with earnings growth
potential that may not be recognized by the market at large. Although themes may
emerge in any Portfolio, securities are generally selected without regard to any
defined industry sector or other similarly defined selection procedure.
Realization of income is not a significant investment consideration. Any income
realized on these Portfolios' investments will be incidental to their
objectives.
 
Q:    ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?

A:    Generally, yes. Portfolio managers seek companies that meet their
      selection criteria, regardless of country of organization or place of
principal business activity. 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. See "Additional Risk Factors" on page 11.
 
Q:    WHAT IS THE MAIN RISK OF INVESTING IN A GROWTH PORTFOLIO?

A:    Since the Growth Portfolios usually invest heavily in common stock, the
      fundamental risk is that the value of the stocks a Portfolio holds might
decrease. Stock values may fluctuate in response to the activities of an
individual company or in response to general market and economic conditions.
Historically, common stocks have provided greater long-term returns and have
entailed greater short-term risks than other investment choices. Smaller or
newer issuers are more likely to realize more substantial growth as well as
suffer more significant losses than larger or more established issuers.
Investments in such companies can be both more volatile and more speculative.
See "Additional Risk Factors" on page 11.
 
Q:    WHAT IS MEANT BY "MARKET CAPITALIZATION"?

A:    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 which may invest in small to medium sized
companies to a greater degree. Although Growth Portfolio, International Growth
Portfolio and Worldwide Growth Portfolio do not emphasize companies of any
particular size, Portfolios with a larger asset base are more likely to invest
in larger, more-established issuers.
 
Q:    HOW DOES A DIVERSIFIED PORTFOLIO DIFFER FROM A NONDIVERSIFIED PORTFOLIO?

A:    Diversification is a means of reducing risk by investing a Portfolio's
      assets 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 share price of a nondiversified portfolio, its share price
can be expected to fluctuate more than a comparable diversified portfolio.
Aggressive Growth Portfolio is a nondiversified portfolio.
 
Q:    HOW DO THESE PORTFOLIOS TRY TO REDUCE RISK?

A:    Diversification of a Portfolio's assets reduces the effect of any single
      holding on its overall portfolio value. A Portfolio may also use futures,
options and other derivative instruments to protect its portfolio from movements
in securities prices and interest rates. The Portfolios may use a variety of
currency hedging techniques, including forward currency contracts, to manage
exchange rate risk when
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    9
<PAGE> 
 
investing directly in foreign markets. See "Additional Risk Factors" on page 11.
In addition, to the extent that a Portfolio holds a larger cash position, it may
not participate in market declines to the same extent as if the Portfolio
remained more fully invested in common stocks.
 
BALANCED PORTFOLIO IS DESIGNED FOR INVESTORS WHO PRIMARILY SEEK GROWTH OF
CAPITAL WITH A DEGREE OF EMPHASIS ON INCOME. IT IS NOT DESIGNED FOR INVESTORS
WHO DESIRE A CONSISTENT LEVEL OF INCOME.
 
Investment Objective:..........................................Growth of Capital
                                                 Including an Emphasis on Income
Primary Holdings:..............................................Common Stocks and
                                                     Income-Producing Securities
Shareholder's Investment Horizon:......................................Long-Term
 
BALANCED PORTFOLIO
The investment objective of this Portfolio is long-term capital growth,
consistent with preservation of capital and balanced by current income. It is a
diversified Portfolio that, under normal circumstances, pursues its objective by
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 senior securities, which include debt securities and preferred
stocks.
 
TYPES OF INVESTMENTS
Balanced Portfolio may invest in the types of investments previously described
on pages 8 and 9. The Portfolio's investment in high-yield/high-risk securities
will not exceed 35% of net assets and investment in mortgage- and asset-backed
securities will not exceed 25% of assets.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN BALANCED PORTFOLIO.
 
Q:    HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF
      BALANCED PORTFOLIO?                                                 

A:    Balanced Portfolio may invest in a combination of common stocks, preferred
      stocks, convertible securities, debt securities and other fixed-income
securities. Balanced Portfolio may shift assets between the growth and income
components of its portfolio 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.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    The growth component of Balanced Portfolio is expected to consist
      primarily of common stocks. The selection criteria for common stocks are
described on page 9. 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 equity securities. Balanced
Portfolio may also find opportunities for capital growth from debt securities
because of anticipated changes in interest rates, credit standing, currency
relationships or other factors.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    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
Balanced Portfolio if they currently pay dividends or a portfolio manager
believes they have the potential for either increasing their dividends or
commencing dividends, if none are currently paid.
 
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 only at the time of purchase of the security. 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's manager believes that market conditions are not favorable for
profitable investing or when the portfolio manager is otherwise unable to locate
favorable investment opportunities, a Portfolio's investments may be hedged to a
greater degree and/or its 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 are a residual - they represent the assets that
remain after a portfolio manager has committed available assets to desirable
investment opportunities. Partly because the portfolio managers act
independently of each other, the cash positions of the Portfolios may vary
significantly. Larger hedged positions and/or larger cash positions may serve as
a means of preserving capital in unfavorable market conditions.
 
Securities that the Portfolios may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolios may
also invest in money market funds (including funds managed by Janus Capital).
When a Portfolio is hedged or its investments in cash or similar investments
increase, it may not participate in stock or bond market advances or declines to
the same extent that it would if the Portfolio was not hedged or remained more
fully invested in stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiver-
 
 10   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
sified. All of the Portfolios (except Aggressive Growth Portfolio) qualify as
diversified funds under the 1940 Act and are subject to the following
diversification requirements:
 
- - As a fundamental policy, no Portfolio may own more than 10% of the outstanding
  voting shares of any issuer.
 
- - As a fundamental policy, with respect to 50% of the total assets of Aggressive
  Growth Portfolio and 75% of the total assets of the other Portfolios, no
  Portfolio will purchase a security of any issuer (other than cash items and
  U.S. government securities, as defined in the 1940 Act) if such purchase would
  cause a Portfolio's holdings of that issuer to amount to more than 5% of that
  Portfolio's total assets.
 
- - No Portfolio will invest more than 25% of its total assets in a single issuer
  (other than U.S. government securities).
 
- - Aggressive Growth Portfolio reserves the right to become a diversified
  portfolio by limiting the investments in which more than 5% of its total net
  assets are invested.
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, each Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, no Portfolio will invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
Each Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may 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 initial investment
decision. Changes are made in a Portfolio whenever its portfolio manager
believes such changes are desirable. Portfolio turnover rates are generally not
a factor in making buy and sell decisions.
 
To a limited extent, a Portfolio may purchase securities in anticipation of
relatively short-term price gains. A Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
Each Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of a Portfolio's net
assets after the time of purchase, the Portfolio will take steps to reduce in an
orderly fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public because of
their terms or because of SEC regulations. Janus Capital will follow guidelines
established by the Trustees of the Trust ("Trustees") in making liquidity
determinations for Rule 144A securities and other securities, including
privately placed commercial paper and municipal lease obligations.
 
BORROWING AND LENDING
Each Portfolio may borrow money and lend securities or other assets, as follows:
 
- - Each Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - Each Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, each Portfolio may lend securities or other assets
  if, as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under an exemptive order received from the SEC, each Portfolio may borrow money
from or lend money to each other and other funds that permit such transactions
and for which Janus Capital serves as investment adviser. All such borrowing and
lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. A Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though 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 underdeveloped or developing
  countries which may have relatively unstable governments 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    11
<PAGE> 
 
  company or that the government may impose taxes or limits on the removal of a
  Portfolio's assets from that country. The Portfolios may invest in emerging
  market countries. Emerging market countries involve greater risks such as
  immature economic structures, national policies restricting investments by
  foreigners, and different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions. Foreign securities purchased indirectly
  (e.g., depositary receipts) are subject to many of the above risks, including
  currency risk, because their values depend on the performance of a foreign
  security denominated in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or potential development, or they may 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 be 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 wider
price fluctuations. Investments in such companies tend to be more volatile and
somewhat more speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
Each Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolios intend to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolios may also use derivative instruments for non-hedging
purposes such as seeking to increase a Portfolio's income or otherwise seeking
to enhance return. Please refer to Appendix A to this Prospectus and the SAI for
a more detailed discussion of these instruments.
 
The use of derivative instruments exposes the Portfolios to additional
investment risks and transaction costs. Risks inherent in the use of derivative
instruments include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that a portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than a Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave a
  Portfolio worse off than if it had not entered into the position.
 
Although the Portfolios' managers believe the use of derivative instruments will
benefit the Portfolios, a Portfolio's performance could be worse than if the
Portfolio had not used such instruments if the portfolio manager's judgement
proves incorrect.
 
When a Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and
 
 12   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
principal payments (i.e., credit risk) than is the case for higher quality
securities. Conversely, the value of higher quality securities may be more
sensitive to interest rate movements than lower rated securities. Issuers of
high-yield/high-risk securities may not be as strong financially as those
issuing bonds with higher credit ratings. Investments in such companies are
considered to be more speculative than higher quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, a Portfolio would
experience a reduction of its income and could expect a decline in the market
value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as higher
quality securities. Sovereign debt of foreign governments is generally rated by
country. Because these ratings do not take into account individual factors
relevant to each issue and may not be updated regularly, Janus Capital may treat
such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, a Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolios may have to sell securities
to have sufficient cash to pay the dividends.
 
SHORT SALES
Each Portfolio may engage in "short sales against the box." This technique
involves selling either a security that a Portfolio owns, or a security
equivalent in kind and amount that a Portfolio has the right to obtain, for
delivery at a specified date in the future. A Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
SPECIAL SITUATIONS
Each Portfolio may invest in "special situations" from time to time. 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.
Investment in special situations may carry an additional risk of loss in the
event that the anticipated development does not occur or does not attract the
expected attention.
 
See Appendix A for risks associated with certain other investments.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    13
<PAGE> 
 
                                 MANAGEMENT OF
                                 THE PORTFOLIOS
 
TRUSTEES
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to each Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolios to the
officers of the Trust and meet at least quarterly to review the Portfolios'
investment policies, performance, expenses and other business affairs.
 
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 has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
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. From
1989 to 1993, he was a Project Director for the National Security Archives, a
nonprofit research organization. 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. Mr. Craig previously managed Balanced Portfolio from September
1993 through April 1996 and has co-managed Janus Venture Fund since February 1,
1997. Mr. Craig previously managed Janus Venture Fund from its inception to
December 1993 and Janus Balanced Fund from December 1993 to December 1995. He
holds a Bachelor of Arts in Business from the University of Alabama and a Master
of Arts in Finance from the Wharton School of the University of Pennsylvania.
 
JAMES P. GOFF is Executive Vice President and portfolio manager of Aggressive
Growth Portfolio which he has managed since inception. Mr. Goff joined Janus
Capital in 1988 and has managed Janus Enterprise Fund since its inception. Mr.
Goff co-managed or managed Janus Venture Fund from December 1993 to February 1,
1997. He holds a Bachelor of Arts in Economics from Yale University and is a
Chartered Financial Analyst.
 
HELEN YOUNG HAYES is Executive Vice President and portfolio manager of Worldwide
Growth Portfolio and co-manager of International Growth Portfolio which she has
managed or co-managed since inception. Ms. Hayes joined Janus Capital in 1987
and has managed or co-managed Janus Worldwide Fund and Janus Overseas Fund since
their inceptions. She holds a Bachelor of Arts in Economics from Yale University
and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS is Executive Vice President and portfolio manager of Balanced
Portfolio, which he has managed since May 1996 and Equity Income Portfolio,
which he has managed since inception. He is 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 June 1996.
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 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
 
 14   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
Business at Duke University and a Bachelor's Degree in Economics and Political
Science from Tufts University. He is a Chartered Financial Analyst.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment activities
in a manner that Janus Capital believes is not detrimental to the Portfolios or
Janus Capital's other advisory clients. See the SAI for more detailed
information.
 
MANAGEMENT EXPENSES
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):
 
<TABLE>
<CAPTION>
                                AVERAGE DAILY
                                  NET ASSETS        ANNUAL RATE
       FEE SCHEDULE              OF PORTFOLIO      PERCENTAGE(%)
- ----------------------------------------------------------------
<S>                           <C>                  <C>
Growth,                       First $300 Million       0.75
Aggressive Growth,            Next $200 Million        0.70
International Growth,         Over $500 Million        0.65
Worldwide Growth and
Balanced Portfolios
- ----------------------------------------------------------------
</TABLE>
 
 Janus Capital has agreed to reduce each Portfolio's advisory fee to the extent
 that such fee exceeds the effective rate of the Janus retail fund
 corresponding to such Portfolio. Janus Capital may terminate this fee
 reduction or any of the expense limitations set forth above at any time upon
 at least 90 days' notice to the Trustees. The effective rate is the advisory
 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 .65%, .72%, .66%, .65% and .73%, respectively, for
 the quarter ended March 31, 1998.
 
Differences in the actual management fees incurred by the Portfolios are due
primarily to variances in the asset sizes of the corresponding retail funds. As
asset size increases, the annual rate of the management fee rate declines in
accordance with the above schedule. 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. The fee schedule for the Portfolios was
effective July 1, 1997.
 
During the most recent fiscal year the Portfolios paid the following management
fees expressed as a percentage of average net assets: Growth Portfolio .65%;
Aggressive Growth Portfolio .73%; International Growth Portfolio .67%; Worldwide
Growth Portfolio .66%; and Balanced Portfolio .76%.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of each Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for a Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for a Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolios' Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for a Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses serving to reduce those expenses. The SAI
further explains the selection of broker-dealers.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolios with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust consists of twelve separate series, five of which are offered by this
Prospectus.
 
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 contracts and
life insurance contracts as well as certain qualified retirement plans.
Retirement Shares of each Portfolio are offered by separate prospectus and are
available only to participant directed qualified plans using plan service
providers that are compensated for providing distribution and/or recordkeeping
and other administrative services provided to plan participants. Because the
expenses of each class may differ, the
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    15
<PAGE> 
 
performance of each class is expected to differ. If you would like additional
information about the Retirement Shares, please call 1-800-525-0020.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific class or Portfolio or for the Trust as a
whole for purposes such as electing or removing Trustees, terminating or
reorganizing the Trust, changing fundamental policies, or for any other purpose
requiring a shareholder vote under the 1940 Act. Separate votes are taken by
each class or Portfolio only if a matter affects or requires the vote of only
that class or Portfolio or the interest of a class or Portfolio in the matter
differs from the interest of the other class or Portfolios of the Trust. As a
shareholder, you are entitled to one vote for each share that you own.
 
An insurance company issuing a variable contract invested in Shares of a
Portfolio will request voting instructions from variable contract holders. Under
current law, the insurance company must vote all Shares held by the separate
account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Each Portfolio's Shares are available only to variable annuity and variable life
separate accounts of insurance companies that are unaffiliated with Janus
Capital and to certain qualified retirement plans. Retirement Shares of the
Portfolios (offered through a separate prospectus) are available to certain
participant directed qualified plans. Although the Portfolios do not currently
anticipate any disadvantages to policy owners arising out of the fact that 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 anticipated disadvantages or material irreconcilable conflicts 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 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 securities at disadvantageous prices. In addition, the Trustees
may refuse to sell shares of any Portfolio to any separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve any Portfolio's investment objective
by investing all of that Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to that Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolios with
service at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of a Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per Share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of Shares outstanding.
 
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.
 
 16   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<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. 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. EACH CLASS OF EACH PORTFOLIO MAKES SEMIANNUAL 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 BE AUTOMATICALLY
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. Distributed
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 in the Shares depends on the
features of the variable insurance contracts purchased from a participating
insurance company. Further information may be found 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 as a foreign tax credit.
If such an 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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    17
<PAGE> 
 
                              SHAREHOLDER'S GUIDE
 
INVESTORS MAY NOT PURCHASE OR REDEEM SHARES OF THE PORTFOLIOS DIRECTLY. SHARES
MAY BE PURCHASED OR REDEEMED ONLY THROUGH VARIABLE INSURANCE CONTRACTS OFFERED
BY THE SEPARATE ACCOUNTS OF PARTICIPATING INSURANCE COMPANIES OR THROUGH
QUALIFIED RETIREMENT PLANS. 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.
 
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.
 
All investments in the Portfolios are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by a Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by a Portfolio.
 
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. Any Portfolio may
discontinue sales of its shares if management believes that a substantial
further increase may adversely affect that Portfolio's ability to achieve its
investment objective. In such event, however, it is anticipated that existing
policy owners and plan participants invested in that Portfolio would be
permitted to continue to authorize investment in such Portfolio and to reinvest
any dividends or capital gains distributions.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolios that they have authorized for
investment. Each report will show the investments owned by each Portfolio and
the market values thereof, as well as other information about the Portfolios and
their operations. The Trust's fiscal year ends December 31.
 
 18   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                                   APPENDIX A
 
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 objective 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 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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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 to provide cash to satisfy unusually high redemption
requests or for other temporary or emergency purposes.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    19
<PAGE> 
 
RULE 144A SECURITIES are securities that are not registered for sale to the
general public under the Securities Act of 1933, but that may be resold to
certain institutional investors.
 
STANDBY COMMITMENTS are obligations purchased by a Portfolio from a dealer that
give the Portfolio the option to sell a security to the dealer at a specified
price.
 
STEP COUPON BONDS are debt securities that trade at a discount from their face
value and pay coupon interest. The discount from the face value depends on the
time remaining until cash payments begin, prevailing interest rates, liquidity
of the security and the perceived credit quality of the issuer.
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a 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
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 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 of comparable 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 non-dollar denominated securities or to
reduce the impact of currency appreciation on purchases of non-dollar
denominated 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
 
 20   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
instrument or index. For example, upon reset the interest rate payable on a
security may go down when the underlying index has risen. Certain inverse
floaters may have an interest rate reset mechanism that multiplies the effects
of change in the underlying index. Such mechanism may increase the volatility of
the security's market value.
 
OPTIONS are the right, but not the obligation, to buy or sell a specified amount
of securities or other assets on or before a fixed date at a predetermined
price. The Portfolios may purchase and write put and call options on securities,
securities indices and foreign currencies.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    21
<PAGE> 

                               JANUS ASPEN SERIES
                                   PROSPECTUS
                                Growth Portfolio
                          Aggressive Growth Portfolio
                         International Growth Portfolio
                           Worldwide Growth Portfolio
                               Balanced Portfolio


                                 [JANUS LOGO]



            P.O. Box 173375 o Denver, CO 80217-3375 o 1-800-525-1068
                                     (5/98)
<PAGE> 
 
                               JANUS ASPEN SERIES
                          GROWTH AND INCOME PORTFOLIO
 
                                   Prospectus
 
                                  MAY 1, 1998
 
Growth and Income Portfolio (the "Portfolio") is a no-load, diversified mutual
fund that seeks long-term growth of capital with a limited emphasis on income.
Although the Portfolio normally invests at least 25% of its assets in securities
that have income potential, it emphasizes equity securities selected for their
growth potential. The Portfolio is a series of Janus Aspen Series (the "Trust")
and currently offers two classes of Shares. The Institutional Shares are sold
under the name "Janus Aspen Series." The Trust is registered with the Securities
and Exchange Commission as an open-end management investment company. The
Portfolio is recently organized and has a limited operating history.
 
The Institutional Shares (the "Shares") of the Portfolio offered by this
Prospectus are issued and redeemed only in connection with investment in and
payments under variable annuity contracts and variable life insurance contracts
(collectively, "variable insurance contracts"), as well as certain qualified
retirement plans.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Each variable insurance contract
involves fees and expenses not described in this Prospectus. The Portfolio may
not be available in connection with a particular contract. See the accompanying
contract prospectus for information regarding contract fees and expenses and any
restrictions on purchases or allocations.
 
This Prospectus contains information about the Portfolio that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolio. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Portfolio is contained in the
Statement of Additional Information ("SAI") dated May 1, 1998, which is filed
with the Securities and Exchange Commission ("SEC") and is incorporated by
reference into this Prospectus. The SAI is available upon request and without
charge by writing or calling your insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO         MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                            <C>
THE PORTFOLIO AT A GLANCE
Brief description of the Portfolio...........    2
 
EXPENSE INFORMATION..........................    3
THE PORTFOLIO IN DETAIL
The Portfolio's Investment Objective and
   Policies..................................    5
General Portfolio Policies...................    7
Additional Risk Factors......................    9
MANAGEMENT OF THE PORTFOLIO
Investment Adviser and Portfolio Manager.....   13
Personal Investing...........................   14
Portfolio Transactions.......................   14
Management Expenses and Expense Limits.......   14
Other Service Providers......................   15
Other Information............................   15
DISTRIBUTIONS AND TAXES
Distributions................................   18
Taxes........................................   18
PERFORMANCE TERMS
An Explanation of Performance Terms..........   20
SHAREHOLDER'S GUIDE
Purchases....................................   21
Redemptions..................................   21
Shareholder Communications...................   21
APPENDIX A
Glossary of Investment Terms.................   22
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIO AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolio
and its investment emphasis. A more detailed discussion of the Portfolio's
investment objective and policies begins on page 5.
 
INVESTMENT OBJECTIVE
The investment objective of the Portfolio is long-term capital growth and
current income.
 
PRIMARY HOLDINGS
A diversified portfolio that emphasizes equity securities selected for their
growth potential, although the Portfolio will normally invest at least 25% of
its assets in securities that the portfolio manager believes have income
potential.
 
SHAREHOLDER'S INVESTMENT HORIZON
The Portfolio is designed for long-term investors who seek growth of capital
with a limited emphasis on income. The Portfolio is not designed for investors
who desire a consistent level of income nor is it a short-term trading vehicle
and should not be relied upon for short-term financial needs.
 
PORTFOLIO ADVISER
Janus Capital Corporation ("Janus Capital") serves as the Portfolio's investment
adviser. Janus Capital has been in the investment advisory business for over 27
years and currently manages approximately $80 billion in assets.
 
PORTFOLIO MANAGER
David J. Corkins
 
PORTFOLIO INCEPTION
May 1998
 
 2   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Portfolio in understanding the various costs
and expenses that you will bear directly or indirectly as an investor in the
Portfolio. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN THE PORTFOLIO
SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A DESCRIPTION OF
COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT DEDUCTIONS AT THE
SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT MAY BE INCURRED
UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES
 
<TABLE>
         <S>                                              <C>
         Maximum sales load imposed on purchases          None
         Maximum sales load imposed on reinvested
           dividends                                      None
         Deferred sales charges on redemptions            None
         Redemption fees                                  None
         Exchange fee                                     None
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998    3
<PAGE> 
 
ANNUAL PORTFOLIO OPERATING EXPENSES
(after fee waivers and reductions)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<S>                                                           <C>   
 
 ----------------------------------------------------------------------
Management Fee(1)                                             0.67%
Other Expenses(1)                                             0.30%
 ----------------------------------------------------------------------
Total Portfolio Operating Expenses(1)                         0.97%
 ----------------------------------------------------------------------
</TABLE>
 
(1) The fees and expenses in the table above are based on the estimated gross
    expenses before estimated expense offset arrangements that the Shares of the
    Portfolio expect to incur in their initial fiscal year, net of fee
    reductions or waivers from Janus Capital. Fee reductions 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. Without such waivers or reductions, the
    Management Fee, Other Expenses and Total Portfolio Operating Expenses are
    estimated to be 0.75%, 0.30% and 1.05%, respectively. Janus Capital may
    modify or terminate the waivers or reductions at any time upon at least 90
    days' notice to the Trustees.
 
EXAMPLE
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------
                                                             1 YEAR   3 YEARS
- ---------------------------------------------------------------------------------
<S>                                                          <C>      <C>     
Assume you invest $1,000, the Shares of the Portfolio
returns 5% annually and its expense ratio remains as listed
above. The example shows the operating expenses that you
would indirectly bear as an investor in the Shares of the
Portfolio.                                                    $10       $31
 --------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
                              FINANCIAL HIGHLIGHTS
 
No Financial Highlights are presented for the Shares because the Shares did not
commence operations before May 1, 1998.
 
 4   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998
<PAGE> 
 
                            THE PORTFOLIO IN DETAIL
 
This section takes a closer look at the Portfolio's investment objectives,
policies and the securities in which it invests. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques and refer
to Appendix A for a more detailed description of the Portfolio's investments
(and certain of the risks associated with those investments). You should
carefully consider your own investment goals, time horizon and risk tolerance
before investing in the Portfolio.
 
The Portfolio's investment objectives and policies are similar to those of Janus
Growth and Income Fund, a Janus retail fund. Although it is anticipated that the
Portfolio and its corresponding retail fund will hold similar securities,
differences in asset size and cash flow needs as well as the relative weightings
of securities selections may result in differences in investment performance.
Expenses of the Portfolio and its corresponding retail fund are expected to
differ.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including the Portfolio's investment objectives, are
not fundamental and may be changed by the Portfolio's Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in the Portfolio's objectives or policies, you should
consider whether the Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
 
INVESTMENT OBJECTIVE
The investment objective of the Portfolio is long-term capital growth and
current income. It is a diversified portfolio that, under normal circumstances,
pursues its objective by investing up to 75% of its assets in equity securities
selected primarily for their growth potential and at least 25% of its assets in
securities that the portfolio manager believes have income potential. The
Portfolio normally emphasizes the growth component. However, in unusual
circumstances (such as those described under "Cash Position" on page 7), the
Portfolio may reduce the growth component of its portfolio to 25% of its assets.
 
TYPES OF INVESTMENTS
The Portfolio invests primarily in common stocks of domestic and foreign
companies. The Portfolio may invest to a lesser degree in other types of
securities including preferred stock, warrants, convertible securities and debt
securities when its portfolio manager perceives an opportunity for capital
growth from such securities or to receive a return on idle cash. The Portfolio
may purchase securities on a when-issued, delayed delivery or forward commitment
basis. The Portfolio may invest up to 25% of its assets in mortgage- and
asset-backed securities, up to 10% of its assets in zero coupon, pay-in-kind and
step coupon securities, and without limit in indexed/structured securities. The
Portfolio will invest less than 35% of its assets in high-yield/high-risk
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
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998    5
<PAGE> 
 
include depositary receipts or shares, and passive foreign investment companies.
The Portfolio may use futures, options and other derivatives for hedging
purposes or for non-hedging purposes such as seeking to enhance return. See
"Additional Risk Factors" on page 9 for a discussion of the risks associated
with foreign investing and derivatives.
 
See Appendix A for a further description of the Portfolio's investments.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE PORTFOLIO.
 
Q:   HOW ARE EQUITY SECURITIES SELECTED?

A:   The growth component of the Portfolio is expected to consist primarily of
     common stocks. The selection criteria for common stocks are described
     below. Because income is a part of the investment objective of the
Portfolio, the portfolio manager may consider dividend-paying characteristics in
selecting equity securities for this Portfolio. The Portfolio may also find
opportunities for capital growth from debt securities because of anticipated
changes in interest rates, credit standing, currency relationships or other
factors.
 
The portfolio manager will invest in common stocks to the extent he believes
that the relevant market environment favors profitable investing in those
securities. The portfolio manager generally takes a "bottom up" approach to
building the portfolio. In other words, he seeks to identify individual
companies with earnings growth potential that may not be recognized by the
market at large. Although themes may emerge in the Portfolio, securities are
generally selected without regard to any defined industry sector or other
similarly defined selection procedure.
 
Q:   HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENT OF THE
     PORTFOLIO?                                                             

A:   The Portfolio may invest in a combination of common stocks, preferred
     stocks, convertible securities, debt securities and other fixed-income
     securities. The Portfolio may shift assets between the growth and income
components of its portfolio 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.

Q:   WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF THE PORTFOLIO?

A:   The income component of the 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. Investors in the Portfolio should keep in
mind that the Portfolio is not designed to produce a consistent level of income.
 
 6   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998
<PAGE> 
 
Q:   ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?

A:   Generally, yes. The portfolio manager seeks companies that meet his
     selection criteria, regardless of country of organization or place of
     principal business activity. 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. See "Additional Risk Factors" on page 9.
 
Q:   HOW DOES THE PORTFOLIO TRY TO REDUCE RISK?

A:   Diversification of the Portfolio's assets reduces the effect of any single
     holding on its overall portfolio value. The Portfolio may use futures,
     options and other derivative instruments to protect the portfolio from
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. See "Additional Risk Factors" on page 9. In addition,
to the extent that the Portfolio holds a larger cash position, it might not
participate in market declines to the same extent as if it had remained more
fully invested in common stocks.
 
GENERAL PORTFOLIO POLICIES
The Portfolio will follow the general policies listed below in investing its
portfolio assets. The percentage limitations included in these policies and
elsewhere in this Prospectus apply at the time of purchase of the security. 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's manager believes that market conditions are not favorable
for profitable investing or when the portfolio manager is otherwise unable to
locate favorable investment opportunities, the Portfolio's investments may be
hedged to a greater degree and/or its 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 are a residual - they represent the assets
that remain after a portfolio manager has committed available assets to
desirable investment opportunities. Larger hedged positions and/or larger cash
positions may serve as a means of preserving capital in unfavorable market
conditions.
 
Securities that the Portfolio may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolio may
also invest in money market funds (including funds managed by Janus Capital).
When a Portfolio is hedged or its investments in cash or similar investments
increase, it may not participate in stock or bond market advances or declines to
the same extent that it would if the Portfolio was not hedged or remained more
fully invested in stocks or bonds.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998    7
<PAGE> 
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. The Portfolio qualifies as a
diversified fund under the 1940 Act and is subject to the following
requirements:
 
- - As a fundamental policy, the Portfolio may not own more than 10% of the
  outstanding voting shares of any issuer.
 
- - As a fundamental policy, with respect to 75% of its total assets, the
  Portfolio will not purchase a security of any issuer (other than cash items
  and U.S. government securities, as defined in the 1940 Act) if such purchase
  would cause the Portfolio's holdings of that issuer to amount to more than 5%
  of the Portfolio's total assets.
 
- - The Portfolio will invest no more than 25% of its total assets in a single
  issuer (other than U.S. government securities).
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, the Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, the Portfolio will not invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
The Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may result from
liquidity needs, securities having reached a price or yield objective,
anticipated 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. Changes are made in the Portfolio whenever its portfolio
manager believes such changes are desirable. The portfolio turnover rate is
generally not a factor in making buy and sell decisions. The Portfolio's
turnover rate is not expected to exceed 200%.
 
To a limited extent, the Portfolio may purchase securities in anticipation of
relatively short-term price gains. 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. 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.
 
ILLIQUID INVESTMENTS
The Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of net assets after
the time of purchase, the Portfolio will take steps to reduce in an orderly
fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public
 
 8   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998
<PAGE> 
 
because of their terms or because of SEC regulations. Janus Capital will follow
guidelines established by the Trustees of the Trust ("Trustees") in making
liquidity determinations for Rule 144A securities and certain other securities,
including privately placed commercial paper.
 
BORROWING AND LENDING
The Portfolio may borrow money and lend securities or other assets, as follows:
 
- - The Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - The Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, the Portfolio may lend securities or other assets if,
  as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, the Portfolio may
borrow money from or lend money to other funds that permit such transactions and
for which Janus Capital serves as investment adviser. All such borrowing and
lending will be subject to the above limits.
 
ADDITIONAL RISK FACTORS
SPECIAL SITUATIONS
The Portfolio may invest in "special situations" from time to time. A special
situation arises when, in the opinion of the Portfolio's portfolio 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.
Investment in special situations may carry an additional risk of loss in the
event that the anticipated development does not occur or does not attract the
expected attention.
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but most foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. The Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though the security increases in value in its home
  country. U.S. dollar denominated securities of foreign issuers may also be
  affected by currency risk.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO    MAY 1, 1998    9
<PAGE> 
 
- - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject to heightened
  political and economic risks, particularly in underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolio may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk, because their values depend
on the performance of a foreign security denominated in its home currency.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
The Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively, "derivative instruments"). The Portfolio intends to use most
derivative instruments primarily to hedge the value of its portfolio holdings
against potential adverse movements in securities prices, foreign currency
markets or interest rates. To a limited extent, the Portfolio may also use
derivative instruments for non-hedging purposes such as seeking to increase the
Portfolio's income or otherwise seeking to enhance return. Please refer to
Appendix A to this Prospectus and the SAI for a more detailed discussion of
these instruments.
 
The use of derivative instruments exposes the Portfolio to additional investment
risks and transaction costs. Risks inherent in the use of derivative instruments
include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the directions that the portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
 10   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than the Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave the
  Portfolio worse off than if it had not entered into the position.
 
Although the portfolio manager believes the use of derivative instruments will
benefit the Portfolio, the Portfolio's performance could be worse than if it had
not used such instruments if the portfolio manager's judgment proves incorrect.
 
When the Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT SECURITIES RATED
BELOW INVESTMENT GRADE BY THE PRIMARY RATING AGENCIES (SUCH AS, STANDARD &
POOR'S RATINGS SERVICES AND MOODY'S INVESTORS SERVICE, INC.).
 
The value of lower quality securities generally is more dependent on the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for higher quality securities. Conversely, the value of higher
quality securities may be more sensitive to interest rate movements than lower
quality securities. Issuers of high-yield securities may not be as strong
financially as those issuing bonds with higher credit ratings. Investments in
such companies are considered to be more speculative than higher quality
investments.
 
Issuers of high-yield securities may be more vulnerable to real or perceived
economic changes (for instance, an economic downturn or prolonged period of
rising interest rates), political changes or adverse developments specific to
the issuer. Adverse economic, political or other developments may impair the
issuer's ability to service principal and interest obligations, to meet
projected business goals and to obtain additional financing, particularly if the
issuer is highly leveraged. In the event of a default, the Portfolio would
experience a reduction of its income and could expect a decline in the market
value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    11
<PAGE> 
 
laws may also have a greater negative impact on the market for lower quality
securities. Unrated debt, while not necessarily of lower quality than rated
securities, may not have as broad a market as rated securities. Sovereign debt
of foreign governments is generally rated by country. Because these ratings do
not take into account individual factors relevant to each issue and may not be
updated regularly, Janus Capital may treat such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, the Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolio may have to sell securities
to have sufficient cash to pay the dividends.
 
Please refer to the SAI for a description of bond rating categories.
 
SHORT SALES
The Portfolio may engage in "short sales against the box." This technique
involves selling either a security that the Portfolio owns, or a security
equivalent in kind and amount to the security sold short that the Portfolio has
the right to obtain, for delivery at a specified date in the future. The
Portfolio may enter into a short sale against the box to hedge against
anticipated declines in the market price of portfolio securities. If the value
of the securities sold short increases prior to the scheduled delivery date, the
Portfolio loses the opportunity to participate in the gain.
 
See Appendix A for risks associated with certain other investments.
 
 12   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
                          MANAGEMENT OF THE PORTFOLIO
 
TRUSTEES
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to the Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolio to the officers
of the Trust and meet at least quarterly to review the Portfolio's investment
policies, performance, expenses and other business affairs.
 
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 its investment portfolio and other business affairs.
 
Janus Capital has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
The Portfolio pays all of its expenses not assumed by Janus Capital, including
transfer agent and custodian fees and expenses, legal and auditing fees,
registration fees and expenses, and independent Trustees' fees and expenses and
certain other expenses. 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.
 
PORTFOLIO MANAGER
DAVID J. CORKINS is Executive Vice President and portfolio manager of the
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
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    13
<PAGE> 
 
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.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment activities
in a manner that Janus Capital believes is not detrimental to the Portfolio or
Janus Capital's other advisory clients. See the SAI for more detailed
information.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of the Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for the Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolio or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for the Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolio's Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for the Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses. The SAI further explains the selection of
broker-dealers.
 
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):
 
<TABLE>
<CAPTION>
                                                AVERAGE DAILY NET         ANNUAL RATE
                  FEE SCHEDULE                 ASSETS OF PORTFOLIO       PERCENTAGE(%)
- --------------------------------------------------------------------------------
    <S>                                        <C>                       <C>
                                               First $300 Million        0.75*
                                               Next $200 Million         0.70
                                               Next $200 Million         0.65
    -----------------------------------------------------------------------------------
</TABLE>
 
 * Janus Capital has agreed to reduce the Portfolio's advisory fee to the
   extent that such fee exceeds the effective rate of Janus Growth and Income
   Fund, the Janus retail fund corresponding to the Portfolio. Janus Capital
   may terminate this fee reduction at any time upon at least 90 days' notice
   to the Trustees. The effective rate is the advisory 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 .67% for the quarter ended March 31, 1998. In addition, Janus
   Capital has agreed to limit the expenses of the Portfolio's Shares to an
   annual rate of 1.25% of average net assets through at least May 1, 1999.
- --------------------------------------------------------------------------------
 
 14   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
As asset size increases, the annual rate of the management fee declines in
accordance with the above schedule. 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.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolio with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is an open-end management investment company organized as a Delaware
business trust on May 20, 1993. The Portfolio has been established as a separate
series of the Trust.
 
The Portfolio currently offers two classes of shares, one of which, the
Institutional Shares, are offered pursuant to this prospectus. The Institutional
Shares of the Portfolio, as well as other Janus Aspen Series - Institutional
Shares 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 are offered by a separate prospectus and are available
only to participant directed qualified plans using plan service providers that
are compensated for providing distribution and/or recordkeeping and other
administrative services to plan participants. Because the expenses of each class
may differ, the performance in each class is expected to differ. If you would
like additional information about the Retirement Shares, please call
1-800-525-0020.
 
SHAREHOLDER MEETINGS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific Portfolio or for the Trust as a whole for
purposes such as electing or removing Trustees, terminating or reorganizing the
Trust, changing fundamental policies, or for any other purpose requiring a
shareholder vote under the 1940 Act. Separate votes are taken by each class or
Portfolio only if a matter affects or requires the vote of only that class or
Portfolio or the interest of the class or Portfolio in the matter differs from
the interest of the other class or Portfolios of the Trust. As a shareholder,
you are entitled to one vote for each share that you own.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    15
<PAGE> 
 
An insurance company issuing a variable contract invested in shares of the
Portfolio will request voting instructions from variable contract holders. Under
current law, the insurance company must vote all shares held by the separate
account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Portfolio shares 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. Although the Portfolio does
not currently anticipate any disadvantages to policy owners arising out of the
fact that the Portfolio offers its shares to such entities, there is a
possibility that disadvantages could occur or that a material conflict may
arise. The Trustees monitor events in order to identify any anticipated
disadvantages or material irreconcilable conflicts that may arise 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 plans to withdraw its investments in the Portfolio
or substitute shares of another Portfolio. As a result, the Portfolio may be
forced to sell securities at disadvantageous prices. In addition, the Trustees
may refuse to sell shares of the Portfolio to any separate account or may
suspend or terminate the offering of shares of the Portfolio 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve the Portfolio's investment objective
by investing all of the Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to the Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolio with service
at current levels.
 
THE VALUATION OF SHARES
The net asset value ("NAV") of the Shares of the Portfolio is determined at the
close of the regular trading session of the New York Stock Exchange (the "NYSE")
(normally 4:00 p.m., New York time) each day that the NYSE is open. NAV per
Share is determined by dividing the total value of the securities and other
assets, less liabilities, by the total number of Shares outstanding.
 
 16   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
Securities are valued at market value or, if market information 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.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    17
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. EACH CLASS OF THE PORTFOLIO MAKES SEMIANNUAL 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 THE SHARES OF THE PORTFOLIO WILL BE
AUTOMATICALLY REINVESTED INTO ADDITIONAL SHARES OF THE PORTFOLIO.
 
HOW DISTRIBUTIONS AFFECT THE PORTFOLIO'S 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 drops by the amount of the distribution, net
of any subsequent market fluctuations. For example, assume that on December 31,
the Shares of the Portfolio declared a dividend in the amount of $0.25 per
share. If the price of the 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 in the Shares depends on the features of
the variable insurance contracts purchased from a participating insurance
company. Further information may be found 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 as a foreign tax credit.
If such an election is not made, any foreign taxes paid or accrued will
represent an expense to the Portfolio which will reduce its investment income.
 
 18   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
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.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    19
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe the Portfolio's performance. You may see references to these terms in
our newsletters, advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of the Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. The Portfolio
generally measures performance in terms of total return.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. Cumulative total return is generally quoted for more
than one year (e.g., the life of the Portfolio). A cumulative total return does
not show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in the Portfolio's return and are
not the same as actual annual results.
 
THE PORTFOLIO IMPOSES NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
COMPUTATIONS. TOTAL RETURN FIGURES OF THE PORTFOLIO INCLUDE THE EFFECT OF
DEDUCTING THE PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES
ATTRIBUTABLE TO ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES
ARE BASED UPON HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE
PERFORMANCE. INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT
SHARES, WHEN REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
 20   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<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 A VARIABLE INSURANCE CONTRACT AND ON HOW TO SELECT THE
PORTFOLIO AS AN INVESTMENT OPTION FOR A CONTRACT OR A QUALIFIED PLAN.
 
PURCHASES
Purchases of Portfolio 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's separate account or to your plan documents for information on how to
invest in the Shares of the Portfolio.
 
All investments in the Portfolio are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by the Portfolio. Investments will be processed at the NAV next
calculated after an order is received and accepted by the Portfolio.
 
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 the size that
would disrupt the management of the Portfolio. The Portfolio may discontinue
sales of its shares if management believes that a substantial further increase
may adversely affect the Portfolio's ability to achieve its investment
objective. In such event, however, it is anticipated 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 distribution.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolio. Each report will show the investments
owned by the Portfolio and market values thereof, as well as other information
about the Portfolio and its operations. The Trust's fiscal year ends December
31.
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    21
<PAGE> 
 
                                   APPENDIX A
 
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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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 (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
 
 22   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
through to shareholders on a pro rata basis. These securities involve prepayment
risk, which is the risk that the underlying mortgages or other debt may be
refinanced or paid off prior to their maturities during periods of declining
interest rates. In that case, the portfolio manager may have to reinvest the
proceeds from the securities at a lower rate. Potential market gains on a
security subject to prepayment risk may be more limited than potential market
gains on a comparable security that is not subject to prepayment risk.
 
PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign corporations which
generate certain amounts of passive income or hold certain amounts of assets for
the production of passive income. Passive income includes dividends, interest,
royalties, rents and annuities. To avoid taxes and interest that the Portfolio
must pay if these investments are profitable, the Portfolio may make various
elections permitted by the tax laws. These elections could require that the
Portfolio recognize taxable income, which 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    23
<PAGE> 
 
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.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a specified formula, usually with reference to some interest rate
index or market 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 bears the risk of market value fluctuations
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 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 of comparable maturity.
 
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. The Portfolio may also
enter into forward contracts to purchase or sell securities or other financial
indices.
 24   JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998
<PAGE> 
 
FUTURES CONTRACTS are contracts that obligate the buyer to receive and the
seller to deliver an instrument or money at a specified price on a specified
date. The Portfolio may buy and sell futures contracts on foreign currencies,
securities and financial indices including interest rates or an index of U.S.
government, foreign government, equity or fixed-income securities. The Portfolio
may also buy options on futures contracts. An option on a futures contract gives
the buyer the right, but not the obligation, to buy or sell a futures contract
at a specified price on or before a specified date. Futures contracts and
options on futures are standardized and traded on designated exchanges.
 
INDEXED/STRUCTURED SECURITIES are typically short- to intermediate-term debt
securities whose value at maturity or interest rate is linked to currencies,
interest rates, equity securities, indices, commodity prices or other financial
indicators. Such securities may be positively or negatively indexed (i.e., their
value may increase or decrease if the reference index or instrument
appreciates). Indexed/structured securities may have return characteristics
similar to direct investments in the underlying instruments and may be more
volatile than the underlying instruments. The Portfolio bears the market risk of
an investment in the underlying instruments, as well as the credit risk of the
issuer.
 
INTEREST RATE SWAPS involve the exchange by two parties of their respective
commitments to pay or receive interest (e.g., an exchange of floating rate
payments for fixed rate payments).
 
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.
 
JANUS ASPEN SERIES PROSPECTUS -- GROWTH AND INCOME PORTFOLIO   MAY 1, 1998    25
<PAGE> 
 
                      This page intentionally left blank.
 
<PAGE>
  
                               JANUS ASPEN SERIES
                                   PROSPECTUS
                          Growth and Income Portfolio




                                 [JANUS LOGO]



            P.O. Box 173375 o Denver, CO 80217-3375 o 1-800-525-3713
                                    (5/98)
<PAGE> 
 
                               JANUS ASPEN SERIES
                           SHORT-TERM BOND PORTFOLIO
 
                                   Prospectus
 
                                  MAY 1, 1998
 
Short-Term Bond Portfolio (the "Portfolio") is a diversified mutual fund that
seeks a high level of current income while minimizing interest rate risk by
investing in shorter term fixed-income securities. Its average weighted maturity
is normally less than three years. The Portfolio pursues its objective by
investing primarily in short- and intermediate-term fixed-income securities.
 
The Portfolio is closed to new investors effective May 1, 1998. Janus Capital
Corporation ("Janus Capital") anticipates liquidation of the Portfolio soon
after the required regulatory approvals are obtained. In anticipation of the
proposed liquidation of the Portfolio, the Portfolio may begin to shorten its
dollar-weighted average portfolio effective maturity and to invest in
lower-yielding more liquid securities and cash to facilitate an orderly
liquidation of the Portfolio's assets.
 
The Portfolio is a series of Janus Aspen Series (the "Trust"). The Institutional
Shares are sold under the name "Janus Aspen Series." The Trust is registered
with the Securities and Exchange Commission ("SEC") as an open-end management
investment company. The Institutional Shares of the Portfolio (the "Shares") are
offered by this prospectus in connection with investment in and payments under
variable annuity contracts and variable life insurance contracts (collectively
"variable insurance contracts"), as well as certain qualified retirement plans.
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. 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.
 
This Prospectus contains information about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolio. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS              MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                            <C>
THE PORTFOLIO AT A GLANCE
Brief description of the Portfolio...........    2
EXPENSE INFORMATION
The Portfolio's annual operating expenses....    4
Financial Highlights - a summary of financial
  data.......................................    5
PERFORMANCE TERMS
An explanation of performance terms..........    7
THE PORTFOLIO IN DETAIL
Investment Objective and Policies............    8
General Portfolio Policies...................   10
Additional Risk Factors......................   12
MANAGEMENT OF THE PORTFOLIO
Investment Adviser and Investment
   Personnel.................................   17
Personal Investing...........................   18
Management Expenses and Expense Limits.......   18
Portfolio Transactions.......................   18
Other Service Providers......................   19
Other Information............................   19
DISTRIBUTIONS AND TAXES
Distributions................................   21
Taxes........................................   21
SHAREHOLDER'S GUIDE
Purchases....................................   23
Redemptions..................................   23
Shareholder Communications...................   23
APPENDIX A
Glossary of Investment Terms.................   24
APPENDIX B
Explanation of Ratings Categories............   28
</TABLE>
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIO AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolio
and its investment emphasis. A more detailed discussion of the Portfolio's
investment objective and policies begins on page 8.
 
INVESTMENT OBJECTIVE:
The investment objective of the Portfolio is to seek a high a level of current
income as is consistent with the preservation of capital.
 
PRIMARY HOLDINGS:
A diversified portfolio that pursues its investment objective by investing
primarily in short- and intermediate-term fixed income securities. Under normal
circumstances, it is expected that the Portfolio's dollar-weighted average
portfolio effective maturity will not exceed three years.
 
SHAREHOLDER INVESTMENT HORIZON:
The Portfolio is designed for those investors who primarily seek current income.
 
FUND ADVISER:
Janus Capital Corporation ("Janus Capital") serves as the Portfolio's investment
adviser. Janus Capital has been in the investment advisory business for over 27
years and currently manages approximately $80 billion in assets.
 
PORTFOLIO MANAGER:
Sandy R. Rufenacht
 
PORTFOLIO INCEPTION:
September 1993
 
 2   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolio in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES
 
<TABLE>
         <S>                                              <C>
         Maximum sales load imposed on purchases          None
         Maximum sales load imposed on reinvested
           dividends                                      None
         Deferred sales charges on redemptions            None
         Redemption fee                                   None
         Exchange fee                                     None
</TABLE>
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998    3
<PAGE> 
 
ANNUAL OPERATING EXPENSES
(after fee waivers)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<S>                                                           <C>   
- -----------------------------------------------------------------------
MANAGEMENT FEE                                                0.39%
OTHER EXPENSES                                                0.26%
- -----------------------------------------------------------------------
TOTAL OPERATING EXPENSES                                      0.65%
- -----------------------------------------------------------------------
</TABLE>
 
(1) The fees and expenses in the table above are based on gross expenses of the
    Shares before expense offset arrangements for the fiscal year ended December
    31, 1997. The information is net of fee waivers from Janus Capital. Without
    such waivers, the Management Fee, Other Expenses and Total Operating
    Expenses for the Shares would have been 0.65%, 0.26%, and 0.91%,
    respectively. Janus Capital may modify or terminate the waivers at any time
    upon at least 90 days' notice to the Trustees.
 
EXAMPLE
 
<TABLE>
<CAPTION>
- --------------------------------------------------------------------------------------
                                             1 YEAR   3 YEARS   5 YEARS   10 YEARS
- --------------------------------------------------------------------------------------
<S>                                          <C>      <C>       <C>       <C>      
Assume you invest $1,000, the Shares of the
Portfolio return 5% annually and the
expense ratio remains as listed above. The
example shows the operating expenses that
you would indirectly bear as an investor in
the Shares of the Portfolio.                   $7       $21       $36       $81
- --------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
 4   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise noted, the information below is for fiscal periods ending on
December 31 of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolio's financial statements since its inception. Their report
is included in the Portfolio's Annual Report, which is incorporated by reference
into the SAI. A DETAILED EXPLANATION OF THE FINANCIAL HIGHLIGHTS CAN BE FOUND ON
PAGE 6.
 
<TABLE>
<CAPTION>
                                                               SHORT-TERM BOND PORTFOLIO
              INSTITUTIONAL SHARES                  1997       1996       1995      1994     1993(1)
- --------------------------------------------------------------------------------------------------------
<C>  <S>                                           <C>        <C>        <C>       <C>       <C>    
 1.  NET ASSET VALUE, BEGINNING OF PERIOD            $9.97     $10.03     $9.72     $9.93    $10.00
- --------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                            1.85        .42       .60       .35       .11
 3.  Net gains or (losses) on securities (both
     realized and unrealized)                       (1.19)      (.03)       .31     (.26)     (.08)
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 4.  Total from investment operations                  .66        .39       .91       .09       .03
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     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)         (1.75)      (.44)     (.60)     (.30)     (.10)
 6.  Tax return of capital distributions                --         --        --        --        --
 7.  Distributions (from capital gains)                 --      (.01)        --        --        --
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 8.  Total distributions                            (1.75)      (.45)     (.60)     (.30)     (.10)
- --------------------------------------------------------------------------------------------------------
 9.  NET ASSET VALUE, END OF PERIOD                  $8.88      $9.97    $10.03     $9.72     $9.93
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10.  Total return*                                   6.72%      3.98%     9.54%     0.92%     0.30%
- --------------------------------------------------------------------------------------------------------
11.  Net assets, end of period (in thousands)       $2,735    $11,901    $3,187    $2,902      $502
12.  Average net assets for the period (in
     thousands)                                    $12,407     $7,168    $2,727    $1,774      $492
13.  Ratio of gross expenses to average net
     assets**                                        0.65%(6)   0.66%(5)  0.70%(4)    N/A       N/A
14.  Ratio of net expenses to average net
     assets**                                        0.65%      0.65%     0.65%     0.65%(3)  0.65%(2)
15.  Ratio of net investment income to average
     net assets**                                    5.83%      5.44%     6.02%     5.00%     3.57%
16.  Portfolio turnover rate**                        204%       416%      417%      256%       91%
17.  Average commission rate                           N/A        N/A       N/A       N/A       N/A
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</TABLE>
 
 * Total return not annualized for periods of less than one year.
**( )Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) The ratio was 5.33% before waiver of certain fees incurred by the Portfolio.
(3) The ratio was 1.40% before waiver of certain fees incurred by the Portfolio.
(4) The ratio was 1.37% before waiver of certain fees incurred by the Portfolio.
(5) The ratio was 0.84% before waiver of certain fees incurred by the Portfolio.
(6) The ratio was 0.91% before waiver of certain fees incurred by the Portfolio.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998    5
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights table. The table contains important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolio's
Annual Report contains additional information about the Portfolio's performance,
including a comparison to an appropriate securities index. To request a copy of
the Annual Report, please call or write your insurance company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of the Portfolio. It is
computed by adding the value of all of the Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between line 1 and line 9 in the Financial
Highlights table represents the change in value of a Share of the Portfolio over
the fiscal period, but not its total return.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by the Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that the Portfolio paid
from net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities the Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that the Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of the Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolio was not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in the Portfolio's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is the Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of the Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of the Portfolio's securities.
 
AVERAGE COMMISSION RATE is the total of the Portfolio's agency commission paid
on equity securities trades divided by the number of shares purchased and sold.
 
 6   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe the Portfolio's performance. You may see references to these terms in
our newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of the Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. The Portfolio
generally measures performance in terms of yield.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights table shows total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of the Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in the Portfolio's return and are
not the same as actual annual results.
 
YIELD shows the rate of income the Shares earn on investments as a percentage of
the Share price. It is calculated by dividing the Portfolio's net investment
income for a 30-day period by the average number of shares entitled to receive
dividends and dividing the result by the Share's NAV per share at the end of the
30-day period. Yield does not include changes in NAV.
 
Yields are calculated according to standardized SEC formulas and may not equal
the income on an investor's account. Yield is usually quoted on an annualized
basis. An annualized yield represents the amount you would earn if you remained
in the Portfolio for a year and the Shares of the Portfolio continued to have
the same yield for the entire year.
 
THE PORTFOLIO IMPOSES NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
COMPUTATIONS. YIELD AND TOTAL RETURN FIGURES OF THE PORTFOLIO INCLUDE THE EFFECT
OF DEDUCTING THE PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES
ATTRIBUTABLE TO ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES
ARE BASED UPON HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE
PERFORMANCE. INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT
SHARES, WHEN REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998    7
<PAGE> 
 
                            THE PORTFOLIO IN DETAIL
 
This section takes a closer look at the Portfolio's investment objective,
policies and the securities in which it invests. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed description of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before investing in the Portfolio.
 
The Portfolio's investment objective and policies are similar to those of Janus
Short-Term Bond Fund, a retail fund managed by Janus Capital. Although it is
anticipated that the Portfolio and Janus Short-Term Bond Fund will hold similar
securities, differences in asset size and cash flow needs as well as the
relative weightings of securities selections may result in differences in
investment performance. Expenses of the Portfolio and Janus Short-Term Bond 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 contract fees and
expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including the Portfolio's investment objective, are
not fundamental and may be changed by the Portfolio's Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in the Portfolio's objective or policies, you should
consider whether the Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
 
THE PORTFOLIO IS DESIGNED FOR THOSE INVESTORS WHO PRIMARILY SEEK CURRENT INCOME.
 
SHORT-TERM BOND PORTFOLIO
Investment Objective:.....................................................Income
Primary Holdings:....................................Income-Producing Securities
Shareholder's Investment Horizon:....................Short- to Intermediate-Term
 
INVESTMENT OBJECTIVE
The investment objective of the Portfolio is to seek as high a level of current
income as is consistent with preservation of capital. The Portfolio pursues its
objective by investing primarily in short- and intermediate-term fixed-income
securities. Under normal circumstances, it is expected that the Portfolio's
dollar-weighted average portfolio effective maturity will not exceed three
years.
 
Effective maturity is the weighted average period over which a security's
principal is expected to be paid, and differs from stated maturity in that it
estimates the effect of expected principal prepayments and call provisions.
Targeting effective maturity provides additional flexibility in portfolio
management but, all else being equal, could result in higher volatility than a
fund targeting a stated maturity or maturity range. See the question and answer
section below for a more detailed discussion of the Portfolio's maturity policy.
 
 8   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998
<PAGE> 
 
Short-Term Bond Portfolio will normally invest at least 65% of its assets in
debt securities. Subject to this policy and subject to its maturity limits, the
Portfolio may invest in a wide variety of income-producing securities including
corporate bonds and notes, government securities, indexed/structured securities,
preferred stock, income-producing common stocks, debt securities that are
convertible or exchangeable into equity securities, and debt securities that
carry with them the right to acquire equity securities as evidenced by warrants
attached to or acquired with the securities. The Portfolio may invest to a
lesser degree in common stocks, other equity securities or debt securities that
are not currently paying dividends or interest.
 
TYPES OF INVESTMENTS
Short-Term Bond Portfolio may purchase securities on a when-issued, delayed
delivery or forward commitment basis. The Portfolio's investments in mortgage-
and asset-backed securities will not exceed 25% of assets and its investments in
zero coupon, pay-in-kind and step coupon securities will not exceed 10% of
assets. The Portfolio may invest without limit in indexed/structured securities.
The Portfolio will invest less than 35% of its net assets in
high-yield/high-risk securities. The Portfolio may invest without limit in
foreign securities, including those of corporate and government issuers. In
addition, the Portfolio may use futures, options and other derivatives for
hedging purposes or for non-hedging purposes such as seeking to enhance return.
See "Additional Risk Factors" on page 12.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN SHORT-TERM BOND PORTFOLIO.
 
Q:   HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
A:   A fundamental risk associated with any fund that invests in fixed-income
     securities (e.g., a bond fund) is the risk that the value of the securities
     it holds will rise or fall as interest rates change. 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.
A bond fund's average-weighted effective maturity and its duration are measures
of how the portfolio may react to interest rate changes.
 
Q:   WHAT IS MEANT BY THE PORTFOLIO'S "AVERAGE-WEIGHTED EFFECTIVE MATURITY"?
A:   The stated maturity of a bond is the date when the issuer must repay the
     bond's entire principal value to an investor, such as the Portfolio. Some
     types of bonds, such as mortgage-backed securities, which are subject to
prepayment risk, and securities with call provisions, may also have an
"effective maturity" that is shorter than the stated date. With respect to GNMA
securities and other mortgage-backed securities, effective maturity is likely to
be substantially less than the stated maturities of the mortgages in the
underlying pools. With respect to obligations with call provisions, effective
maturity is typically the next call date on which the obligation reasonably may
be expected to be called. Securities without prepayment or call provisions
generally have
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS         MAY 1, 1998    9
<PAGE> 
 
an effective maturity equal to their stated maturity. Dollar-weighted effective
maturity is calculated by averaging the effective maturity of bonds held by a
Portfolio with each effective maturity "weighted" according to the percentage of
net assets that it represents.
 
  
Q:   WHAT IS MEANT BY THE PORTFOLIO'S "DURATION"?
A:   A bond's duration indicates the time it will take an investor to recoup his
     or her 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 fund is calculated
by averaging the duration of bonds held by the Portfolio 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.
 
  
Q:   HOW DOES THE PORTFOLIO MANAGE INTEREST RATE RISK?
A:   The Portfolio may vary its average-weighted effective maturity 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 its portfolio manager expects interest rates to rise and longer when its
portfolio manager expects interest rates to fall. The Portfolio may also use
futures, options and other derivatives to manage interest rate risk. See
"Additional Risk Factors" on page 12.
 
  
Q:   WHAT IS MEANT BY "CREDIT QUALITY"?
A:   Credit quality measures the likelihood that the issuer will meet its
     obligations on a bond. One of the fundamental risks associated with all
     bond 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.
 
  
Q:   HOW IS CREDIT QUALITY MEASURED?
A:   Ratings published by nationally recognized rating agencies such as Standard
     & Poor's Rating Services ("Standard & Poor's") and Moody's Investors
     Service, Inc. ("Moody's") 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 Appendix B for a description
of rating categories.
 
GENERAL PORTFOLIO POLICIES
In investing its assets, the Portfolio will follow the general policies listed
below. The percentage limitations included in these policies and elsewhere in
this Prospectus apply only at the time of purchase of the security. For example,
if the Portfolio exceeds a
 
 10   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
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's manager believes that market conditions are not favorable
for profitable investing or when the portfolio manager is otherwise unable to
locate favorable investment opportunities, the Portfolio's investments may be
hedged to a greater degree and/or its 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 are a residual - they represent the assets
that remain after the portfolio manager has committed available assets to
desirable investment opportunities. A larger hedged position and/or larger cash
position may serve as a means of preserving capital in unfavorable market
conditions.
 
Securities that the Portfolio may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolio may
also invest in money market funds (including funds managed by Janus Capital).
When the Portfolio is hedged or its cash or similar investments increase, it may
not participate in stock or bond market advances or declines to the same extent
that it would if the Portfolio was hedged or remained more fully invested in
stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. The Portfolio qualifies as a
diversified fund under the 1940 Act and is subject to the following
diversification requirements:
 
- - As a fundamental policy, the Portfolio may not own more than 10% of the
  outstanding voting shares of any issuer.
 
- - As a fundamental policy, with respect to 75% of its total assets, the
  Portfolio will not purchase a security of any issuer (other than cash items
  and U.S. government securities, as defined in the 1940 Act) if such purchase
  would cause the Portfolio's holdings of that issuer to amount to more than 5%
  of the Portfolio's total assets.
 
- - The Portfolio will not invest more than 25% of its total assets in a single
  issuer (other than U.S. government securities).
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, the Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, the Portfolio will not invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    11
<PAGE> 
 
PORTFOLIO TURNOVER
The Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may 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 initial investment
decision. Changes are made in the Portfolio whenever its portfolio manager
believes such changes are desirable. The portfolio turnover rate is generally
not a factor in making buy and sell decisions.
 
To a limited extent, the Portfolio may purchase securities in anticipation of
relatively short-term price gains. The Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
The Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of the Portfolio's
net assets after the time of purchase, the Portfolio will take steps to reduce
in an orderly fashion its holdings of illiquid securities. An illiquid
investment is a security or other position that cannot be disposed of quickly in
the normal course of business. Some securities cannot be sold to the U.S. public
because of their terms or because of SEC regulations. Janus Capital will follow
guidelines established by the Trustees of the Trust ("Trustees") in making
liquidity determinations for Rule 144A securities and other securities,
including privately placed commercial paper and municipal lease obligations.
 
BORROWING AND LENDING
The Portfolio may borrow money and lend securities or other assets, as follows:
 
- - The Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - The Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, the Portfolio may lend securities or other assets if,
  as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, the Portfolio may
borrow money from or lend money to other funds that permit such transactions and
for which Janus Capital serves as investment adviser. All such borrowing and
lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
 12   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. The Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though 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 underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolio may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk, because their values depend
on the performance of a foreign security denominated in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or potential development, or they may 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 be subject to intense
competition from
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    13
<PAGE> 
 
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 wider price fluctuations.
Investments in such companies tend to be more volatile and somewhat more
speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
The Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolio intends to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolio may also use derivative instruments for non-hedging
purposes such as seeking to increase its income or otherwise seeking to enhance
return. Please refer to Appendix A to this Prospectus and the SAI for a more
detailed discussion of these instruments.
 
The use of derivative instruments exposes the Portfolio to additional investment
risks and transaction costs. Risks inherent in the use of derivative instruments
include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that the portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than the Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately-negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave the
  Portfolio worse off than if it had not entered into the position.
 
Although the Portfolio's manager believes the use of derivative instruments will
benefit the Portfolio, the Portfolio's performance could be worse than if the
Portfolio had not used such instruments if the portfolio manager's judgement
proves incorrect.
 
When the Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover.
 
 14   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
Segregating assets could diminish the Portfolio's return due to the opportunity
losses of foregoing other potential investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for higher quality securities. Conversely, the value of higher
quality securities may be more sensitive to interest rate movements than lower
rated securities. Issuers of high-yield/high-risk securities may not be as
strong financially as those issuing bonds with higher credit ratings.
Investments in such companies are considered to be more speculative than higher
quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, the Portfolio
would experience a reduction of its income and could expect a decline in the
market value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as higher
quality securities. Sovereign debt of foreign governments is generally rated by
country. Because these ratings do not take into account individual factors
relevant to each issue and may not be updated regularly, Janus Capital may treat
such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, the Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolio may have to sell securities
to have sufficient cash to pay the dividends.
 
SHORT SALES
The Portfolio may engage in "short sales against the box." This technique
involves selling either a security that the Portfolio owns, or a security
equivalent in kind and amount that the Portfolio has the right to obtain, for
delivery at a specified date in the future. The Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    15
<PAGE> 
 
SPECIAL SITUATIONS
The Portfolio may invest in "special situations" from time to time. 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.
Investment in special situations may carry an additional risk of loss in the
event that the anticipated development does not occur or does not attract the
expected attention.
 
See Appendix A for risks associated with certain other investments.
 
 16   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
                          MANAGEMENT OF THE PORTFOLIO
 
TRUSTEES
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to the Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolio to the officers
of the Trust and meet at least quarterly to review the Portfolio's investment
policies, performance, expenses and other business affairs.
 
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 has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
INVESTMENT PERSONNEL
PORTFOLIO MANAGER
SANDY R. RUFENACHT is Executive Vice President and portfolio manager of the
Portfolio, which he has managed since May 1996. He is also the portfolio manager
of High-Yield Portfolio which he has managed or co-managed since October 1996.
He previously co-managed Flexible Income Portfolio from January 1997 to May 1,
1998. Mr. Rufenacht joined Janus Capital in 1990 and has managed Janus
Short-Term Bond Fund since January 1996. He is also the portfolio manager of
Janus High-Yield Fund. He
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    17
<PAGE> 
 
previously co-managed Janus Flexible Income Fund from June 1996 to May 1, 1998.
He holds a Bachelor of Arts in Business from the University of Northern
Colorado.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment activities
in a manner that Janus Capital believes is not detrimental to the Portfolio or
Janus Capital's other advisory clients. See the SAI for more detailed
information.
 
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):
 
<TABLE>
<CAPTION>
  AVERAGE DAILY NET     ANNUAL RATE    EXPENSE LIMIT*
 ASSETS OF PORTFOLIO   PERCENTAGE (%)  PERCENTAGE (%)
 -----------------------------------------------------
 <S>                   <C>             <C>
 First $300 Million    .65             .65
 Over $300 Million     .55
 -----------------------------------------------------
</TABLE>
 
  *Janus Capital may terminate or modify the expense limitation at any time upon
   at least 90 days' notice to the Trustees.
 
As asset size increases, the annual rate of the management fee declines in
accordance with the above schedule. 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.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of the Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for the Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for the Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolio's Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for the Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses serving to reduce those expenses. The SAI
further explains the selection of broker-dealers.
 
 18   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolio with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust consists of twelve separate series, one of which is offered by this
Prospectus.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for the class or Portfolio or for the Trust as a whole
for purposes such as electing or removing Trustees, terminating or reorganizing
the Trust, changing fundamental policies, or for any other purpose requiring a
shareholder vote under the 1940 Act. Separate votes are taken by the class or
Portfolio only if a matter affects or requires the vote of only the class or
Portfolio or the interest of a class or Portfolio in the matter differs from the
interest of the other class or Portfolios of the Trust. As a shareholder, you
are entitled to one vote for each share that you own.
 
An insurance company issuing a variable contract invested in Shares of the
Portfolio will request voting instructions from variable contract holders. Under
current law, the insurance company must vote all Shares held by the separate
account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
The Portfolio's Shares 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
participant directed qualified plans. Although the Portfolio does not currently
anticipate any disadvantages to policy owners arising out of the fact that it
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
anticipated disadvantages or material irreconcilable conflicts that may arise
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 plans to withdraw its investment in the Portfolio
or substitute shares of another portfolio of the
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    19
<PAGE> 
 
Trust. If this occurs, the Portfolio may be forced to sell securities at
disadvantageous prices. In addition, the Trustees may refuse to sell shares of
the Portfolio to any separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve the Portfolio's investment objective
by investing all of the Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to the Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolio with service
at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of the Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of shares outstanding.
 
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.
 
 20   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. THE SHARES OF THE PORTFOLIO MAKES SEMIANNUAL 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 THE SHARES OF THE PORTFOLIO WILL BE
AUTOMATICALLY 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 the Portfolio declared a dividend in the
amount of $0.25 per share. If the price of the Portfolio's Share 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 in the Shares depends on the features of
the variable insurance contracts purchased from a participating insurance
company. Further information may be found 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 as a foreign tax credit.
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.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    21
<PAGE> 
 
The Portfolio does not expect to pay any federal income or excise taxes because
it intends to meet certain requirements of the Internal Revenue Code. In
addition, the Portfolio intends to qualify under the Internal Revenue Code with
respect to the diversification requirements related to the tax-deferred status
of insurance company separate accounts.
 
 22   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<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.
 
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's
separate account or your plan documents for information on how to invest in the
Shares of the Portfolio.
 
All investments in the Portfolio are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by the Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by the Portfolio.
 
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. The Portfolio may discontinue
sales of its shares if management believes that a substantial further increase
may adversely affect the Portfolio's ability to achieve its investment
objective. In such event, however, it is anticipated 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.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolio. Each report will show the investments
owned by the Portfolio and the market values thereof, as well as other
information about the Portfolio and its operations. The Trust's fiscal year ends
December 31.
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    23
<PAGE> 
 
                                   APPENDIX A
 
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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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
 
 24   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
through to shareholders on a pro rata basis. These securities involve prepayment
risk, which is the risk that the underlying mortgages or other debt may be
refinanced or paid off prior to their maturities during periods of declining
interest rates. In that case, the portfolio manager may have to reinvest the
proceeds from the securities at a lower rate. Potential market gains on a
security subject to prepayment risk may be more limited than potential market
gains on a comparable security that is not subject to prepayment risk.
 
PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign corporations which
generate certain amounts of passive income or hold certain amounts of assets for
the production of passive income. Passive income includes dividends, interest,
royalties, rents and annuities. To avoid taxes and interest that the Portfolio
must pay if these investments are profitable, the Portfolio may make various
elections permitted by the tax laws. These elections could require that the
Portfolio recognize taxable income, which 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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 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.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    25
<PAGE> 
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a specified formula, usually with reference to some interest rate
index or market 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 bears the risk of market value fluctuations
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 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 of comparable 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
 
 26   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<PAGE> 
 
Portfolio may enter into forward currency contracts to hedge against declines in
the value of non-dollar denominated securities or to reduce the impact of
currency appreciation on purchases of non-dollar denominated securities. The
Portfolio 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 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.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    27
<PAGE> 
 
                                   APPENDIX B
 
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>
 
 28   JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998
<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 the
portfolio manager determines that such securities are the equivalent of
investment grade securities. Securities that have received different ratings
from more than one agency are considered investment grade if at least one agency
has rated the security investment grade.
 
JANUS ASPEN SERIES SHORT-TERM BOND PORTFOLIO PROSPECTUS        MAY 1, 1998    29
<PAGE> 
                               
                   
                                     
                               JANUS ASPEN SERIES
                                   PROSPECTUS
                            Short-Term Bond Portfolio

                                     [LOGO]

            P.O. Box 173375 o Denver, CO 80217-3375 o 1-800-525-3713
                                     (5/98)
<PAGE> 
 
                               JANUS ASPEN SERIES
 
                                   Prospectus
 
                                  MAY 1, 1998
 
This prospectus describes ten mutual funds with a variety of investment
objectives, including growth of capital, current income and a combination of
growth and income (the "Portfolios"). Each Portfolio is a series of Janus Aspen
Series (the "Trust") and currently offers two classes of shares. The
Institutional Shares are sold under the name "Janus Aspen Series." The Trust is
registered with the Securities and Exchange Commission ("SEC") as an open-end
management investment company. The Institutional Shares of each Portfolio
(collectively, the "Shares") are offered by this prospectus in connection with
investment in and payments under variable annuity contracts and variable life
insurance contracts (collectively "variable insurance contracts"), as well as
certain qualified retirement plans. Janus Capital Corporation ("Janus Capital")
serves as investment adviser to each Portfolio. Janus Capital has been in the
investment advisory business for over 27 years and currently manages
approximately $80 billion in assets.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Each variable insurance contract
involves fees and expenses 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 about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolios. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
FLEXIBLE INCOME PORTFOLIO AND HIGH-YIELD PORTFOLIO MAY INVEST ALL OF THEIR
ASSETS IN HIGH-YIELD CORPORATE DEBT SECURITIES, COMMONLY KNOWN AS "JUNK BONDS."
SEE "ADDITIONAL RISK FACTORS" ON PAGE 26 FOR THE RISKS ASSOCIATED WITH INVESTING
IN THESE SECURITIES.
 
AN INVESTMENT IN THE MONEY MARKET PORTFOLIO IS NEITHER INSURED NOR GUARANTEED BY
THE U.S. GOVERNMENT. THERE IS NO ASSURANCE THAT THE MONEY MARKET PORTFOLIO WILL
BE ABLE TO MAINTAIN A STABLE NET ASSET VALUE OF $1.00 PER SHARE.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                      This page intentionally left blank.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                            <C>
PORTFOLIOS AT A GLANCE
Brief description of the Portfolios..........    2
EXPENSE INFORMATION
Each Portfolio's annual operating expenses...    5
Financial Highlights - a summary of financial
  data.......................................    6
PERFORMANCE TERMS
An explanation of performance terms..........   14
THE PORTFOLIOS IN DETAIL
Investment Objectives and Policies of the
   Growth Combination and Fixed-Income
   Portfolios................................   15
General Portfolio Policies of the Portfolios
  other than Money Market Portfolio..........   24
Additional Risk Factors......................   26
MONEY MARKET PORTFOLIO
Investment Objectives, Policies and
   Techniques................................   30
MANAGEMENT OF THE PORTFOLIOS
Investment Adviser and Investment
   Personnel.................................   35
Personal Investing...........................   37
Portfolio Transactions.......................   37
Management Expenses and Expense Limits.......   38
Other Service Providers......................   39
Other Information............................   39
DISTRIBUTIONS AND TAXES
Distributions................................   42
Taxes........................................   42
SHAREHOLDER'S GUIDE
Purchases....................................   44
Redemptions..................................   44
Shareholder Communications...................   45
APPENDIX A
Glossary of Investment Terms.................   46
APPENDIX B
Explanation of Rating Categories.............   50
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIOS AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolios
and their investment emphasis. A more detailed discussion of the Portfolios'
investment objectives and policies begins on page 15.
 
GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on companies with larger
market capitalizations.
Inception: September 1993
Manager: James P. Craig, III
Assistant Managers: David C. Decker
                    Blaine P. Rollins
 
AGGRESSIVE GROWTH PORTFOLIO
Focus: A nondiversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on securities issued by
medium-sized companies.
Inception: September 1993
Manager: James P. Goff
 
CAPITAL APPRECIATION PORTFOLIO
Focus: A nondiversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of issuers of any size.
Inception: May 1997
Manager: Scott W. Schoelzel
 
INTERNATIONAL GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign issuers.
Inception: May 1994
Managers:Laurence J. Chang
         Helen Young Hayes
 
WORLDWIDE GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign and domestic issuers.
Inception: September 1993
Manager: Helen Young Hayes
Assistant Manager: Laurence J. Chang
 
 2   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
BALANCED PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital, balanced
by current income. The 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.
Inception: September 1993
Manager: Blaine P. Rollins
 
EQUITY INCOME PORTFOLIO
Focus: A diversified portfolio that seeks current income and long-term growth of
capital by investing primarily in income-producing equity securities.
Inception: May 1997
Manager: Blaine P. Rollins
 
FLEXIBLE INCOME PORTFOLIO
Focus: A diversified portfolio that seeks to maximize total return from a
combination of income and capital appreciation by investing primarily in
income-producing securities.
Inception: September 1993
Manager: Ronald V. Speaker
 
HIGH-YIELD PORTFOLIO
Focus: A diversified portfolio that seeks high current income as its primary
objective. Capital appreciation is a secondary objective when consistent with
the primary objective. The Portfolio seeks to achieve these objectives by
investing primarily in high-yield/high-risk fixed-income securities.
Inception: May 1996
Manager: Sandy R. Rufenacht
 
MONEY MARKET PORTFOLIO
Focus: A money market mutual fund that seeks maximum current income to the
extent consistent with stability of capital. The Portfolio seeks to achieve this
objective by investing primarily in high quality debt obligations and
obligations of financial institutions.
Inception: May 1995
Manager: Sharon S. Pichler
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    3
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolios in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES
    (applicable to each Portfolio)
 
<TABLE>
         <S>                                              <C>
         Maximum sales load imposed on purchases          None
         Maximum sales load imposed on reinvested
           dividends                                      None
         Deferred sales charges on redemptions            None
         Redemption fee                                   None
         Exchange fee                                     None
</TABLE>
 
 4   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
ANNUAL OPERATING EXPENSES (after fee waivers and reductions)(1)
 
(expressed as a percentage of average net assets)
 
<TABLE>
<CAPTION>
- ------------------------------------------------------------------------------------
                                       MANAGEMENT     OTHER      TOTAL OPERATING
                                          FEE        EXPENSES       EXPENSES
- ------------------------------------------------------------------------------------
<S>                                    <C>           <C>         <C>             
Growth Portfolio                         0.65%        0.05%           0.70%
Aggressive Growth Portfolio              0.73%        0.03%           0.76%
Capital Appreciation Portfolio(2)        0.23%        1.03%           1.26%
International Growth Portfolio           0.67%        0.29%           0.96%
Worldwide Growth Portfolio               0.66%        0.08%           0.74%
Balanced Portfolio                       0.76%        0.07%           0.83%
Equity Income Portfolio(2)               0.00%        1.25%           1.25%
Flexible Income Portfolio                0.65%        0.10%           0.75%
High-Yield Portfolio                     0.00%        1.00%           1.00%
Money Market Portfolio                   0.22%        0.28%           0.50%
 -----------------------------------------------------------------------------------
</TABLE>
 
(1) Management fees for Growth, Aggressive Growth, Capital Appreciation,
    International Growth, Worldwide Growth, Balanced and Equity Income
    Portfolios reflect a reduced fee schedule effective July 1, 1997. The
    management fee for each of these Portfolios reflects the new rate applied to
    net assets as of December 31, 1997. Other expenses are based on gross
    expenses of the Shares before expense offset arrangements for the fiscal
    year ended December 31, 1997. The information for each Portfolio other than
    the Flexible Income Portfolio is net of fee waivers or reductions from Janus
    Capital. Fee reductions for the Growth, Aggressive Growth, Capital
    Appreciation, International Growth, Worldwide Growth, Balanced and Equity
    Income Portfolios reduce the management fee to the level of the
    corresponding Janus retail fund. Other waivers, if applicable, are first
    applied against the management fee and then against other expenses. Without
    such waivers or reductions, the Management Fee, Other Expenses and Total
    Operating Expenses for the Shares would have been 0.74%, 0.04%, and 0.78%
    for Growth Portfolio; 0.74%, 0.04%, and 0.78% for Aggressive Growth
    Portfolio; 1.14%, 1.05%, and 2.19% for Capital Appreciation Portfolio;
    0.79%, 0.29% and 1.08% for International Growth Portfolio; 0.72%, 0.09% and
    0.81% for Worldwide Growth Portfolio; 0.77%, 0.06% and 0.83% for Balanced
    Portfolio; 0.76%, 4.99%, and 5.75% for Equity Income Portfolio; 0.75%, 2.52%
    and 3.27% for High-Yield Portfolio; and 0.25%, 0.30% and 0.55% for Money
    Market Portfolio, respectively. Janus Capital may modify or terminate the
    waivers or reductions at any time upon at least 90 days' notice to the
    Trustees.
(2) Based on expenses incurred during the initial fiscal period.
 
EXAMPLE
Assume you invest $1,000, the Shares of the Portfolios return 5% annually and
each Portfolio's expense ratio remains as listed above. The example below shows
the operating expenses that you would indirectly bear as an investor in the
Shares of the Portfolios.
 
<TABLE>
<CAPTION>
- --------------------------------------------------------------------------------------
                                             1 YEAR   3 YEARS   5 YEARS   10 YEARS
- --------------------------------------------------------------------------------------
<S>                                          <C>      <C>       <C>       <C>     
Growth Portfolio                              $ 7       $22       $39       $ 87
Aggressive Growth Portfolio                   $ 8       $24       $42       $ 94
Capital Appreciation Portfolio                $13       $40       $69       $152
International Growth Portfolio                $10       $31       $53       $118
Worldwide Growth Portfolio                    $ 8       $24       $41       $ 92
Balanced Portfolio                            $ 8       $26       $46       $103
Equity Income Portfolio                       $13       $40       $69       $151
Flexible Income Portfolio                     $ 8       $24       $42       $ 93
High-Yield Portfolio                          $10       $32       $55       $122
Money Market Portfolio                        $ 5       $16       $28       $ 63
 -------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    5
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise noted, the information below is for fiscal periods ending on
December 31st of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolios' financial statements since their inception. Their report
is included in the Portfolios' Annual Report, which is incorporated by reference
into the SAI. A detailed explanation of the Financial Highlights can be found on
page 13.
 
<TABLE>
<CAPTION>
                                                                GROWTH PORTFOLIO
          INSTITUTIONAL SHARES               1997        1996        1995        1994         1993(1)
<S>                                        <C>         <C>         <C>         <C>            <C>
 -----------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF PERIOD     $15.51      $13.45      $10.57      $10.32         $10.00
 -----------------------------------------------------------------------------------------------------
 INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                       .15         .17         .28         .09            .03
 3. Net gains or (losses) on securities
    (both realized and unrealized)             3.34        2.29        2.90         .20            .32
 -----------------------------------------------------------------------------------------------------
 4. Total from investment operations           3.49        2.46        3.18         .29            .35
 -----------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment
    income)                                   (.15)       (.17)       (.30)       (.04)          (.03)
 6. Tax return of capital distributions          --          --          --          --             --
 7. Distributions (from capital gains)        (.37)       (.23)          --          --             --
 -----------------------------------------------------------------------------------------------------
 8. Total distributions                       (.52)       (.40)       (.30)       (.04)          (.03)
 -----------------------------------------------------------------------------------------------------
 9. NET ASSET VALUE, END OF PERIOD           $18.48      $15.51      $13.45      $10.57         $10.32
 -----------------------------------------------------------------------------------------------------
10. Total return*                            22.75%      18.45%      30.17%       2.76%          3.50%
 -----------------------------------------------------------------------------------------------------
11. Net assets, end of period (in
    thousands)                             $608,281    $325,789    $126,911     $43,549         $7,482
12. Average net assets for the period (in
    thousands)                             $477,914    $216,125     $77,344     $26,464         $3,191
13. Ratio of gross expenses to average
    net assets**                              0.70%(7)    0.69%(6)    0.78%(5)      N/A            N/A
14. Ratio of net expenses to average net
    assets**                                  0.69%       0.69%       0.76%       0.88%(2)(4)    0.25%(3)
15. Ratio of net investment income to
    average net assets**                      0.91%       1.39%       1.24%       1.45%          2.54%
16. Portfolio turnover rate**                  122%         87%        185%        169%           162%
17. Average commission rate                 $0.0500     $0.0466         N/A         N/A            N/A
 -----------------------------------------------------------------------------------------------------
</TABLE>
 
*  Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) 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.
(3) The ratio was 2.16% for Growth Portfolio, before waiver of certain fees
    and/or voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(4) The ratio was 1.23% for Growth Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
(5) The ratio was 0.98% for Growth Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
(6) The ratio was 0.83% for Growth Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
(7) The ratio was 0.78%, for Growth Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
 
 6   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                                                                CAPITAL
                                                                                              APPRECIATION
                                               AGGRESSIVE GROWTH PORTFOLIO                     PORTFOLIO
    INSTITUTIONAL SHARES         1997        1996        1995        1994         1993(1)       1997(2)
<S>                            <C>         <C>         <C>         <C>            <C>         <C>
 ---------------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE,
    BEGINNING OF PERIOD          $18.24      $17.08      $13.62      $11.80         $10.00        $10.00
 ---------------------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT
  OPERATIONS:
 2. Net investment income            --          --         .24         .11            .01           .05
 3. Net gains or (losses) on
    securities (both realized
    and unrealized)                2.31        1.36        3.47        1.82           1.80          2.61
 ---------------------------------------------------------------------------------------------------------
 4. Total from investment
    operations                     2.31        1.36        3.71        1.93           1.81          2.66
 ---------------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net
    investment income)               --          --       (.25)       (.11)          (.01)         (.04)
 6. Tax return of capital
    distributions                    --       (.01)          --          --             --            --
 7. Distributions (from
    capital gains)                   --       (.19)          --          --             --            --
 ---------------------------------------------------------------------------------------------------------
 8. Total distributions                       (.20)       (.25)       (.11)          (.01)         (.04)
 ---------------------------------------------------------------------------------------------------------
 9. NET ASSET VALUE, END OF
    PERIOD                       $20.55      $18.24      $17.08      $13.62         $11.80        $12.62
 ---------------------------------------------------------------------------------------------------------
10. Total return*                12.66%       7.95%      27.48%      16.33%         18.05%        26.60%
 ---------------------------------------------------------------------------------------------------------
11. Net assets, end of period
    (in thousands)             $508,198    $383,693    $185,911     $41,289         $1,985        $6,833
12. Average net assets for
    the period (in thousands)  $418,464    $290,629    $107,582     $14,152         $1,091        $2,632
13. Ratio of gross expenses
    to average net assets**       0.76%(8)    0.76%(7)    0.86%(6)      N/A            N/A         1.26%(8)
14. Ratio of net expenses to
    average net assets**          0.76%       0.76%       0.84%       1.05%(3)(5)    0.25%(4)      1.25%
15. Ratio of net investment
    income to average net
    assets**                    (0.10%)     (0.27%)       0.58%       2.18%          0.34%         1.43%
16. Portfolio turnover rate**      130%         88%        155%        259%            31%          101%
17. Average commission rate     $0.0367     $0.0347         N/A         N/A            N/A       $0.0375
 ---------------------------------------------------------------------------------------------------------
</TABLE>
 
*  Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) May 1, 1997 (inception) to December 31, 1997.
(3) 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.
(4) The ratio was 5.79% for Aggressive Growth Portfolio, before waiver of
    certain fees and/or voluntary reduction of adviser's fees to the effective
    rate of the corresponding Janus retail fund.
(5) The ratio was 1.14% for Aggressive Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(6) The ratio was 0.93%, for Aggressive Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(7) The ratio was 0.83% for Aggressive Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(8) The ratio was 0.78% and 2.19%, respectively for Aggressive Growth and
    Capital Appreciation Portfolios, before waiver of certain fees and/or
    voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    7
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                           INTERNATIONAL GROWTH PORTFOLIO
               INSTITUTIONAL SHARES                   1997        1996        1995      1994(2)
<S>                                                 <C>         <C>         <C>         <C>
 -----------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF PERIOD              $15.72      $11.95       $9.72      $10.00
 -----------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                                .11         .05         .09       (.09)
 3. Net gains or (losses) on securities (both
    realized and unrealized)                            2.80        4.06        2.16       (.19)
 -----------------------------------------------------------------------------------------------
 4. Total from investment operations                    2.91        4.11        2.25       (.28)
 -----------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment income)             (.11)       (.11)       (.02)          --
 6. Tax return of capital distributions                   --          --          --          --
 7. Distributions (from capital gains)                 (.01)       (.23)          --          --
 8. Distributions (in excess of realized gains)        (.03)          --          --          --
 -----------------------------------------------------------------------------------------------
 9. Total distributions                                (.15)       (.34)       (.02)          --
 -----------------------------------------------------------------------------------------------
10. NET ASSET VALUE, END OF PERIOD                    $18.48      $15.72      $11.95       $9.72
 -----------------------------------------------------------------------------------------------
11. Total return*                                     18.51%      34.71%      23.15%     (2.80%)
 -----------------------------------------------------------------------------------------------
12. Net assets, end of period (in thousands)        $161,091     $27,192      $1,608      $1,353
13. Average net assets for the period (in
    thousands)                                       $96,164      $7,437      $1,792      $1,421
14. Ratio of gross expenses to average net
    assets**                                           0.96%(6)    1.26%(5)    2.69%(4)      N/A
15. Ratio of net expenses to average net assets**      0.96%       1.25%       2.50%       2.50%(3)
16. Ratio of net investment income to average net
    assets**                                           0.70%       0.62%     (0.80%)     (1.30%)
17. Portfolio turnover rate**                            86%         65%        211%        275%
18. Average commission rate                          $0.0241     $0.0305         N/A         N/A
 -----------------------------------------------------------------------------------------------
</TABLE>
 
*  Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(2) May 2, 1994 (inception) to December 31, 1994.
(3) The ratio was 4.67% for International Growth Portfolio, before waiver of
    certain fees and/or voluntary reduction of adviser's fees to the effective
    rate of the corresponding Janus retail fund.
(4) The ratio was 3.57% for International Growth Portfolio, before waiver of
    certain fees and/or voluntary reduction of adviser's fees to the effective
    rate of the corresponding Janus retail fund.
(5) The ratio was 2.21% for International Growth Portfolio, before waiver of
    certain fees and/or voluntary reduction of adviser's fees to the effective
    rate of the corresponding Janus retail fund.
(6) The ratio was 1.08%, for International Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
 
 8   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                           WORLDWIDE GROWTH PORTFOLIO
          INSTITUTIONAL SHARES               1997         1996        1995        1994         1993(1)
<S>                                       <C>           <C>         <C>         <C>            <C>
 ------------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF PERIOD      $19.44      $15.31      $12.07      $11.89         $10.00
 ------------------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                        .16         .16         .11         .04            .02
 3. Net gains or (losses) on securities
    (both realized and unrealized)              4.14        4.27        3.19         .14           1.89
 ------------------------------------------------------------------------------------------------------
 4. Total from investment operations            4.30        4.43        3.30         .18           1.91
 ------------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment
    income)                                    (.17)       (.17)       (.06)          --          (.01)
 6. Dividends (in excess of net
    investment income)                         (.02)          --          --          --             --
 7. Tax return of capital distributions           --          --          --          --             --
 8. Distributions (from capital gains)         (.16)       (.13)          --          --          (.01)
 ------------------------------------------------------------------------------------------------------
 9. Total distributions                        (.35)       (.30)       (.06)          --          (.02)
 ------------------------------------------------------------------------------------------------------
10. NET ASSET VALUE, END OF PERIOD            $23.39      $19.44      $15.31      $12.07         $11.89
 ------------------------------------------------------------------------------------------------------
11. Total return*                             22.15%      29.04%      27.37%       1.53%         19.10%
 ------------------------------------------------------------------------------------------------------
12. Net assets, end of period (in
    thousands)                            $1,576,548    $582,603    $108,563     $37,728         $4,856
13. Average net assets for the period
    (in thousands)                        $1,148,951    $304,111     $59,440     $22,896         $2,200
14. Ratio of gross expenses to average
    net assets**                               0.74%(7)    0.80%(6)    0.90%(5)      N/A            N/A
15. Ratio of net expenses to average net
    assets**                                   0.74%       0.80%       0.87%       1.18%(2)(4)    0.25%(3)
16. Ratio of net investment income to
    average net assets**                       0.67%       0.83%       0.95%       0.50%          0.84%
17. Portfolio turnover rate**                    80%         62%        113%        217%            57%
18. Average commission rate                  $0.0337     $0.0345         N/A         N/A            N/A
 ------------------------------------------------------------------------------------------------------
</TABLE>
 
*  Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) 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.
(3) The ratio was 2.71%, for Worldwide Growth Portfolio, before waiver of
    certain fees and/or voluntary reduction of adviser's fees to the effective
    rate of the corresponding Janus retail fund.
(4) The ratio was 1.49%, for Worldwide Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(5) The ratio was 1.09%, for Worldwide Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(6) The ratio was 0.91%, for Worldwide Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
(7) The ratio was 0.81%, for Worldwide Growth Portfolio, before voluntary
    reduction of adviser's fees to the effective rate of the corresponding Janus
    retail fund.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    9
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                                                                 EQUITY
                                                                                                 INCOME
                                                        BALANCED PORTFOLIO                      PORTFOLIO
        INSTITUTIONAL SHARES            1997       1996       1995       1994        1993(1)     1997(2)
<S>                                   <C>         <C>        <C>        <C>          <C>        <C>
- ---------------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF
  PERIOD                                $14.77     $13.03     $10.63    $10.64        $10.00      $10.00
- ---------------------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                  .34        .32        .17       .15           .08         .01
 3. Net gains or (losses) on
    securities (both realized and
    unrealized)                           2.89       1.81       2.45     (.06)           .64        3.46
- ---------------------------------------------------------------------------------------------------------
 4. Total from investment operations      3.23       2.13       2.62       .09           .72        3.47
- ---------------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment
  income)                                (.35)      (.30)      (.22)     (.10)         (.08)       (.01)
 6. Tax return of capital
  distributions                             --         --         --        --            --          --
 7. Distributions (from capital
  gains)                                 (.18)      (.09)         --        --            --          --
- ---------------------------------------------------------------------------------------------------------
 8. Total distributions                  (.53)      (.39)      (.22)     (.10)         (.08)       (.01)
- ---------------------------------------------------------------------------------------------------------
 9. NET ASSET VALUE, END OF PERIOD      $17.47     $14.77     $13.03    $10.63        $10.64      $13.46
- ---------------------------------------------------------------------------------------------------------
10. Total return*                       22.10%     16.18%     24.79%     0.84%         7.20%      34.70%
- ---------------------------------------------------------------------------------------------------------
11. Net assets, end of period (in
  thousands)                          $362,409    $85,480    $14,021    $3,153          $537      $3,047
12. Average net assets for the
    period (in thousands)             $176,432    $43,414     $5,739    $2,336          $521      $1,101
13. Ratio of gross expenses to
    average net assets**                 0.83%(8)   0.94%(7)   1.37%(6)    N/A           N/A       1.25%(8)
14. Ratio of net expenses to average
    net assets**                         0.82%      0.92%      1.30%     1.57%(3)(5)   0.25%(4)    1.25%
15. Ratio of net investment income
    to average net assets**              2.87%      2.92%      2.41%     1.90%         2.69%       0.35%
16. Portfolio turnover rate**             139%       103%       149%      158%          126%        128%
17. Average commission rate            $0.0456    $0.0426        N/A       N/A           N/A     $0.0382
- ---------------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) May 1, 1997 (inception) to December 31, 1997.
(3) 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.
(4) The ratio was 7.92% for Balanced Portfolio, before waiver of certain fees
    and/or voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(5) The ratio was 1.74% for Balanced Portfolio, before voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
(6) The ratio was 1.55% for the Balanced Portfolio, before voluntary reduction
    of adviser's fees to the effective rate of the Janus Balanced Fund.
(7) The ratio was 1.07% for the Balanced Portfolio, voluntary reduction of
    adviser's fees to the effective rate of the Janus Balanced Fund.
(8) The ratio was 0.83% and 5.75%, respectively, for Balanced and Equity Income
    Portfolios, before waiver of certain fees and/or voluntary reduction of
    adviser's fees to the effective rate of the corresponding Janus retail fund.
 
 10   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                           FLEXIBLE INCOME PORTFOLIO
             INSTITUTIONAL SHARES                1997      1996      1995      1994      1993(1)
<S>                                             <C>       <C>       <C>       <C>        <C>
- ------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF PERIOD         $11.24    $11.11     $9.48     $9.97    $10.00
- ------------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                           .67       .74       .53       .47       .11
 3. Net gains or (losses) on securities (both
    realized and unrealized)                        .62       .24      1.70     (.56)     (.04)
- ------------------------------------------------------------------------------------------------
 4. Total from investment operations               1.29       .98      2.23     (.09)       .07
- ------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment income)        (.64)     (.72)     (.60)     (.40)     (.10)
 6. Tax return of capital distributions              --        --        --        --        --
 7. Distributions (from capital gains)            (.11)     (.13)        --        --        --
- ------------------------------------------------------------------------------------------------
 8. Total distributions                           (.75)     (.85)     (.60)     (.40)     (.10)
- ------------------------------------------------------------------------------------------------
 9. NET ASSET VALUE, END OF PERIOD               $11.78    $11.24    $11.11     $9.48     $9.97
- ------------------------------------------------------------------------------------------------
10. Total return*                                11.76%     9.19%    23.86%   (0.91%)     0.70%
- ------------------------------------------------------------------------------------------------
11. Net assets, end of period (in thousands)    $54,098   $25,315   $10,831    $1,924      $538
12. Average net assets for the period (in
  thousands)                                    $36,547   $17,889    $5,556    $1,636      $497
13. Ratio of gross expenses to average net
  assets**                                        0.75%     0.84%     1.07%       N/A       N/A
14. Ratio of net expenses to average net
  assets**                                        0.75%     0.83%     1.00%     1.00%(3)  1.00%(2)
15. Ratio of net investment income to average
  net assets**                                    6.90%     7.31%     7.46%     5.49%     3.77%
16. Portfolio turnover rate**                      119%      250%      236%      234%      508%
17. Average commission rate                         N/A       N/A       N/A       N/A       N/A
- ------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) The ratio was 5.27%, for Flexible Income Portfolio, before waiver of certain
    fees.
(3) The ratio was 1.35%, for Flexible Income Portfolio, before waiver of certain
    fees.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    11
<PAGE> 
 
(continued)
 
<TABLE>
<CAPTION>
                                                    HIGH-YIELD
                                                    PORTFOLIO            MONEY MARKET PORTFOLIO
             INSTITUTIONAL SHARES                1997      1996(1)     1997       1996      1995(4)
<S>                                             <C>        <C>        <C>        <C>        <C>
- ---------------------------------------------------------------------------------------------------
 1. NET ASSET VALUE, BEGINNING OF PERIOD         $10.83    $10.00       $1.00      $1.00     $1.00
- ---------------------------------------------------------------------------------------------------
  INCOME FROM INVESTMENT OPERATIONS:
 2. Net investment income                           .70       .43         .05        .05       .04
 3. Net gains or (losses) on securities (both
    realized and unrealized)                        .99       .80          --         --        --
- ---------------------------------------------------------------------------------------------------
 4. Total from investment operations               1.69      1.23         .05        .05       .04
- ---------------------------------------------------------------------------------------------------
  LESS DISTRIBUTIONS:
 5. Dividends (from net investment income)        (.68)     (.40)       (.05)      (.05)     (.04)
 6. Tax return of capital distributions                        --                     --        --
 7. Distributions (from capital gains)            (.06)        --          --
- ---------------------------------------------------------------------------------------------------
 8. Total distributions                           (.74)     (.40)       (.05)      (.05)     (.04)
- ---------------------------------------------------------------------------------------------------
 9. NET ASSET VALUE, END OF PERIOD               $11.78    $10.83       $1.00      $1.00     $1.00
- ---------------------------------------------------------------------------------------------------
10. Total return*                                15.98%    12.40%       5.17%      5.05%     3.63%
- ---------------------------------------------------------------------------------------------------
11. Net assets, end of period (in thousands)     $2,914      $783     $15,374     $6,016    $1,735
12. Average net assets for the period (in
  thousands)                                     $1,565      $459      $8,926     $3,715    $1,543
13. Ratio of gross expenses to average net
  assets**                                        1.00%(3)  1.01%(2)    0.50%      0.50%     0.50%
14. Ratio of net expenses to average net
  assets**                                        1.00%     1.00%       0.50%(7)   0.50%(6)  0.50%(5)
15. Ratio of net investment income to average
  net assets**                                    7.98%     5.74%       5.17%      4.93%     5.30%
16. Portfolio turnover rate**                      299%      301%         N/A        N/A       N/A
17. Average commission rate                         N/A       N/A         N/A        N/A       N/A
- ---------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) May 1, 1996 (inception) to December 31, 1996.
(2) The ratio was 6.29% for High-Yield Portfolio before waiver of certain fees
    incurred by the Portfolio.
(3) The ratio was 3.27% for High-Yield Portfolio before waiver of certain fees
    incurred by the Portfolio.
(4) May 1, 1995 (inception) to December 31, 1995.
(5) The ratio was 1.07% before waiver of certain fees incurred by Money Market
    Portfolio.
(6) The ratio was 0.78% before waiver of certain fees incurred by Money Market
    Portfolio.
(7) The ratio was 0.55% before waiver of certain fees incurred by Money Market
    Portfolio.
 
 12   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights tables. The tables contain important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolios'
Annual Report contains additional information about each Portfolio's
performance, including a comparison to an appropriate securities index. To
request a copy of the Annual Report, please call or write your insurance
company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of a Portfolio. It is
computed by adding the value of all of a Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between NET ASSET VALUE, BEGINNING OF PERIOD,
and NET ASSET VALUE, END OF PERIOD in the Financial Highlights tables represents
the change in value of a Share of a Portfolio over the fiscal period, but not
its total return. The NAV of the Money Market Portfolio's Shares is expected to
be $1.00.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by a Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that a Portfolio paid from
net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities a Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that a Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of a Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolios were not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in a Portfolio's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is a Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of a Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of a Portfolio's securities. The
Money Market Portfolio does not calculate portfolio turnover.
 
AVERAGE COMMISSION RATE is the total of a Portfolio's agency commission paid on
equity securities trades divided by the number of shares purchased and sold.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    13
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe a Portfolio's performance. You may see references to these terms in our
newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of a Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. Growth
Portfolio, Aggressive Growth Portfolio, International Growth Portfolio,
Worldwide Growth Portfolio, Capital Appreciation Portfolio, Balanced Portfolio
and Equity Income Portfolio generally measure performance in terms of total
return, while Flexible Income Portfolio, High-Yield Portfolio and Money Market
Portfolio generally use yield.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights tables show total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of a Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in a Portfolio's return and are not
the same as actual annual results.
 
YIELD shows the rate of income the Shares earn on investments as a percentage of
the Share price. It is calculated by dividing net investment income for a 30-day
period (7-day period for the Money Market Portfolio) by the average number of
Shares entitled to receive dividends and dividing the result by the Share's NAV
per share at the end of the 30-day period. Yield does not include changes in
NAV.
 
Yields are calculated according to standardized SEC formulas and may not equal
the income on an investor's account. Yield is usually quoted on an annualized
basis. An annualized yield represents the amount you would earn if you remained
in a Portfolio for a year and the Shares of that Portfolio continued to have the
same yield for the entire year.
 
EFFECTIVE YIELD is similar to yield in that it is calculated over the same time
frame, but instead the net investment income is compounded and then annualized.
Due to the compounding effect, the effective yield will normally be higher than
the yield.
 
THE PORTFOLIOS IMPOSE NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
OR YIELD COMPUTATIONS. YIELD AND TOTAL RETURN FIGURES OF THE PORTFOLIOS INCLUDE
THE EFFECT OF DEDUCTING EACH PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES
AND EXPENSES ATTRIBUTABLE TO ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO
PERFORMANCE FIGURES ARE BASED UPON HISTORICAL RESULTS AND ARE NOT INTENDED TO
INDICATE FUTURE PERFORMANCE. INVESTMENT RETURNS AND NET ASSET VALUE WILL
FLUCTUATE SO THAT SHARES, WHEN REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR
ORIGINAL COST.
 
 14   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                            THE PORTFOLIOS IN DETAIL
 
This section takes a closer look at the Portfolios' investment objectives,
policies and the securities in which they invest. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed description of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before choosing a Portfolio.
 
Each Portfolio has an investment objective and policies that are similar to
those of a Janus retail fund, as illustrated in the chart on this page. Although
it is anticipated that each Portfolio and its corresponding retail fund will
hold similar securities, differences in asset size and cash flow needs as well
as the relative weightings of securities selections may result in differences in
investment performance. 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 contract fees and
expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including each Portfolio's investment objective, are
not fundamental and may be changed by the Portfolios' Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in a Portfolio's objective or policies, you should
consider whether that Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
 EACH OF THE PORTFOLIOS HAS A SIMILAR INVESTMENT OBJECTIVE AND SIMILAR
 INVESTMENT POLICIES TO AN EXISTING JANUS RETAIL FUND.
 
 Growth Portfolio....................................................Janus Fund
 Aggressive Growth Portfolio..............................Janus Enterprise Fund
 Capital Appreciation Portfolio..............................Janus Olympus Fund
 International Growth Portfolio.............................Janus Overseas Fund
 Worldwide Growth Portfolio................................Janus Worldwide Fund
 Balanced Portfolio.........................................Janus Balanced Fund
 Equity Income Portfolio...............................Janus Equity Income Fund
 Flexible Income Portfolio...........................Janus Flexible Income Fund
 High-Yield Portfolio.....................................Janus High-Yield Fund
 Money Market Portfolio.................................Janus Money Market Fund
 
GROWTH PORTFOLIO, AGGRESSIVE GROWTH PORTFOLIO, CAPITAL APPRECIATION PORTFOLIO,
INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO ARE DESIGNED FOR
LONG-TERM INVESTORS WHO SEEK GROWTH OF CAPITAL ONLY AND WHO CAN TOLERATE THE
GREATER RISKS ASSOCIATED WITH COMMON STOCK INVESTMENTS.
 
GROWTH PORTFOLIOS
Investment Objective:..........................................Growth of Capital
Primary Holdings:..................................................Common Stocks
Shareholder's Investment Horizon:......................................Long-Term
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    15
<PAGE> 
 
GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective by investing primarily in common stocks of
issuers of any size. This Portfolio generally invests in larger, more
established issuers.
 
AGGRESSIVE GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a nondiversified portfolio that pursues its investment objective by normally
investing at least 50% of its equity assets in securities issued by medium-sized
companies. Medium-sized companies are those whose market capitalizations fall
within the range of companies in the S&P MidCap 400 Index (the "MidCap Index").
Companies whose capitalization falls outside this range after the Portfolio's
initial purchase continue to be considered medium-sized companies for the
purpose of this policy. As of December 31, 1997, the MidCap Index included
companies with capitalizations between approximately $213 million and $13.7
billion. The range of the MidCap Index is expected to change on a regular basis.
Subject to the above policy, the Portfolio may also invest in smaller or larger
issuers.
 
CAPITAL APPRECIATION PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a nondiversified portfolio that pursues its objective by investing primarily in
common stocks of issuers of any size, which may include larger well-established
issuers and/or smaller emerging growth companies.
 
INTERNATIONAL GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a diversified portfolio that pursues its objective primarily through investments
in common stocks of issuers located outside the United States. The Portfolio has
the flexibility to invest on a worldwide basis in companies and other
organizations of any size, regardless of country of organization or place of
principal business activity. The 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 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
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective primarily through investments in common
stocks of foreign and domestic issuers. The Portfolio has the flexibility to
invest on a worldwide basis in companies and other organizations of any size,
regardless of country of organization or place of principal business activity.
Worldwide Growth 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.
 
 16   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
TYPES OF INVESTMENTS
Each of these Portfolios invests primarily in common stocks of foreign and
domestic companies. However, the percentage of each Portfolio's assets invested
in common stocks will vary and each Portfolio may at times hold substantial
positions in cash equivalents or interest bearing securities. See "General
Portfolio Policies" on page 24. Each Portfolio may invest to a lesser degree in
other types of securities including preferred stocks, warrants, convertible
securities and debt securities when its portfolio manager perceives an
opportunity for capital growth from such securities or to receive a return on
idle cash. Some securities that the Portfolios purchase may be on a when-
issued, delayed delivery or forward commitment basis. The Portfolios may invest
up to 25% of their assets in mortgage- and asset-backed securities, up to 10% of
their assets in zero coupon, pay-in-kind and step coupon securities, and without
limit in indexed/ structured securities. No Growth Portfolio will invest 35% or
more of its assets in high-yield/high-risk securities.
 
Although Worldwide Growth Portfolio and International Growth Portfolio are
committed to foreign investing, Growth Portfolio, Aggressive Growth Portfolio
and Capital Appreciation Portfolio may also invest without limit in foreign
equity and debt securities. The Portfolios may invest directly in foreign
securities denominated in a foreign currency and not publicly traded in the
United States. Other ways of investing in foreign securities include depositary
receipts or shares, and passive foreign investment companies ("PFICs"). These
Portfolios may use futures, options and other derivatives for hedging purposes
or for non-hedging purposes such as seeking to enhance return. See "Additional
Risk Factors" on page 26 for a discussion of the risks associated with foreign
investing and derivatives.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE GROWTH PORTFOLIOS.
 
Q:   HOW ARE COMMON STOCKS SELECTED?
                                    
A:   Each of these Portfolios invests substantially all of its assets in common
     stocks to the extent its portfolio manager believes that the relevant
     market environment favors profitable investing in those securities.
Portfolio managers generally take a "bottom up" approach to building their
portfolios. In other words, they seek to identify individual companies with
earnings growth potential that may not be recognized by the market at large.
Although themes may emerge in any Portfolio, securities are generally selected
without regard to any defined industry sector or other similarly defined
selection procedure. Realization of income is not a significant investment
consideration. Any income realized on these Portfolios' investments will be
incidental to their objectives.
 
Q:   ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
                                                             
A:   Generally, yes. Portfolio managers seek companies that meet their selection
     criteria, regardless of country of organization or place of principal
     business activity. Foreign securities are generally selected on a
stock-by-stock basis without regard to any defined allocation among countries or
geographic regions. However, certain

JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    17
<PAGE> 
 
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. See "Additional Risk Factors" on
page 26.
 
Q:   WHAT IS THE MAIN RISK OF INVESTING IN A GROWTH PORTFOLIO?
                                                              
A:   Since the Growth Portfolios usually invest heavily in common stocks, the
     fundamental risk is that the value of the stocks a Portfolio holds might
     decrease. Stock values may fluctuate in response to the activities of an
individual company or in response to general market and economic conditions.
Historically, common stocks have provided greater long-term returns and have
entailed greater short-term risks than other investment choices. Smaller or
newer issuers are more likely to realize more substantial growth as well as
suffer more significant losses than larger or more established issuers.
Investments in such companies can be both more volatile and more speculative.
See "Additional Risk Factors" on page 26.
 
Q:   WHAT IS MEANT BY "MARKET CAPITALIZATION"?
                                              
A:   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 which may invest in small-
to medium-sized companies to a greater degree. Although Growth Portfolio,
Capital Appreciation Portfolio, International Growth Portfolio and Worldwide
Growth Portfolio do not emphasize companies of any particular size, Portfolios
with a larger asset base are more likely to invest in larger, more-established
issuers.
 
Q:   HOW DOES A DIVERSIFIED PORTFOLIO DIFFER FROM A NONDIVERSIFIED PORTFOLIO?
 
A:   Diversification is a means of reducing risk by investing a Portfolio's
     assets 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 share price of a nondiversified portfolio, its share price
can be expected to fluctuate more than a comparable diversified portfolio.
Aggressive Growth Portfolio and Capital Appreciation Portfolio are
nondiversified portfolios.
 
Q:   HOW DO THESE PORTFOLIOS TRY TO REDUCE RISK?
                                                
A:   Diversification of a Portfolio's assets reduces the effect of any single
     holding on its overall portfolio value. A Portfolio may also use futures,
     options and other derivative instruments to protect its portfolio from
movements in securities' prices and interest rates. The Portfolios may use a
variety of currency hedging techniques, including forward currency contracts, to
manage exchange rate risk when investing directly in foreign markets. See
"Additional Risk Factors" on page 26. In addition, to the extent that a
Portfolio holds a larger cash position, it may not participate in market
 
 18   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
declines to the same extent as if the Portfolio remained more fully invested in
common stocks.
 
BALANCED PORTFOLIO AND EQUITY INCOME PORTFOLIO ARE DESIGNED FOR INVESTORS WHO
PRIMARILY SEEK GROWTH OF CAPITAL INCLUDING AN EMPHASIS ON INCOME. THEY ARE NOT
DESIGNED FOR INVESTORS WHO DESIRE A CONSISTENT LEVEL OF INCOME.
 
COMBINATION PORTFOLIOS
Investment Objective:..........................................Growth of Capital
                                                 Including an Emphasis on Income
Primary Holdings:..................Common Stocks and Income-Producing Securities
Shareholder's Investment Horizon:......................................Long-Term
 
BALANCED PORTFOLIO
The investment objective of this Portfolio is long-term capital growth,
consistent with preservation of capital and balanced by current income. It is a
diversified portfolio that, under normal circumstances, pursues its objective by
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 senior securities, which include debt securities and preferred
stocks.
 
EQUITY INCOME PORTFOLIO
The investment objective of this Portfolio is current income and long-term
growth of capital. It is a diversified portfolio that pursues its objective by
normally investing at least 65% of its invested assets in income-producing
equity securities. Equity securities include common stocks, preferred stocks,
warrants and securities convertible into common or preferred stocks. Growth
potential is a significant investment consideration and the Portfolio may hold
securities selected solely for their growth potential.
 
TYPES OF INVESTMENTS
Both of the Combination Portfolios may invest in a combination of common stocks,
preferred stocks, convertible securities, debt securities and other fixed-income
securities. The Combination Portfolios may invest in the types of investments
previously described under "Growth Portfolios" on page 15. Although each of the
Combination Portfolios places some emphasis on the income objective, investors
should keep in mind that the Combination Portfolios are not designed to produce
a consistent level of income.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE COMBINATION PORTFOLIOS.
 
Q:   HOW DO THE COMBINATION PORTFOLIOS DIFFER FROM EACH OTHER?
                                                              
A:   Equity Income Portfolio emphasizes investments in dividend-paying common
     stocks and other equity securities characterized by relatively greater
     price stability. Balanced Portfolio places a greater emphasis on the income
component of its portfolio and invests to a greater degree in debt securities
and preferred stock. As a result, it is expected to be the less volatile of the
Combination Portfolios.
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    19
<PAGE> 
 
Q:   HOW DOES EQUITY INCOME PORTFOLIO TRY TO LIMIT PORTFOLIO VOLATILITY?
                                                                        
A:   Equity Income Portfolio seeks to provide a lower level of volatility than
     the stock market at large, as measured by the S&P 500. The lower volatility
     by this Portfolio is expected to result primarily from investments in
dividend-paying common stocks and other equity securities characterized by
relatively greater price stability. The greater price stability sought by Equity
Income Portfolio may be characteristic of companies that generate above average
free cash flows. A company may use free cash flows for a number of purposes
including commencing or increasing dividend payments, repurchasing its own stock
or retiring outstanding debt. The portfolio manager also considers growth
potential in selecting this Portfolio's securities and may hold securities
selected solely for their growth potential.
 
Q:   HOW ARE EQUITY SECURITIES SELECTED?
                                        
A:   The growth component of Balanced Portfolio is expected to consist primarily
     of common stocks and Equity Income Portfolio invests substantially all of
     its assets in common stocks. The selection criteria for common stocks are
described on page 17. Because income is a part of the investment objective of
the Combination Portfolios, a portfolio manager may consider dividend-paying
characteristics to a greater degree in selecting equity securities for these
Portfolios. The Combination Portfolios may also find opportunities for capital
growth from debt securities because of the anticipated changes in interest
rates, credit standing, currency relationships or other factors.
 
Q:   HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF 
     BALANCED PORTFOLIO?                                                  
 
A:   Balanced Portfolio may shift assets between the growth and income
     components of its portfolio based on its 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.
 
Q:   WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED  
     PORTFOLIO?                                                         
                                                                        
A:   The income component of the 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
Balanced Portfolio if they currently pay dividends or a portfolio manager
believes they have the potential for either increasing their dividends or
commencing dividends, if none are currently paid.
 
 20   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
FLEXIBLE INCOME PORTFOLIO AND HIGH-YIELD PORTFOLIO ARE DESIGNED FOR THOSE
INVESTORS WHO PRIMARILY SEEK CURRENT INCOME.
 
FIXED-INCOME PORTFOLIOS
Investment Objective:
  Flexible Income Portfolio.........................................Total Return
  High-Yield Portfolio....................................................Income
Primary Holdings:....................................Income-Producing Securities
Shareholder's Investment Horizon:
  Flexible Income Portfolio and High-Yield
Portfolio.............................................Intermediate- to Long-Term
 
FLEXIBLE INCOME PORTFOLIO
The investment objective of this Portfolio is to obtain maximum total return,
consistent with preservation of capital. The Portfolio pursues its objective
primarily through investments in income-producing securities. Total return is
expected to result from a combination of current income and capital
appreciation, although income will normally be the dominant component of total
return. As a fundamental policy, this Portfolio will invest at least 80% of its
assets in income-producing securities.
 
Flexible Income Portfolio may invest in a wide variety of income-producing
securities including corporate bonds and notes, government securities,
index/structured securities, preferred stock, income-producing common stocks,
debt securities that are convertible or exchangeable into equity securities, and
debt securities that carry with them the right to acquire equity securities as
evidenced by warrants attached to or acquired with the securities. The Portfolio
may invest to a lesser degree in common stocks, other equity securities or debt
securities that are not currently paying dividends or interest. The Portfolio
may purchase securities of any maturity and quality and the average maturity and
quality of its portfolio may vary substantially.
 
Flexible Income Portfolio may invest without limit in foreign securities,
including those of corporate and government issuers. The Portfolio may invest
without limit in high-yield/high-risk securities and may have substantial
holdings of such securities. The Portfolio may invest without limit in mortgage-
and asset-backed securities and in zero coupon, pay-in-kind and step coupon
securities. The risks of foreign securities and high-yield securities are
described under "Additional Risk Factors" on page 26.
 
The Portfolio may purchase defaulted debt securities if, in the opinion of Janus
Capital it appears likely that the issuer may resume interest payments or other
advantageous developments appear likely in the near term. Defaulted debt
securities may be illiquid and subject to the Portfolio's limit on illiquid
investments.
 
HIGH-YIELD PORTFOLIO
The primary investment objective of this Portfolio is to obtain high current
income. Capital appreciation is a secondary objective when consistent with its
primary objective. Capital appreciation may result, for example, from an
improvement in the credit standing of an issuer whose securities are held by
this Portfolio or from a general lowering of interest rates, or both. This
Portfolio pursues its objectives by investing primarily in high-yield/high-risk
fixed-income securities. This Portfolio will normally invest at least 65% of its
total assets in those securities. In addition, the Portfolio may
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    21
<PAGE> 
 
invest in all of the types of securities previously described under Flexible
Income Portfolio.
 
The high yields sought by this Portfolio are expected to result primarily from
investments in longer-term, lower quality corporate bonds, commonly referred to
as "junk" bonds. This Portfolio considers lower quality securities to be
securities rated below investment grade by established rating agencies or
unrated securities of comparable quality. Securities rated BB or lower by
Standard & Poor's Ratings Services ("Standard & Poor's") or Ba or lower by
Moody's Investors Service, Inc. ("Moody's") are below investment grade. Lower
quality securities are often considered to be more speculative and involve
greater risk of default or price changes due to changes in interest rates,
economic conditions and the issuer's credit-worthiness. As a result, their
market prices tend to fluctuate more than higher quality securities of
comparable maturity. Additional risks of lower quality securities are described
under "Additional Risk Factors" on page 26.
 
TYPES OF INVESTMENTS
In addition to the investment policies described above, each Fixed-Income
Portfolio may purchase securities on a when-issued, delayed delivery or forward
commitment basis. In addition, each Portfolio may use futures, options and other
derivatives for hedging purposes or for non-hedging purposes such as seeking to
enhance return. See "Additional Risk Factors" on page 26. When its portfolio
manager is unable to locate investment opportunities with favorable risk/reward
characteristics, the cash position of any Portfolio may increase and the
Portfolio may have substantial holdings of cash or cash equivalent short-term
obligations. See "General Portfolio Policies" on page 24.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE FIXED-INCOME PORTFOLIOS.
 
Q:   HOW DO INTEREST RATES AFFECT THE VALUE OF MY INVESTMENT?
                                                             
A:   A fundamental risk associated with any fund that invests in fixed-income
     securities (e.g., a bond fund) is the risk that the value of the securities
     it holds will rise or fall as interest rates change. 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 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 portfolio may react to interest rate changes.
 
Q:   WHAT IS MEANT BY A PORTFOLIO'S "AVERAGE-WEIGHTED EFFECTIVE MATURITY"?
                                                                          
A:   The stated maturity of a bond is the date when the issuer must repay the
     bond's entire principal value to an investor, such as a Portfolio. Some
     types of bonds,
 
 22   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
such as mortgage-backed securities, which are subject to prepayment risk, and
securities with call provisions, may also have an "effective maturity" that is
shorter than the stated date. With respect to GNMA securities and other
mortgage-backed securities, effective maturity is likely to be substantially
less than the stated maturities of the mortgages in the underlying pools. With
respect to obligations with call provisions, effective maturity is typically the
next call date on which the obligation reasonably may be expected to be called.
Securities without prepayment or call provisions generally have an effective
maturity equal to their stated maturity. Dollar-weighted effective maturity is
calculated by averaging the effective maturity of bonds held by a Portfolio with
each effective maturity "weighted" according to the percentage of net assets
that it represents.
 
Q:   WHAT IS MEANT BY A PORTFOLIO'S "DURATION"?
                                               
A:   A bond's duration indicates the time it will take an investor to recoup his
     or her 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 fund is calculated
by averaging the duration of bonds held by a Portfolio with each duration
"weighted" according to the percentage of net assets that it represents. Because
duration accounts for interest payments, a Portfolio's duration is usually
shorter than its average maturity.
 
Q:   HOW DO FLEXIBLE INCOME PORTFOLIO AND HIGH-YIELD PORTFOLIO MANAGE INTEREST
     RATE RISK?                                                               
                                                                              
A:   Each of these Portfolios may vary the average-weighted effective maturity
     of its portfolio to reflect its portfolio manager's analysis of interest
     rate trends and other factors. A Portfolio's average-weighted effective
maturity will tend to be shorter when its portfolio manager expects interest
rates to rise and longer when its portfolio manager expects interest rates to
fall. The Portfolios may also use futures, options and other derivatives to
manage interest rate risk. See "Additional Risk Factors" on page 26.
 
Q:   WHAT IS MEANT BY "CREDIT QUALITY"?
 
A:   Credit quality measures the likelihood that the issuer will meet its
     obligations on a bond. One of the fundamental risks associated with all
     bond 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.
 
Q:   HOW IS CREDIT QUALITY MEASURED?
                                    
A:   Ratings published by nationally recognized rating agencies such as Standard
     & Poor's and Moody's 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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    23
<PAGE> 
 
Lower rated bonds generally pay higher yields to compensate investors for the
associated risk. Please refer to Appendix B for a description of rating
categories.
 
Q:   WHAT IS A HIGH-YIELD/HIGH-RISK SECURITY?
A:   A high-yield 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.
 
Q:   WHAT RISKS DO HIGH-YIELD/HIGH-RISK SECURITIES PRESENT?
A:   High-yield securities are often considered to be more speculative and
     involve greater risk of default or price changes due to changes in economic
     and industry conditions and the issuer's creditworthiness. Their market
prices tend to fluctuate more than higher quality securities as a result of
changes in these factors.
 
The default rate of lower quality debt securities is likely to be higher when
issuers have difficulty meeting projected goals or obtaining additional
financing. This could occur during economic recessions or periods of high
interest rates. In addition, there may be a smaller market for lower quality
securities than for higher quality securities, making lower quality securities
more difficult to sell promptly at an acceptable price.
 
The junk bond market can experience sudden and sharp price swings. Because
Flexible Income Portfolio and High-Yield Portfolio may invest a significant
portion of their portfolios in high-yield/high-risk securities, investors in
such Portfolios should be willing to tolerate a corresponding increase in the
risk of significant and sudden changes in NAV.
 
GENERAL PORTFOLIO POLICIES OF THE PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
Unless otherwise stated, each of the following policies applies to all of the
Portfolios other than the Money Market Portfolio. The percentage limitations
included in these policies and elsewhere in this Prospectus apply only at the
time of purchase of the security. 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's manager believes that market conditions are not favorable for
profitable investing or when the portfolio manager is otherwise unable to locate
favorable investment opportunities, a Portfolio's investments may be hedged to a
greater degree and/or its 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 are a residual - they represent the assets that
remain after a portfolio manager has committed available assets to desirable
investment opportunities. Partly because the portfolio managers act
independently of each other, the cash positions of the Portfolios may vary
significantly. Larger hedged positions and/or larger cash positions may serve as
a means of preserving capital in unfavorable market conditions.

 24   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
Securities that the Portfolios may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolios may
also invest in money market funds (including funds managed by Janus Capital).
When a Portfolio is hedged or its investments in cash or similar investments
increase, it may not participate in stock or bond market advances or declines to
the same extent that it would if the Portfolio was not hedged or remained more
fully invested in stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. All of the Portfolios (except
Aggressive Growth and Capital Appreciation Portfolio) qualify as diversified
funds under the 1940 Act and are subject to the following diversification
requirements:
 
- - As a fundamental policy, no Portfolio may own more than 10% of the outstanding
  voting shares of any issuer.
 
- - As a fundamental policy, with respect to 50% of the total assets of Aggressive
  Growth Portfolio and Capital Appreciation Portfolio and 75% of the total
  assets of the other Portfolios, no Portfolio will purchase a security of any
  issuer (other than cash items and U.S. government securities, as defined in
  the 1940 Act) if such purchase would cause a Portfolio's holdings of that
  issuer to amount to more than 5% of that Portfolio's total assets.
 
- - No Portfolio will invest more than 25% of its total assets in a single issuer
  (other than U.S. government securities).
 
- - Aggressive Growth Portfolio and Capital Appreciation Portfolio reserve the
  right to become diversified portfolios by limiting the investments in which
  more than 5% of their total net assets are invested.
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, each Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, no Portfolio will invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
Each Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may 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 initial investment
decision. Changes are made in a Portfolio whenever its portfolio manager
believes such changes are desirable. Portfolio turnover rates are generally not
a factor in making buy and sell decisions.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    25
<PAGE> 
 
To a limited extent, a Portfolio may purchase securities in anticipation of
relatively short-term price gains. A Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
Each Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of a Portfolio's net
assets after the time of purchase, the Portfolio will take steps to reduce in an
orderly fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public because of
their terms or because of SEC regulations. Janus Capital will follow guidelines
established by the Trustees of the Trust ("Trustees") in making liquidity
determinations for Rule 144A securities and other securities, including
privately placed commercial paper and municipal lease obligations.
 
BORROWING AND LENDING
Each Portfolio may borrow money and lend securities or other assets, as follows:
 
- - Each Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - Each Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, each Portfolio may lend securities or other assets
  if, as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, each Portfolio may
borrow money from or lend money to each other and other funds that permit such
transactions and for which Janus Capital serves as investment adviser. All such
borrowing and lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS OF THE PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. A Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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
 
 26   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
  denominated security, its value may be worth less in U.S. dollars even though
  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 underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolios may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk, because their values depend
on the performance of a foreign security denominated in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or potential development, or they may 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 be 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 wider
price fluctuations. Investments in such companies tend to be more volatile and
somewhat more speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
Each Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    27
<PAGE> 
 
options on securities, financial indices and foreign currencies ("options"),
forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolios intend to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolios may also use derivative instruments for non-hedging
purposes such as seeking to increase a Portfolio's income or otherwise seeking
to enhance return. Please refer to Appendix A to this Prospectus and the SAI for
a more detailed discussion of these instruments.
 
The use of derivative instruments exposes the Portfolios to additional
investment risks and transaction costs. Risks inherent in the use of derivative
instruments include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that a portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than a Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately-negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave a
  Portfolio worse off than if it had not entered into the position.
 
Although the portfolios' managers believe the use of derivative instruments will
benefit the Portfolios, a Portfolio's performance could be worse than if the
Portfolio had not used such instruments if the portfolio manager's judgement
proves incorrect.
 
When a Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for
 
 28   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
higher quality securities. Conversely, the value of higher quality securities
may be more sensitive to interest rate movements than lower rated securities.
Issuers of high-yield/ high-risk securities may not be as strong financially as
those issuing bonds with higher credit ratings. Investments in such companies
are considered to be more speculative than higher quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, a Portfolio would
experience a reduction of its income and could expect a decline in the market
value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as higher
quality securities. Sovereign debt of foreign governments is generally rated by
country. Because these ratings do not take into account individual factors
relevant to each issue and may not be updated regularly, Janus Capital may treat
such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, a Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolios may have to sell securities
to have sufficient cash to pay the dividends.
 
Please refer to Appendix B for a description of bond rating categories.
 
SHORT SALES
Each Portfolio may engage in "short sales against the box." This technique
involves selling either a security that a Portfolio owns, or a security
equivalent in kind and amount that a Portfolio has the right to obtain, for
delivery at a specified date in the future. A Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
SPECIAL SITUATIONS
Each Portfolio may invest in "special situations" from time to time. 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    29
<PAGE> 
 
security. Investment in special situations may carry an additional risk of loss
in the event that the anticipated development does not occur or does not attract
the expected attention.
 
See Appendix A for risks associated with certain other investments.
 
MONEY MARKET PORTFOLIO IS DESIGNED FOR INVESTORS WHO PRIMARILY SEEK MAXIMUM
CURRENT INCOME TO THE EXTENT CONSISTENT WITH STABILITY OF CAPITAL.
 
MONEY MARKET PORTFOLIO
 
INVESTMENT OBJECTIVE
The Portfolio's investment objective is to seek maximum current income to the
extent consistent with stability of capital. There can be no assurance that the
Portfolio will achieve its investment objective or be able to maintain a stable
net asset value of $1.00 per share.
 
INVESTMENT POLICIES
The Portfolio will invest only in eligible high quality, short-term money market
instruments that present minimal credit risks, as determined by Janus Capital,
the Portfolio's investment adviser, pursuant to procedures adopted by the
Trustees. The Portfolio may invest only in U.S. dollar-denominated instruments
that have a remaining maturity of 397 days or less (as calculated pursuant to
Rule 2a-7 under the Investment Company Act of 1940) and will maintain a
dollar-weighted average portfolio maturity of 90 days or less.
 
Except to the limited extent permitted by Rule 2a-7 and except for U.S.
Government Securities (as defined below), the Portfolio will not invest more
than 5% of its total assets in the securities of any one issuer. Investment in
demand features, guarantees and other types of instruments are subject to the
diversification limits under Rule 2a-7. The Portfolio may not invest more than
25% of its total assets in any one industry, except that this limit does not
apply to U.S. Government Securities, bank obligations or municipal securities.
To ensure adequate liquidity, the Portfolio may not invest more than 10% of its
net assets in illiquid securities, including repurchase agreements maturing in
more than seven days (unless subject to a demand feature) and certain time
deposits that are subject to early withdrawal penalties and mature in more than
seven days. Janus Capital determines and monitors the liquidity of portfolio
securities under the supervision of the Trustees.
 
RATINGS
High quality money market instruments include those that (i) are rated (or, if
unrated, are issued by an issuer with comparable outstanding short-term debt
that is rated) in one of the two highest rating categories for short-term debt
by any two nationally recognized statistical rating organizations ("NRSROs") or,
if only one NRSRO has issued a rating, by that NRSRO or (ii) are otherwise
unrated and determined by Janus Capital to be of comparable quality. The
Portfolio will invest at least 95% of its total assets in securities in the
highest rating category (as determined pursuant to Rule 2a-7). Descriptions of
the rating categories of Standard & Poor's, Moody's, and certain other NRSROs
are
 
 30   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
contained in Appendix B. A further description of the Money Market Portfolio's
investment policies is included in the Money Market Portfolio's SAI.
 
Although the Portfolio only invests in high quality money market instruments, an
investment in the Portfolio is subject to risk even if all securities in its
portfolio are paid in full at maturity. All money market instruments, including
U.S. Government Securities, can change in value as a result of changes in
interest rates, the issuer's actual or perceived creditworthiness or the
issuer's ability to meet its obligations.
 
TYPES OF INVESTMENTS
The Portfolio pursues its objective by investing primarily in high quality debt
obligations and obligations of financial institutions. It may invest in U.S.
Government Securities (as defined below) and municipal securities, although the
Portfolio expects to invest in such securities to a lesser degree.
 
DEBT OBLIGATIONS
The Portfolio may invest in debt obligations of domestic issuers, including
commercial paper (short-term promissory notes issued by companies to finance
their, or their affiliates', current obligations), notes and bonds, and variable
amount master demand notes. The payment obligations on these instruments may be
backed by securities, swap agreements or other assets, by a guarantee of a third
party or solely by the unsecured promise of the issuer to make payments when
due. The Portfolio may invest in privately issued commercial paper or other
securities that are restricted as to disposition under the federal securities
laws. In general, sales of these securities may not be made absent registration
under the Securities Act of 1933 (the "1933 Act") or the availability of an
appropriate exemption therefrom. Pursuant to Section 4(2) of the 1933 Act or
Rule 144A adopted under the 1933 Act, however, some of these securities are
eligible for resale to institutional investors, and accordingly, Janus Capital
may determine that a liquid market exists for such a security pursuant to
guidelines adopted by the Trustees.
 
OBLIGATIONS OF FINANCIAL INSTITUTIONS
The Portfolio may invest in obligations of financial institutions. Examples of
obligations in which it may invest include negotiable certificates of deposit,
bankers' acceptances, time deposits and other obligations of U.S. banks
(including savings and loan associations) having total assets in excess of one
billion dollars and U.S. branches of foreign banks having total assets in excess
of ten billion dollars. The Portfolio may also invest in Eurodollar and Yankee
bank obligations as discussed below and other U.S. dollar-denominated
obligations of foreign banks having total assets in excess of ten billion
dollars that Janus Capital believes are of an investment quality comparable to
obligations of U.S. banks in which the Portfolio may invest.
 
Certificates of deposit represent an institution's obligation to repay funds
deposited with it that earn a specified interest rate over a given period.
Bankers' acceptances are negotiable obligations of a bank to pay a draft which
has been drawn by a customer and are usually backed by goods in international
trade. Time deposits are non-negotiable deposits with a banking institution that
earn a specified interest rate over a given period. Fixed time deposits, which
are payable at the stated maturity date and bear a fixed rate of interest,
generally may be withdrawn on demand by the Portfolio but may be subject to
early withdrawal penalties that could reduce the Portfolio's yield. Unless there
is a
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    31
<PAGE> 
 
readily available market for them, time deposits that are subject to early
withdrawal penalties and that mature in more than seven days will be treated as
illiquid securities.
 
Eurodollar bank obligations are dollar-denominated certificates of deposit or
time deposits issued outside the U.S. capital markets by foreign branches of
U.S. banks and by foreign banks. Yankee bank obligations are dollar-denominated
obligations issued in the U.S. capital markets by foreign banks.
 
Foreign, Eurodollar (and to a limited extent, Yankee) bank obligations are
subject to certain sovereign risks. One such risk is the possibility that a
foreign government might prevent dollar-denominated funds from flowing across
its borders. Other risks include: adverse political and economic developments in
a foreign country; the extent and quality of government regulation of financial
markets and institutions; the imposition of foreign withholding taxes; and
expropriation or nationalization of foreign issuers.
 
U.S. GOVERNMENT SECURITIES
The Portfolio may invest without limit in U.S. Government Securities. U.S.
Government Securities shall have the meaning set forth in the 1940 Act. The 1940
Act defines U.S. Government Securities to include securities issued or
guaranteed by the U.S. government, its agencies and instrumentalities. U.S.
Government Securities may also include repurchase agreements collateralized by
and municipal securities escrowed with or refunded with U.S. Government
Securities. U.S. Government Securities in which the Portfolio may invest include
U.S. Treasury securities and obligations issued or guaranteed by U.S. government
agencies and instrumentalities that are backed by the full faith and credit of
the U.S. government, such as those guaranteed by the Small Business
Administration or issued by the Government National Mortgage Association. In
addition, U.S. Government Securities in which the Portfolio may invest include
securities supported primarily or solely by the creditworthiness of the issuer,
such as securities of the Federal National Mortgage Association, the Federal
Home Loan Mortgage Corporation and the Tennessee Valley Authority. There is no
guarantee that the U.S. government will support securities not backed by its
full faith and credit. Accordingly, although these securities have historically
involved little risk of loss of principal if held to maturity, they may involve
more risk than securities backed by the full faith and credit of the U.S.
government.
 
MUNICIPAL SECURITIES
The municipal securities in which the Portfolio may invest include municipal
notes and short-term municipal bonds. Municipal notes are generally used to
provide for the issuer's short-term capital needs and generally have maturities
of 397 days or less. The Portfolio may also invest in high quality participation
interests in municipal securities. A more detailed description of various types
of municipal securities is contained in Appendix B in the SAI.
 
Yields on municipal securities are dependent on a variety of factors, including
the general conditions of the money market and of the municipal bond and
municipal note markets, the size of a particular offering, the maturity of the
obligation and the rating of the issue. Obligations of issuers of municipal
securities are subject to the provisions of bankruptcy, insolvency and other
laws affecting the rights and remedies of creditors, such as the Bankruptcy
Reform Act of 1978, as amended. Therefore, the possibility exists that, as a
result of litigation or other conditions, the ability of any issuer to pay,
 
 32   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
when due, the principal of and interest on its municipal securities may be
materially affected.
 
PARTICIPATION INTERESTS
The Portfolio may invest in participation interests in any type of security in
which the Portfolio may invest. A participation interest gives a Portfolio an
undivided interest in the underlying securities in the proportion that the
Portfolio's participation interest bears to the total principal amount of the
underlying securities. Participation interests usually carry a demand feature,
as described below, backed by a letter of credit or guarantee of the institution
that issued the interests permitting the holder to tender them back to the
institution.
 
DEMAND FEATURES
The Portfolio may invest in securities that are subject to puts and stand-by
commitments ("demand features"). Demand features give the Portfolio the right to
resell securities at specified periods prior to their maturity dates to the
seller or to some third party at an agreed-upon price or yield. Securities with
demand features may involve certain expenses and risks, including the inability
of the issuer of the instrument to pay for the securities at the time the
instrument is exercised, non-marketability of the instrument and differences
between the maturity of the underlying security and the maturity of the
instrument. Securities may cost more with demand features than without them.
Demand features can serve three purposes: to shorten the maturity of a variable
or floating rate security, to enhance the instrument's credit quality and to
provide a source of liquidity. Demand features are often issued by third party
financial institutions, generally domestic and foreign banks. Accordingly, the
credit quality and liquidity of the Portfolio's investments may be dependent in
part on the credit quality of the banks supporting its investments. This will
result in exposure to risks pertaining to the banking industry, including the
foreign banking industry. Brokerage firms and insurance companies also provide
certain liquidity and credit support.
 
VARIABLE AND FLOATING RATE SECURITIES
The securities in which the Portfolio invests may have variable or floating
rates of interest. These securities pay interest at rates that are adjusted
periodically according to a specified formula, usually with reference to some
interest rate index or market interest rate. Securities with ultimate maturities
of greater than 397 days may be purchased only pursuant to Rule 2a-7. Under that
Rule, only those long-term instruments that have demand features which comply
with certain requirements and certain variable rate U.S. Government Securities
may be purchased. Similar to fixed rate debt instruments, variable and floating
rate instruments are subject to changes in value based on changes in market
interest rates or changes in the issuer's or guarantor's creditworthiness. The
rate of interest on securities purchased by the Portfolio may be tied to
short-term Treasury or other government securities or indices on securities that
are permissible investments of the Portfolio, as well as other money market
rates of interest. The Portfolio will not purchase securities whose values are
tied to interest rates or indices that are not appropriate for the duration and
volatility standards of a money market fund.
 
MORTGAGE- AND ASSET-BACKED SECURITIES
The Portfolio may purchase fixed or adjustable rate mortgage-backed securities
issued by the Government National Mortgage Association, Federal National
Mortgage Association
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    33
<PAGE> 
 
or the Federal Home Loan Mortgage Corporation, or other governmental or
government-related entities. In addition, the Portfolio may purchase other
asset-backed securities, including securities backed by automobile loans,
equipment leases or credit card receivables. These securities directly or
indirectly represent a participation in, or are secured by and payable from,
fixed or adjustable rate mortgage or other loans which may be secured by real
estate or other assets. Unlike traditional debt instruments, payments on these
securities include both interest and a partial payment of principal. Prepayments
of the principal of underlying loans may shorten the effective maturities of
these securities and may result in the Portfolio having to reinvest proceeds at
a lower interest rate.
 
REPURCHASE AGREEMENTS
The Portfolio may seek additional income by entering into collateralized
repurchase agreements. Repurchase agreements are transactions in which the
Portfolio purchases securities and simultaneously commits to resell those
securities to the seller at an agreed-upon price on an agreed-upon future date.
The resale price reflects a market rate of interest that is not related to the
coupon rate or maturity of the purchased securities.
 
REVERSE REPURCHASE AGREEMENTS
The Portfolio may enter into reverse repurchase agreements. Reverse repurchase
agreements are transactions in which the Portfolio sells a security and
simultaneously commits to repurchase that security from the buyer at an agreed
upon price on an agreed upon future date. This technique will be used primarily
for temporary or emergency purposes, such as meeting redemption requests.
 
DELAYED DELIVERY SECURITIES
The Portfolio may purchase securities on a when-issued or delayed delivery
basis. Securities so purchased are subject to market price fluctuation from the
time of purchase but no interest on the securities accrues to the Portfolio
until delivery and payment for the securities take place. Accordingly, the value
of the securities on the delivery date may be more or less than the purchase
price. Forward commitments will be entered into only when the Portfolio has the
intention of taking possession of the securities, but it may sell the securities
before the settlement date if deemed advisable.
 
BORROWING AND LENDING
The Portfolio may borrow money for temporary or emergency purposes in amounts up
to 25% of its total assets. It may not mortgage or pledge securities except to
secure permitted borrowings. As a fundamental policy, the Portfolio will not
lend securities or other assets if, as a result, more than 25% of its total
assets would be lent to other parties. The Portfolio does not currently intend
to engage in securities lending; however, under the terms of an exemptive order
received from the SEC, the Portfolio may borrow money from or lend money to
other funds that permit such transactions and are advised by Janus Capital.
 
 34   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                                 MANAGEMENT OF
                                 THE PORTFOLIOS
 
TRUSTEES
 
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to each Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolios to the
officers of the Trust and meet at least quarterly to review the Portfolios'
investment policies, performance, expenses and other business affairs.
 
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 has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
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. From
1989 to 1993, he was a Project Director for the National Security Archives, a
nonprofit research organization. He is a Chartered Financial Analyst.
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    35
<PAGE> 
 
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. Mr. Craig previously managed Balanced Portfolio from
September 1993 through April 1996. 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 and Janus
Balanced Fund from December 1993 to December 1995. 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.
 
SHARON S. PICHLER is Executive Vice President and portfolio manager of Money
Market Portfolio, which she has managed since inception. She also has managed
Janus Money Market Fund, Janus Government Money Market Fund and Janus Tax-Exempt
Money Market Fund since inception. She holds a Bachelor of Arts in Economics
from Michigan State University and a Master of Business Administration from the
University of Texas at San Antonio. Ms. Pichler is a Chartered Financial
Analyst.
 
BLAINE P. ROLLINS is Executive Vice President and portfolio manager of Balanced
Portfolio, which he has managed since May 1996 and Equity Income Portfolio,
which he has managed since inception. He is an assistant portfolio manager of
Growth Portfolio. Mr. Rollins joined Janus Capital in 1990 and has managed Janus
Balanced Fund since January 1996 and Janus Equity Income Fund since June 1996.
He has been an assistant portfolio manager of Janus Fund since January 1995. He
gained experience as a fixed-income trader and equity research analyst prior to
managing Balanced Portfolio. He holds a Bachelor of Science in Finance from the
University of Colorado and is a Chartered Financial Analyst.
 
SANDY R. RUFENACHT is Executive Vice President and portfolio manager of
High-Yield Portfolio, which he has managed or co-managed since October 1996. He
is also the Executive Vice President and portfolio manager of Short-Term Bond
Portfolio which he has managed since May 1996. He previously co-managed Flexible
Income Portfolio from January 1997 to May 1998. Mr. Rufenacht joined Janus
Capital in 1990 and has managed Janus Short-Term Bond Fund since January 1996.
He is also the portfolio manager of Janus High-Yield Fund. He previously
co-managed Janus Flexible Income Fund from June 1996 to February 1998. He holds
a Bachelor of Arts in Business from the University of Northern Colorado.
 
 36   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
SCOTT W. SCHOELZEL is Executive Vice President and portfolio manager of Capital
Appreciation Portfolio, which he has managed since its inception. He is
portfolio manager of Janus Twenty Fund, which he has managed since August 1997.
He previously managed Janus Olympus Fund from its inception to August 1997. Mr.
Schoelzel joined Janus Capital in January 1994. From 1991 to 1993, Mr. Schoelzel
was a portfolio manager at Founders Asset Management, Denver, Colorado. He holds
a Bachelor of Arts in Business from Colorado College.
 
RONALD V. SPEAKER is Executive Vice President and portfolio manager of Flexible
Income Portfolio which he has managed or co-managed since its inception. He
previously served as co-manager of High-Yield Portfolio, from its inception to
May 1998. He managed Short-Term Bond Portfolio from its inception through April
1996. Mr. Speaker joined Janus Capital in 1986. He has managed or co-managed
Janus Flexible Income Fund since December 1991 and previously managed both Janus
Short-Term Bond Fund and Janus Federal Tax-Exempt Fund from inception through
December 1995. He previously managed or co-managed Janus High-Yield Fund from
its inception to May 1998. He holds a Bachelor of Arts in Finance from the
University of Colorado and is a Chartered Financial Analyst.
 
In January 1997, Mr. Speaker settled an SEC administrative action involving two
personal trades made by him in January of 1993. Without admitting or denying the
allegations, Mr. Speaker agreed to civil money penalty, disgorgement, and
interest payments totaling $37,199 and to a 90-day suspension which ended on
April 25, 1997.
 
ASSISTANT PORTFOLIO 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.
 
PERSONAL INVESTING
 
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment activities
in a manner that Janus Capital believes is not detrimental to the Portfolios or
Janus Capital's other advisory clients. See the SAI for more detailed
information.
 
PORTFOLIO TRANSACTIONS
 
Purchases and sales of securities on behalf of each Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for a Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    37
<PAGE> 
 
portfolio transactions. Janus Capital may also consider payments made by brokers
effecting transactions for a Portfolio i) to the Portfolio or ii) to other
persons on behalf of the Portfolio for services provided to the Portfolio for
which it would be obligated to pay. The Portfolios' Trustees have authorized
Janus Capital to place portfolio transactions on an agency basis with a
broker-dealer affiliated with Janus Capital. When transactions for a Portfolio
are effected with that broker-dealer, the commissions payable by the Portfolio
are credited against certain Portfolio operating expenses serving to reduce
those expenses. The SAI further explains the selection of broker-dealers.
 
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):
 
<TABLE>
<CAPTION>
                                     AVERAGE DAILY NET    ANNUAL RATE    EXPENSE LIMIT
             FEE SCHEDULE           ASSETS 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
    -----------------------------------------------------------------------------------
    Aggressive Growth Portfolio     First $300 Million        0.75       N/A(1)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    Capital Appreciation Portfolio  First $300 Million        0.75       1.25(1)(2)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    International Growth Portfolio  First $300 Million        0.75       N/A(1)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    Worldwide Growth Portfolio      First $300 Million        0.75       N/A(1)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    Balanced Portfolio              First $300 Million        0.75       N/A(1)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    Equity Income Portfolio         First $300 Million        0.75       1.25(1)(2)
                                    Next $200 Million         0.70
                                    Over $500 Million         0.65
    -----------------------------------------------------------------------------------
    Flexible Income Portfolio       First $300 Million        0.65       1.00(3)
                                    Over $300 Million         0.55
    -----------------------------------------------------------------------------------
    High-Yield Portfolio            First $300 Million        0.75       1.00(3)
                                    Over $300 Million         0.65
    -----------------------------------------------------------------------------------
    Money Market Portfolio          All Asset Levels          0.25       0.50(3)
    -----------------------------------------------------------------------------------
</TABLE>
 
 (1) Janus Capital has agreed to reduce Growth, Aggressive Growth, Capital
     Appreciation, International Growth, Worldwide Growth, Balanced, and Equity
     Income Portfolio's advisory fee to the extent that such fee exceeds the
     effective rate of the Janus retail fund corresponding to such Portfolio.
     Janus Capital may terminate this fee reduction or any of the expense
     limitations set forth above at any time upon at least 90 days' notice to
     the Trustees. The effective rate is the advisory fee calculated by the
     corresponding retail fund as of the last day of each calendar quarter
     (expressed as an annual rate). The effective rates of Janus Fund, Janus
     Enterprise Fund, Janus Olympus Fund, Janus Overseas Fund, Janus Worldwide
     Fund, Janus Balanced Fund and Janus Equity Income Fund were .65%, .72%,
     .70%, .66%, .65%, .73% and .75%, respectively, for the quarter ended March
     31, 1998.
 (2) Janus Capital has agreed to limit the expenses of Capital Appreciation
     Portfolio and Equity Income Portfolio to 1.25% of average net assets
     through at least May 1, 1999.
 (3) Janus Capital may modify or terminate the expense limitation at any time
     upon at least 90 days' notice to the Trustees.
- --------------------------------------------------------------------------------
 
 38   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
Differences in the actual management fees incurred by the Portfolios are due
primarily to variances in the asset sizes of the corresponding retail funds. As
asset size increases, the annual rate of the management fee rate declines in
accordance with the above schedule (except for the Money Market Portfolio). 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. The fee schedule for the Growth and Combination Portfolios was
effective July 1, 1997.
 
During the most recent fiscal year (or period) the Portfolios paid the following
management fees expressed as a percentage of average net assets: Growth
Portfolio .65%; Aggressive Growth Portfolio .73%; Capital Appreciation Portfolio
 .74%; International Growth Portfolio .67%; Worldwide Growth Portfolio .66%;
Balanced Portfolio .76%; and Equity Income Portfolio .81%.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolios with administrative and other
services.
 
CUSTODIAN FOR PORTFOLIOS OTHER THAN MONEY MARKET PORTFOLIO
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
CUSTODIAN FOR MONEY MARKET PORTFOLIO
UMB Bank, N.A.
P.O. Box 419226
Kansas City, Missouri 64141-6226
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust offers Shares in twelve separate series, ten of which are offered by this
Prospectus.
 
Each Portfolio currently offers two classes of Shares, one of which, the
Institutional Shares, are offered pursuant to this prospectus and are sold under
the name Janus Aspen Series. The Shares offered by this Prospectus are available
only in connection with investment in and payments under variable insurance
contracts as well as certain qualified retirement plans. Retirement Shares of
each Portfolio are offered by separate prospectus and are available only to
participant directed qualified plans using plan service providers
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    39
<PAGE> 
 
that are compensated for providing distribution and/or recordkeeping and other
administrative services provided to plan participants. Because the expenses of
each class may differ, the performance of each class is expected to differ. If
you would like additional information about the Retirement Shares, please call
1-800-525-0020.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific class or Portfolio or for the Trust as a
whole for purposes such as electing or removing Trustees, terminating or
reorganizing the Trust, changing fundamental policies, or for any other purpose
requiring a shareholder vote under the 1940 Act. Separate votes are taken by
each class or Portfolio only if a matter affects or requires the vote of only
that class or Portfolio or the interest of a class or Portfolio in the matter
differs from the interest of the other class or Portfolios of the Trust. As a
shareholder, you are entitled to one vote for each share that you own.
 
An insurance company issuing a variable contract invested in Shares of a
Portfolio will request voting instructions from variable contract holders. Under
current law, the insurance company must vote all Shares held by the separate
account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Each Portfolio's Shares are available only to variable annuity and variable life
separate accounts of insurance companies that are unaffiliated with Janus
Capital and to certain qualified retirement plans. Retirement Shares of the
Portfolios (offered through a separate prospectus) are available to certain
participant directed qualified plans. Although the Portfolios do not currently
anticipate any disadvantages to policy owners arising out of the fact that 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 anticipated disadvantages or material irreconcilable conflicts 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 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 securities at disadvantageous prices. In addition, the Trustees
may refuse to sell shares of any Portfolio to any separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve any Portfolio's investment objective
by investing all of that Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as
 
 40   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
those applicable to that Portfolio. Unless otherwise required by law, this
policy may be implemented by the Trustees without shareholder approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolios with
service at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of a Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per Share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of Shares outstanding.
 
For the Portfolios other than the Money Market Portfolio, 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.
 
For the Money Market Portfolio, portfolio securities are valued at their
amortized cost. Amortized cost valuation involves valuing an instrument at its
cost and thereafter assuming a constant amortization to maturity (or such other
date as permitted by Rule 2a-7) of any discount or premium. If fluctuating
interest rates cause the market value of a portfolio to deviate more than 1/2 of
1% from the value determined on the basis of amortized cost, the Trustees will
consider whether any action, such as adjusting the Share's NAV to reflect
current market conditions, should be initiated to prevent any material dilutive
effect on shareholders.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    41
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS.
 
PORTFOLIOS OTHER THAN MONEY
MARKET PORTFOLIO
 
Each class of each Portfolio, other than Money Market Portfolio, makes
semi-annual distributions in June and December of substantially all of its
investment income and an annual distribution in June of its net realized gains,
if any. All dividends and capital gains distributions from Shares of a Portfolio
will automatically be reinvested into additional Shares of that Portfolio.
 
HOW DISTRIBUTIONS AFFECT NAV
Distributions, other than daily income dividends, are paid to shareholders as of
the record date of the distribution of a Portfolio, regardless of how long the
shares have been held. Undistributed income and realized gains are included in
the daily NAV of a Portfolio's Shares. The Share price of a Portfolio drops by
the amount of the distribution, net of any subsequent market fluctuations. For
example, assume that on December 31, the Shares of Growth Portfolio declared a
dividend in the amount of $0.25 per share. If the price of Growth Portfolio's
Shares was $10.00 on December 30, the Share price on December 31 would be $9.75,
barring market fluctuations.
 
MONEY MARKET PORTFOLIO
For the Shares of Money Market Portfolio, dividends representing substantially
all of the net investment income and any net realized gains on sales of
securities are declared daily, Saturdays, Sundays and holidays included, and
distributed on the last business day of each month. If a month begins on a
Saturday, Sunday or holiday, dividends for those days are declared at the end of
the preceding month and distributed on the first business day of the month. All
distributions will be automatically reinvested in Shares of the Portfolio.
 
TAXES
TAXES ON DISTRIBUTIONS
Because Shares of the Portfolios may be purchased only through variable
insurance contracts and qualified plans, it is anticipated that any income
dividends or capital gains distributions made by the Shares of a Portfolio will
be exempt from current taxation if left to accumulate within the variable
insurance contract or qualified plan. Generally, withdrawals from such contracts
may be subject to ordinary income tax and, if made before age 59 1/2, a 10%
penalty tax. The tax status of your investment in the Shares
 
 42   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
depends on the features of the variable insurance contracts purchased from a
participating insurance company. Further information may be found 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 as a foreign tax credit.
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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    43
<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. 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.
 
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.
 
All investments in the Portfolios are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by a Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by a Portfolio.
 
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. Any Portfolio may
discontinue sales of its shares if management believes that a substantial
further increase may adversely affect that Portfolio's ability to achieve its
investment objective. In such event, however, it is anticipated that existing
policy owners and plan participants invested in that Portfolio would be
permitted to continue to authorize investment in such Portfolio and to reinvest
any dividends or capital gains distributions.
 
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. 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.
 
 44   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    45
<PAGE> 
 
                                   APPENDIX A
 
GLOSSARY OF INVESTMENT TERMS
This glossary provides a more detailed description of some of the types of
securities and other instruments in which the non-Money Market Portfolios may
invest. These Portfolios may invest in these instruments to the extent permitted
by their investment objective 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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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
 
 46   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
through to shareholders on a pro rata basis. These securities involve prepayment
risk, which is the risk that the underlying mortgages or other debt may be
refinanced or paid off prior to their maturities during periods of declining
interest rates. In that case, a portfolio manager may have to reinvest the
proceeds from the securities at a lower rate. Potential market gains on a
security subject to prepayment risk may be more limited than potential market
gains on a comparable security that is not subject to prepayment risk.
 
PASSIVE FOREIGN INVESTMENT COMPANIES (PFICS) are any foreign corporations which
generate certain amounts of passive income or hold certain amounts of assets for
the production of passive income. Passive income includes dividends, interest,
royalties, rents and annuities. To avoid taxes and interest that the Portfolios
must pay if these investments are profitable, the Portfolios may make various
elections permitted by the tax laws. These elections could require that the
Portfolios recognize taxable income, which in turn must be distributed, before
the securities are sold and before cash is received to pay the distributions.
 
PAY-IN-KIND BONDS are debt securities that normally give the issuer an option to
pay cash at a coupon payment date or give the holder of the security a similar
bond with the same coupon rate and a face value equal to the amount of the
coupon payment that would have been made.
 
PREFERRED STOCK is a class of stock that generally pays dividends at a specified
rate and has 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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    47
<PAGE> 
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a specified formula, usually with reference to some interest rate
index or market 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
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 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 of comparable 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
 
 48   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
Portfolios may enter into forward currency contracts to hedge against declines
in the value of non-dollar denominated securities or to reduce the impact of
currency appreciation on purchases of non-dollar denominated 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 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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    49
<PAGE> 
 
                                   APPENDIX B
 
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 the adviser 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>
<CAPTION>
BOND RATING          EXPLANATION
- ------------------------------------------------------------------------
<S>                  <C>
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>
 
 50   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
MOODY'S INVESTORS SERVICE, INC.
 
<TABLE>
<CAPTION>
BOND RATING          EXPLANATION
- ------------------------------------------------------------------------
<S>                  <C>
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 the
portfolio manager determines that such securities are the equivalent of
investment grade securities. Securities that have received different ratings
from more than one agency are considered investment grade if at least one agency
has rated the security investment grade.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    51
<PAGE> 
 
SECURITIES HOLDINGS BY RATING CATEGORY
During the fiscal year ended December 31, 1997, the percentage of securities
holdings for the Flexible Income Portfolio and High-Yield Portfolio by rating
category based upon a weighted monthly average was:
 
<TABLE>
<CAPTION>
                                          FLEXIBLE INCOME   HIGH-YIELD
           BONDS - S&P RATING                PORTFOLIO      PORTFOLIO
- ----------------------------------------------------------------------
<S>                                       <C>               <C>
AAA                                              21%             0%
AA                                                0%             0%
A                                                15%             0%
BBB                                              12%             0%
BB                                               11%             2%
B                                                27%            74%
CCC                                               1%             6%
CC                                                0%             0%
C                                                 0%             0%
Preferred Stock                                   2%             1%
Cash and Options                                 11%            17%
- ----------------------------------------------------------------------
TOTAL                                           100%           100%
- ----------------------------------------------------------------------
</TABLE>
 
No other Portfolio held 5% or more of its assets in bonds rated below investment
grade for the fiscal year ended December 31, 1997.
 
 52   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
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<PAGE> 
 
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<PAGE> 
                               JANUS RETIREMENT ADVANTAGE (R)
                                   PROSPECTUS
                          A Variable Annuity Issued by
                   Western Reserve Life Assurance Co. of Ohio


                                  [JANUS LOGO]


Please direct all mail and inquiries to:
WESTERN RESERVE LIFE ASSURANCE CO. OF OHIO

Please complete and return Application to:

IF MAILED:
Janus Retirement Advantage
c/o Western Reserve Life Assurance Co. of Ohio
Attn: Annuity Dept.
P.O. Box 9052
Clearwater, FL 33758-9052

IF OVERNIGHT DELIVERY:
Janus Retirement Advantage
c/o Western Reserve Life Assurance Co. of Ohio
Attn: Annuity Dept.
201 Highland Avenue
Largo, FL 33770


                                                                          
                Janus Retirement Advantage (R) o 1-800-504-4440
                                     (5/98)


<PAGE> 
 
                               JANUS ASPEN SERIES
 
                                   Prospectus
 
                                  MAY 1, 1998
 
This prospectus describes four mutual funds with a variety of investment
objectives, including growth of capital, current income and a combination of
growth and income (the "Portfolios"). Each Portfolio is a series of Janus Aspen
Series (the "Trust") and currently offers two classes of shares. The
Institutional Shares are sold under the name "Janus Aspen Series." The Trust is
registered with the Securities and Exchange Commission ("SEC") as an open-end
management investment company. The Institutional Shares of each Portfolio
(collectively, the "Shares") are offered by this prospectus in connection with
investment in and payments under variable annuity contracts and variable life
insurance contracts (collectively "variable insurance contracts"), as well as
certain qualified retirement plans. Janus Capital Corporation ("Janus Capital")
serves as investment adviser to each Portfolio. Janus Capital has been in the
investment advisory business for over 27 years and currently manages
approximately $80 billion in assets.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Each variable insurance contract
involves fees and expenses 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 about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolios. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                                           <C>
PORTFOLIOS AT A GLANCE
Brief description of the Portfolios.........................    2
EXPENSE INFORMATION
Each Portfolio's annual operating expenses..................    3
Financial Highlights - a summary of financial data..........    4
PERFORMANCE TERMS
An explanation of performance terms.........................    6
THE PORTFOLIOS IN DETAIL
Investment Objectives and Policies..........................    7
General Portfolio Policies..................................    9
Additional Risk Factors.....................................   10
MANAGEMENT OF THE PORTFOLIOS
Investment Adviser and Investment Personnel.................   13
Personal Investing..........................................   13
Management Expenses.........................................   14
Portfolio Transactions......................................   14
Other Service Providers.....................................   14
Other Information...........................................   14
DISTRIBUTIONS AND TAXES
Distributions...............................................   16
Taxes.......................................................   16
SHAREHOLDER'S GUIDE
Purchases...................................................   17
Redemptions.................................................   17
Shareholder Communications..................................   17
APPENDIX A
Glossary of Investment Terms................................   18
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIOS AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolios
and their investment emphasis. A more detailed discussion of the Portfolios'
investment objectives and policies begins on page 7.
 
GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on companies with larger
market capitalizations.
Inception: September 1993
Manager: James P. Craig, III
Assistant Managers: David C. Decker
                    Blaine P. Rollins
 
INTERNATIONAL GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign issuers.
Inception: May 1994
Managers: Laurence J. Chang
          Helen Young Hayes
 
WORLDWIDE GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign and domestic issuers.
Inception: September 1993
Manager: Helen Young Hayes
Assistant Manager: Laurence J. Chang
 
BALANCED PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital, balanced
by current income. The 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.
Inception: September 1993
Manager: Blaine P. Rollins
 
 2   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolios in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES (applicable to each Portfolio)
 
<TABLE>
         <S>                                                           <C>
         Maximum sales load imposed on purchases                       None
         Maximum sales load imposed on reinvested dividends            None
         Deferred sales charges on redemptions                         None
         Redemption fee                                                None
         Exchange fee                                                  None
</TABLE>
 
ANNUAL OPERATING EXPENSES (after fee reductions)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<CAPTION>
- -----------------------------------------------------------------------------------------------------------------
                                                             MANAGEMENT         OTHER         TOTAL OPERATING
                                                                 FEE           EXPENSES          EXPENSES
- -----------------------------------------------------------------------------------------------------------------
<S>                                                          <C>           <C>                <C>            
Growth Portfolio                                                0.65%           0.05%              0.70%
International Growth Portfolio                                  0.67%           0.29%              0.96%
Worldwide Growth Portfolio                                      0.66%           0.08%              0.74%
Balanced Portfolio                                              0.76%           0.07%              0.83%
 ----------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Management fees for Growth, International Growth, Worldwide Growth and
    Balanced Portfolios reflect a reduced fee schedule effective July 1, 1997.
    The management fee for each Portfolio reflects the new rate applied to net
    assets as of December 31, 1997. Other expenses are based on the gross
    expenses of the Shares before expense offset arrangements for the fiscal
    year ended December 31, 1997. The information for each Portfolio is net of
    fee reductions from Janus Capital. Fee reductions for Growth, International
    Growth, Worldwide Growth and Balanced Portfolios reduce the management fee
    to the level of the corresponding Janus retail fund. Without such
    reductions, the Management Fee, Other Expenses and Total Operating Expenses
    for the Shares would have been 0.74%, 0.04% and 0.78% for Growth Portfolio;
    0.79%, 0.29%, and 1.08% for International Growth Portfolio; 0.72%, 0.09%,
    and 0.81% for Worldwide Growth Portfolio, and 0.77%, 0.06%, and 0.83% for
    Balanced Portfolio, respectively. Janus Capital may modify or terminate the
    fee reductions at any time upon at least 90 days' notice to the Trustees.
 
EXAMPLE
 
Assume you invest $1,000, the Shares of the Portfolios return 5% annually and
each Portfolio's expense ratio remains as listed above. The example below shows
the operating expenses that you would indirectly bear as an investor in the
Shares of the Portfolios.
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------
                                                             1 YEAR      3 YEARS      5 YEARS      10 YEARS
- ---------------------------------------------------------------------------------------------------------------
<S>                                                          <C>         <C>          <C>          <C>      
Growth Portfolio                                              $ 7          $22          $39          $ 87
International Growth Portfolio                                $10          $31          $53          $118
Worldwide Growth Portfolio                                    $ 8          $24          $41          $ 92
Balanced Portfolio                                            $ 8          $26          $46          $103
 --------------------------------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    3
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise stated, the information below is for fiscal periods ending on
December 31st of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolios' financial statements since their inception. Their report
is included in the Portfolios' Annual Report, which is incorporated by reference
into the SAI. A DETAILED EXPLANATION OF THE FINANCIAL HIGHLIGHTS CAN BE FOUND ON
PAGE 5.
<TABLE>
<CAPTION>
                                                                              GROWTH PORTFOLIO
                 INSTITUTIONAL SHARES                     1997         1996         1995        1994         1993(1)
- --------------------------------------------------------------------------------------------------------------------
<C>  <S>                                                <C>          <C>          <C>          <C>           <C>
 1.  Net asset value, beginning of period                 $15.51       $13.45       $10.57      $10.32       $10.00
- --------------------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                                   .15          .17          .28         .09          .03
 3.  Net gains or (losses) on securities (both
     realized and unrealized)                               3.34         2.29         2.90         .20          .32
- --------------------------------------------------------------------------------------------------------------------
 4.  Total from investment operations                       3.49         2.46         3.18         .29          .35
- --------------------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)                (.15)        (.17)        (.30)       (.04)        (.03)
 6.  Tax return of capital distributions                      --           --           --          --           --
 7.  Distributions (from capital gains)                    (.37)        (.23)           --          --           --
 8.  Distributions (in excess of realized gains)              --           --           --          --           --
- --------------------------------------------------------------------------------------------------------------------
 9.  Total distributions                                   (.52)        (.40)        (.30)       (.04)        (.03)
- --------------------------------------------------------------------------------------------------------------------
10.  Net asset value, end of period                       $18.48       $15.51       $13.45      $10.57       $10.32
- --------------------------------------------------------------------------------------------------------------------
11.  Total return*                                        22.75%       18.45%       30.17%       2.76%        3.50%
- --------------------------------------------------------------------------------------------------------------------
12.  Net assets, end of period (in thousands)           $608,281     $325,789     $126,911     $43,549       $7,482
13.  Average net assets for the period (in thousands)   $477,914     $216,125      $77,344     $26,464       $3,191
14.  Ratio of gross expenses to average net assets**       0.70%(8)     0.69%(7)     0.78%(6)      N/A          N/A
15.  Ratio of net expenses to average net assets**         0.69%        0.69%        0.76%       0.88%(3)(5)  0.25%(4)
16.  Ratio of net investment income to average net
     assets**                                              0.91%        1.39%        1.24%       1.45%        2.54%
17.  Portfolio turnover rate**                              122%          87%         185%        169%         162%
18.  Average commission rate                             $0.0500      $0.0466          N/A         N/A          N/A
- --------------------------------------------------------------------------------------------------------------------
 
<CAPTION>
            INTERNATIONAL GROWTH PORTFOLIO
       1997        1996        1995       1994(2)
 
<C>  <C>          <C>         <C>         <C>       
 1.    $15.72       11.95       $9.72      $10.00   
- ----------------------------------------------------
                                                    
 2.       .11         .05         .09       (.09)   
 3.                                                 
         2.80        4.06        2.16       (.19)   
- ----------------------------------------------------
 4.      2.91        4.11        2.25       (.28)   
- ----------------------------------------------------
                                                    
 5.     (.11)       (.11)       (.02)          --   
 6.        --          --          --          --   
 7.     (.01)       (.23)          --          --   
 8.     (.03)          --          --          --   
- ----------------------------------------------------
 9.     (.15)       (.34)       (.02)          --   
- ----------------------------------------------------
10.    $18.48      $15.72      $11.95       $9.72   
- ----------------------------------------------------
11.    18.51%      34.71%      23.15%     (2.80%)   
- ----------------------------------------------------
12.  $161,091     $27,192      $1,608      $1,353   
13.   $96,164      $7,437      $1,792      $1,421   
14.     0.96%(8)    1.26%(7)    2.69%(6)      N/A   
15.     0.96%       1.25%       2.50%       2.50%(5)
16.                                                 
        0.70%       0.62%     (0.80%)     (1.30%)   
17.       86%         65%        211%        275%   
18.   $0.0241     $0.0305         N/A         N/A   
- ----------------------------------------------------
</TABLE>
<TABLE>
<CAPTION>
                                                                 WORLDWIDE GROWTH PORTFOLIO
            INSTITUTIONAL SHARES                  1997          1996         1995        1994         1993(1)
- -------------------------------------------------------------------------------------------------------------
<C>  <S>                                       <C>            <C>          <C>          <C>           <C>
 1.  Net asset value, beginning of period          $19.44       $15.31       $12.07      $11.89       $10.00
- -------------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                            .16          .16          .11         .04          .02
 3.  Net gains or (losses) on securities
     (both realized and unrealized)                  4.14         4.27         3.19         .14         1.89
- -------------------------------------------------------------------------------------------------------------
 4.  Total from investment operations                4.30         4.43         3.30         .18         1.91
- -------------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)         (.17)        (.17)        (.06)          --        (.01)
 6.  Dividends (in excess of net investment
     income)                                        (.02)           --           --          --           --
 7.  Tax return of capital distributions               --           --           --          --           --
 8.  Distributions (from capital gains)             (.16)        (.13)           --          --        (.01)
- -------------------------------------------------------------------------------------------------------------
 9.  Total distributions                            (.35)        (.30)        (.06)          --        (.02)
- -------------------------------------------------------------------------------------------------------------
10.  Net asset value, end of period                $23.39       $19.44       $15.31      $12.07       $11.89
- -------------------------------------------------------------------------------------------------------------
11.  Total return*                                 22.15%       29.04%       27.37%       1.53%       19.10%
- -------------------------------------------------------------------------------------------------------------
12.  Net assets, end of period (in thousands)  $1,576,548     $582,603     $108,563     $37,728       $4,856
13.  Average net assets for the period (in
     thousands)                                $1,148,951     $304,111      $59,440     $22,896       $2,200
14.  Ratio of gross expenses to average net
     assets**                                       0.74%(8)     0.80%(7)     0.90%(6)      N/A          N/A
15.  Ratio of net expenses to average net
     assets**                                       0.74%        0.80%        0.87%       1.18%(3)(5)  0.25%(4)
16.  Ratio of net investment income to
     average net assets**                           0.67%        0.83%        0.95%       0.50%        0.84%
17.  Portfolio turnover rate**                        80%          62%         113%        217%          57%
18.  Average commission rate                      $0.0337      $0.0345          N/A         N/A          N/A
- -------------------------------------------------------------------------------------------------------------
 
<CAPTION>
                         BALANCED PORTFOLIO
       1997        1996        1995        1994         1993(1)
- -----------------------------------------------------------------
<C>  <C>          <C>         <C>         <C>           <C>      
 1.    $14.77       13.03      $10.63      $10.64       $10.00   
- -----------------------------------------------------------------
                                                                 
 2.       .34         .32         .17         .15          .08   
 3.                                                              
         2.89        1.81        2.45       (.06)          .64   
- -----------------------------------------------------------------
 4.      3.23        2.13        2.62         .09          .72   
- -----------------------------------------------------------------
                                                                 
 5.     (.35)       (.30)       (.22)       (.10)        (.08)   
 6.                                                              
           --          --          --          --           --   
 7.        --          --          --          --           --   
 8.     (.18)       (.09)          --          --           --   
- -----------------------------------------------------------------
 9.     (.53)       (.39)       (.22)       (.10)        (.08)   
- -----------------------------------------------------------------
10.    $17.47       14.77      $13.03      $10.63       $10.64   
- -----------------------------------------------------------------
11.    22.10%      16.18%      24.79%       0.84%        7.20%   
- -----------------------------------------------------------------
12.  $362,409      85,480     $14,021      $3,153         $537   
13.                                                              
     $176,432     $43,414      $5,739      $2,336         $521   
14.                                                              
        0.83%(8)    0.94%(7)    1.37%(6)      N/A          N/A   
15.                                                              
        0.82%       0.92%       1.30%       1.57%(3)(5)  0.25%(4)
16.                                                              
        2.87%       2.92%       2.41%       1.90%        2.69%   
17.      139%        103%        149%        158%         126%   
18.   $0.0456     $0.0426         N/A         N/A          N/A   
- -----------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) May 2, 1994 (inception) to December 31, 1994.
(3) 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.
(4) The ratio was 2.16%, 2.71%, and 7.92%, respectively, for the Growth,
    Worldwide Growth and Balanced Portfolios, before waiver of certain fees
    and/or voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(5) The ratio was 1.23%, 4.67%, 1.49%, and 1.74%, respectively, for the Growth,
    International Growth, Worldwide Growth and Balanced Portfolios, before
    waiver of certain fees and/or voluntary reduction of adviser's fees to the
    effective rate of the corresponding Janus retail fund.
(6) The ratio was 0.98%, 3.57%, 1.09%, and 1.55%, respectively, for the Growth,
    International Growth, Worldwide Growth and Balanced Portfolios, before
    waiver of certain fees and/or voluntary reduction of adviser's fees to the
    effective rate of the corresponding Janus retail fund.
(7) The ratio was 0.83%, 2.21%, 0.91%, and 1.07%, respectively, for the Growth,
    International Growth, Worldwide Growth and Balanced Portfolios, before
    waiver of certain fees and/or voluntary reduction of adviser's fees to the
    effective rate of the corresponding Janus retail fund.
(8) The ratio was 0.78%, 2.19%, 0.81%, and 0.83%, respectively, for the Growth,
    International Growth, Worldwide Growth and Balanced Portfolios, before
    voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
 
 4   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights tables. The tables contain important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolios'
Annual Report contains additional information about each Portfolio's
performance, including a comparison to an appropriate securities index. To
request a copy of the Annual Report, please call or write your insurance
company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of a Portfolio. It is
computed by adding the value of all of a Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between NET ASSET VALUE, BEGINNING OF PERIOD
and NET ASSET VALUE, END OF PERIOD in the Financial Highlights tables represents
the change in value of a Share of a Portfolio over the fiscal period, but not
its total return.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by a Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that a Portfolio paid from
net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities a Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that a Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of a Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolios were not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in a Portfolio's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is a Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of a Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of a Portfolio's securities.
 
AVERAGE COMMISSION RATE is the total of a Portfolio's agency commission paid on
equity securities trades divided by the number of shares purchased and sold.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    5
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe a Portfolio's performance. You may see references to these terms in our
newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of a Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. Growth
Portfolio, International Growth Portfolio, Worldwide Growth Portfolio and
Balanced Portfolio generally measure performance in terms of total return.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights tables show total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of a Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in a Portfolio's return and are not
the same as actual annual results.
 
THE PORTFOLIOS IMPOSE NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
COMPUTATIONS. TOTAL RETURN FIGURES INCLUDE THE EFFECT OF DEDUCTING EACH
PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES ATTRIBUTABLE TO
ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES ARE BASED UPON
HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE PERFORMANCE.
INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT SHARES, WHEN
REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
 6   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                            THE PORTFOLIOS IN DETAIL
 
This section takes a closer look at the Portfolios' investment objectives,
policies and the securities in which they invest. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed discussion of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before choosing a Portfolio.
 
Each Portfolio has an investment objective and policies that are similar to
those of a Janus retail fund, as illustrated in the chart below. Although it is
anticipated that each Portfolio and its corresponding retail fund will hold
similar securities, differences in asset size and cash flow needs as well as the
relative weightings of securities selections may result in differences in
investment performance. 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 contract fees and
expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including each Portfolio's investment objective, are
not fundamental and may be changed by the Portfolios' Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in a Portfolio's objective or policies, you should
consider whether that Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
 
  EACH OF THE PORTFOLIOS HAS A SIMILAR INVESTMENT
  OBJECTIVE AND SIMILAR INVESTMENT POLICIES TO AN EXISTING JANUS RETAIL FUND.
 
Growth Portfolio....................................................Janus Fund
International Growth Portfolio.............................Janus Overseas Fund
Worldwide Growth Portfolio................................Janus Worldwide Fund
Balanced Portfolio.........................................Janus Balanced Fund
 
GROWTH PORTFOLIO, INTERNATIONAL GROWTH PORTFOLIO AND WORLDWIDE GROWTH PORTFOLIO
ARE DESIGNED FOR LONG-TERM INVESTORS WHO SEEK GROWTH OF CAPITAL ONLY AND WHO CAN
TOLERATE THE GREATER RISKS ASSOCIATED WITH COMMON STOCK INVESTMENTS.
 
GROWTH PORTFOLIOS
Investment Objective:..........................................Growth of Capital
Primary Holdings:..................................................Common Stocks
Shareholder's Investment Horizon:......................................Long-Term
 
GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective by investing primarily in common stocks of
issuers of any size. This Portfolio generally invests in larger, more
established issuers.
 
INTERNATIONAL GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a diversified portfolio that pursues its objective primarily through investments
in common stocks of issuers located outside the United States. The Portfolio has
the flexibility to invest on a worldwide basis in companies and other
organizations of any size, regardless of country of organization or place of
principal business activity. The 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 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
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective primarily through investments in common
stocks of foreign and domestic issuers. The Portfolio has the flexibility to
invest on a worldwide basis in companies and other organizations of any size,
regardless of country of organization or place of principal business activity.
Worldwide Growth 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.
 
TYPES OF INVESTMENTS
Each of these Portfolios invests primarily in common stocks of foreign and
domestic companies. However, the percentage of each Portfolio's assets invested
in common stocks will vary and each Portfolio may at times hold substantial
positions in cash equivalents or interest bearing securities. See "General
Portfolio Policies" on page 9. Each Portfolio may invest to a lesser degree in
other types of securities including preferred stocks, warrants, convertible
securities and debt securities when its portfolio manager perceives an
opportunity for capital growth from such securities or to receive a return on
idle cash. Some securities that the Portfolios purchase may be on a when-issued,
delayed delivery or forward commitment basis. The Portfolios may invest up to
25% of their assets in mortgage- and asset-backed securities, up to 10% of their
assets in zero coupon, pay-in-kind and step coupon securities, and without limit
in indexed/structured securities. No Growth Portfolio will invest 35% or more of
its assets in high-yield/high-risk securities.
 
Although International Growth Portfolio and Worldwide Growth Portfolio are
committed to foreign investing, Growth Portfolio may also invest without limit
in foreign equity and debt securities. The Portfolios may invest directly in
foreign
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    7
<PAGE> 
 
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 ("PFICs"). These
Portfolios may use futures, options and other derivatives for hedging purposes
or for non-hedging purposes such as seeking to enhance return. See "Additional
Risk Factors" on page 10 for a discussion of the risks associated with foreign
investing and derivatives.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE GROWTH PORTFOLIOS.
 
Q:    HOW ARE COMMON STOCKS SELECTED?
 
A:    Each of these Portfolios invests substantially all of its assets in common
      stocks to the extent the portfolio manager believes that the relevant
market environment favors profitable investing in those securities. Portfolio
managers generally take a "bottom up" approach to building their portfolios. In
other words, they seek to identify individual companies with earnings growth
potential that may not be recognized by the market at large. Although themes may
emerge in any Portfolio, securities are generally selected without regard to any
defined industry sector or other similarly defined selection procedure.
Realization of income is not a significant investment consideration. Any income
realized on these Portfolios' investments will be incidental to their
objectives.
 
Q:   ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?
 
A:    Generally, yes. Portfolio managers seek companies that meet their
      selection criteria, regardless of country of organization or place of
principal business activity. 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. See "Additional Risk Factors" on page 10.
 
Q:   WHAT IS THE MAIN RISK OF INVESTING IN A GROWTH PORTFOLIO?
 
A:    Since the Growth Portfolios usually invest heavily in common stocks, the
      fundamental risk is that the value of the stocks a Portfolio holds might
decrease. Stock values may fluctuate in response to the activities of an
individual company or in response to general market and economic conditions.
Historically, common stocks have provided greater long-term returns and have
entailed greater short-term risks than other investment choices. Smaller or
newer issuers are more likely to realize more substantial growth as well as
suffer more significant losses than larger or more established issuers.
Investments in such companies can be both more volatile and more speculative.
See "Additional Risk Factors" on page 10.
 
Q:   WHAT IS MEANT BY "MARKET CAPITALIZATION"?
 
A:    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 Growth Portfolio, International Growth Portfolio and Worldwide Growth
Portfolio do not emphasize companies of any particular size, Portfolios with a
larger asset base are more likely to invest in larger, more-established issuers.
 
Q:   HOW DO THESE PORTFOLIOS TRY TO REDUCE RISK?
 
A:    Diversification of a Portfolio's assets reduces the effect of any single
      holding on its overall portfolio value. A Portfolio may also use futures,
options and other derivative instruments to protect its portfolio from movements
in securities' prices and interest rates. The Portfolios may use a variety of
currency hedging techniques, including forward currency contracts, to manage
exchange rate risk when investing directly in foreign markets. See "Additional
Risk Factors" on page 10. In addition, to the extent that a Portfolio holds a
larger cash position, it may not participate in market declines to the same
extent as if the Portfolio remained more fully invested in common stocks.
 
BALANCED PORTFOLIO IS DESIGNED FOR INVESTORS WHO PRIMARILY SEEK GROWTH OF
CAPITAL WITH A DEGREE OF EMPHASIS ON INCOME. IT IS NOT DESIGNED FOR INVESTORS
WHO DESIRE A CONSISTENT LEVEL OF INCOME.
 
Investment Objective:..........................................Growth of Capital
                                                 Including an Emphasis on Income
Primary Holdings:..............................................Common Stocks and
                                                     Income-Producing Securities
Shareholder's Investment Horizon:......................................Long-Term
 
BALANCED PORTFOLIO
The investment objective of this Portfolio is long-term capital growth,
consistent with preservation of capital and balanced by current income. It is a
diversified Portfolio that, under normal circumstances, pursues its objective by
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 senior securities, which include debt securities and preferred
stocks.
 
TYPES OF INVESTMENTS
Balanced Portfolio may invest in a combination of common stocks, preferred
stocks, convertible securities, debt securities and other fixed-income
securities including corporate bonds and notes, government securities and
indexed/structured securities. Balanced Portfolio may invest in the types of
investments previously described under "Growth Portfolios" on page 7. Although
the Portfolio places some emphasis on the income
 
 8   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
objective, investors should keep in mind that Balanced Portfolio is not designed
to produce a consistent level of income.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN BALANCED PORTFOLIO.
 
Q:    HOW ARE ASSETS ALLOCATED BETWEEN THE GROWTH AND INCOME COMPONENTS OF
      BALANCED PORTFOLIO?                                                 

A:    Balanced Portfolio may shift assets between the growth and income
      components of its portfolio 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.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE GROWTH COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    The growth component of Balanced Portfolio is expected to consist
      primarily of common stocks. The selection criteria for common stocks are
described on page 8. 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 equity securities. Balanced
Portfolio may also find opportunities for capital growth from debt securities
because of anticipated changes in interest rates, credit standing, currency
relationships or other factors.
 
Q:    WHAT TYPES OF SECURITIES MAKE UP THE INCOME COMPONENT OF BALANCED
      PORTFOLIO?                                                       

A:    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
Balanced 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.
 
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 only at the time of purchase of the security. 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's manager believes that market conditions are not favorable for
profitable investing or when the portfolio manager is otherwise unable to locate
favorable investment opportunities, a Portfolio's investments may be hedged to a
greater degree and/or its 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 are a residual - they represent the assets that
remain after a portfolio manager has committed available assets to desirable
investment opportunities. Partly because the portfolio managers act
independently of each other, the cash positions of the Portfolios may vary
significantly. Larger hedged positions and/or larger cash positions may serve as
a means of preserving capital in unfavorable market conditions.
 
Securities that the Portfolios may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolios may
also invest in money market funds (including funds managed by Janus Capital).
When a Portfolio is hedged or its investments in cash or similar investments
increase, it may not participate in stock or bond market advances or declines to
the same extent that it would if the Portfolio was not hedged or remained more
fully invested in stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. All of the Portfolios qualify
as diversified funds under the 1940 Act and are subject to the following
diversification requirements:
 
- - As a fundamental policy, no Portfolio may own more than 10% of the outstanding
  voting shares of any issuer.
 
- - As a fundamental policy, with respect to 75% of its total assets, no Portfolio
  will purchase a security of any issuer (other than cash items and U.S.
  government securities, as defined in the 1940 Act) if such purchase would
  cause a Portfolio's holdings of that issuer to amount to more than 5% of that
  Portfolio's total assets.
 
- - No Portfolio will invest more than 25% of its total assets in a single issuer
  (other than U.S. government securities).
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, each Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, no Portfolio will invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
Each Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may result from
liquidity needs, securities having reached a price or yield objective, changes
in interest
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    9
<PAGE> 
 
rates or the credit standing of an issuer, or by reason of economic or other
developments not foreseen at the time of the initial investment decision.
Changes are made in a Portfolio whenever its portfolio manager believes such
changes are desirable. Portfolio turnover rates are generally not a factor in
making buy and sell decisions.
 
To a limited extent, a Portfolio may purchase securities in anticipation of
relatively short-term price gains. A Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
Each Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of a Portfolio's net
assets after the time of purchase, the Portfolio will take steps to reduce in an
orderly fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public because of
their terms or because of SEC regulations. Janus Capital will follow guidelines
established by the Trustees of the Trust ("Trustees") in making liquidity
determinations for Rule 144A securities and other securities, including
privately placed commercial paper and municipal lease obligations.
 
BORROWING AND LENDING
Each Portfolio may borrow money and lend securities or other assets, as follows:
 
- - Each Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - Each Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, each Portfolio may lend securities or other assets
  if, as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, each Portfolio may
borrow money from or lend money to each other and other funds that permit such
transactions and for which Janus Capital serves as investment adviser. All such
borrowing and lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. A Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though 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 underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolios may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk,
 
 10   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
because their values depend on the performance of a foreign security denominated
in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or potential development, or they may 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 be 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 wider
price fluctuations. Investments in such companies tend to be more volatile and
somewhat more speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
Each Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolios intend to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolios may also use derivative instruments for non-hedging
purposes such as seeking to increase a Portfolio's income or otherwise seeking
to enhance return. Please refer to Appendix A to this Prospectus and the SAI for
a more detailed discussion of these instruments.
 
The use of derivative instruments exposes the Portfolios to additional
investment risks and transaction costs. Risks inherent in the use of derivative
instruments include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that a portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than a Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave a
  Portfolio worse off than if it had not entered into the position.
 
Although the Portfolios' managers believe the use of derivative instruments will
benefit the Portfolios, a Portfolio's performance could be worse than if the
Portfolio had not used such instruments if the portfolio manager's judgment
proves incorrect.
 
When a Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for higher quality securities. Conversely, the value of higher
quality securities may be more sensitive to interest rate movements than lower
rated securities. Issuers of high-yield/high-risk securities may not be as
strong financially as those issuing bonds with higher credit ratings.
Investments in such companies are considered to be more speculative than higher
quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, a Portfolio would
experience a reduction of its income and could expect a decline in the market
value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    11
<PAGE> 
 
higher quality securities. Sovereign debt of foreign governments is generally
rated by country. Because these ratings do not take into account individual
factors relevant to each issue and may not be updated regularly, Janus Capital
may treat such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, a Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolios may have to sell securities
to have sufficient cash to pay the dividends.
 
SHORT SALES
Each Portfolio may engage in "short sales against the box." This technique
involves selling either a security that a Portfolio owns, or a security
equivalent in kind and amount that a Portfolio has the right to obtain, for
delivery at a specified date in the future. A Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
SPECIAL SITUATIONS
Each Portfolio may invest in "special situations" from time to time. 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.
Investment in special situations may carry an additional risk of loss in the
event that the anticipated development does not occur or does not attract the
expected attention.
 
See Appendix A for risks associated with certain other investments.
 
 12   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                                 MANAGEMENT OF
                                 THE PORTFOLIOS
 
TRUSTEES
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to each Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolios to the
officers of the Trust and meet at least quarterly to review the Portfolios'
investment policies, performance, expenses and other business affairs.
 
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 has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
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. From
1989 to 1993, he was a Project Director for the National Security Archives, a
nonprofit research organization. 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. Mr. Craig previously managed Balanced Portfolio from September
1993 through April 1996. 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 and Janus Balanced Fund from
December 1993 to December 1995. He holds a Bachelor of Arts in Business from the
University of Alabama and a Master of Arts in Finance from the Wharton School of
the University of Pennsylvania.
 
HELEN YOUNG HAYES is Executive Vice President and portfolio manager of Worldwide
Growth Portfolio and co-manager of International Growth Portfolio, which she has
managed or co-managed since inception. Ms. Hayes joined Janus Capital in 1987
and has managed or co-managed Janus Worldwide Fund and Janus Overseas Fund since
their inceptions. She holds a Bachelor of Arts in Economics from Yale University
and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS is Executive Vice President and portfolio manager of Balanced
Portfolio, which he has managed since May 1996, and Equity Income Portfolio,
which he has managed since inception. He is an assistant portfolio manager of
Growth Portfolio. Mr. Rollins joined Janus Capital in 1990 and has managed both
Janus Balanced Fund since January 1996 and Janus Equity Income Fund since June
1996. 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 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.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    13
<PAGE> 
 
personal investing. Janus Capital's policy requires investment and other
personnel to conduct their personal investment activities in a manner that Janus
Capital believes is not detrimental to the Portfolios or Janus Capital's other
advisory clients. See the SAI for more detailed information.
 
MANAGEMENT EXPENSES
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):
 
<TABLE>
<CAPTION>
                                 AVERAGE DAILY
                                   NET ASSETS       ANNUAL RATE
        FEE SCHEDULE              OF PORTFOLIO     PERCENTAGE(%)
- -----------------------------------------------------------------
<S>                            <C>                 <C>
Growth Portfolio               First $300 Million    0.75
International Growth
  Portfolio                    Next $200 Million     0.70
Worldwide Growth Portfolio
  and                          Over $500 Million     0.65
Balanced Portfolio
- -----------------------------------------------------------------
</TABLE>
 
 Janus Capital has agreed to reduce each Portfolio's advisory fee to the extent
 that such fee exceeds the effective rate of the Janus retail fund
 corresponding to such Portfolio. Janus Capital may terminate this fee
 reduction at any time upon at least 90 days' notice to the Trustees. The
 effective rate is the advisory fee calculated by the corresponding retail fund
 as of the last day of each calendar quarter (expressed as an annual rate). The
 effective rates of Janus Fund, Janus Overseas Fund, Janus Worldwide Fund and
 Janus Balanced Fund were .65%, .66%, .65% and .73%, respectively, for the
 quarter ended March 31, 1998.
 
Differences in the actual management fees incurred by the Portfolios are due
primarily to variances in the asset sizes of the corresponding retail funds. As
asset size increases, the annual rate of the management fee rate declines in
accordance with the above schedule. 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. The fee schedule for the Portfolios was
effective July 1, 1997.
 
During the most recent fiscal year the Portfolios paid the following management
fees expressed as a percentage of average net assets: Growth Portfolio .65%;
International Growth Portfolio .67%; Worldwide Growth Portfolio .66%; and
Balanced Portfolio .76%.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of each Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for a Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for a Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolios' Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for a Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses serving to reduce those expenses. The SAI
further explains the selection of broker-dealers.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolios with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust consists of twelve separate series, four of which are offered by this
Prospectus.
 
Each Portfolio currently offers two classes of Shares, one of which, the
Institutional Shares, are offered pursuant to this Prospectus and are sold under
the name Janus Aspen Series. The Shares offered by this Prospectus are available
only in connection with investment in and payments under variable insurance
contracts as well as certain qualified retirement plans. Retirement Shares of
each Portfolio are offered by separate prospectus and are available only to
participant directed qualified plans using plan service providers that are
compensated for providing distribution and/or recordkeeping and other
administrative services provided to plan participants. Because the expenses of
each class may differ, the performance of each class is expected to differ. If
you would like additional information about the Retirement Shares, please call
1-800-525-0020.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific class or Portfolio or for the Trust as a
whole for purposes such as electing or removing Trustees, terminating or
reorganizing the Trust, changing fundamental policies, or for any other purpose
requiring a shareholder vote under the 1940 Act. Separate votes are taken by
each class or Portfolio only if a matter affects or requires the vote of only
that class or Portfolio or interest of a
 
 14   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
class or Portfolio in the matter differs from the interest of the other class or
Portfolios of the Trust. As a shareholder, you are entitled to one vote for each
share that you own. An insurance company issuing a variable contract invested in
Shares of a Portfolio will request voting instructions from variable contract
holders. Under current law, the insurance company must vote all Shares held by
the separate account in proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Each Portfolio's Shares are available only to variable annuity and variable life
separate accounts of insurance companies that are unaffiliated with Janus
Capital and to certain qualified retirement plans. Retirement Shares of the
Portfolios (offered through a separate prospectus) are available to certain
participant directed qualified plans. Although the Portfolios do not currently
anticipate any disadvantages to policy owners arising out of the fact that 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 anticipated disadvantages or material irreconcilable conflicts that may
arise 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 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 securities at disadvantageous prices. In
addition, the Trustees may refuse to sell shares of any Portfolio to any
separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve a Portfolio's investment objective
by investing all of that Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to that Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolios with
service at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of a Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per Share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of Shares outstanding.
 
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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    15
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. EACH CLASS OF EACH PORTFOLIO MAKES SEMIANNUAL 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 THE SHARES OF A PORTFOLIO WILL BE AUTOMATICALLY
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 in the Shares depends on the
features of the variable insurance contracts purchased from a participating
insurance company. Further information may be found 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 as a foreign tax credit.
If such an 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.
 
 16   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<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. 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.
 
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.
 
All investments in the Portfolios are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by a Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by a Portfolio.
 
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. Any Portfolio may
discontinue sales of its shares if management believes that a substantial
further increase may adversely affect that Portfolio's ability to achieve its
investment objective. In such event, however, it is anticipated that existing
policy owners and plan participants invested in that Portfolio would be
permitted to continue to authorize investment in such Portfolio and to reinvest
any dividends or capital gains distributions.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolios that they have authorized for
investment. Each report will show the investments owned by each Portfolio and
the market values thereof, as well as other information about the Portfolios and
their operations. The Trust's fiscal year ends December 31.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    17
<PAGE> 
 
                                   APPENDIX A
 
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 objective 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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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 to provide cash to satisfy unusually high redemption
requests or for other temporary or emergency purposes.
 
 18   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
RULE 144A SECURITIES are securities that are not registered for sale to the
general public under the Securities Act of 1933, but that may be resold to
certain institutional investors.
 
STANDBY COMMITMENTS are obligations purchased by a Portfolio from a dealer that
give the Portfolio the option to sell a security to the dealer at a specified
price.
 
STEP COUPON BONDS are debt securities that trade at a discount from their face
value and pay coupon interest. The discount from the face value depends on the
time remaining until cash payments begin, prevailing interest rates, liquidity
of the security and the perceived credit quality of the issuer.
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a 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
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 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 of comparable 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 non-dollar denominated securities or to
reduce the impact of currency appreciation on purchases of non-dollar
denominated 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    19
<PAGE> 
 
instrument or index. For example, upon reset the interest rate payable on a
security may go down when the underlying index has risen. Certain inverse
floaters may have an interest rate reset mechanism that multiplies the effects
of change in the underlying index. Such mechanism may increase the volatility of
the security's market value.
 
OPTIONS are the right, but not the obligation, to buy or sell a specified amount
of securities or other assets on or before a fixed date at a predetermined
price. The Portfolios may purchase and write put and call options on securities,
securities indices and foreign currencies.
 
 20   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
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<PAGE> 
                               JANUS ASPEN SERIES
                                   PROSPECTUS
                               Growth Portfolio
                         International Growth Portfolio
                          Worldwide Growth Portfolio
                               Balanced Portfolio


                                 [JANUS LOGO]



            P.O. Box 173375 o Denver, CO 80217-3375 o 1-800-525-1068
                                     (5/98)
<PAGE> 
 
                               JANUS ASPEN SERIES
 
                                   Prospectus
 
                                  MAY 1, 1998
 
This prospectus describes four mutual funds each of which has its own investment
objective and policies (the "Portfolios"). Each Portfolio is a series of Janus
Aspen Series (the "Trust") and currently offers two classes of shares. The
Institutional Shares are sold under the name "Janus Aspen Series." The Trust is
registered with the Securities and Exchange Commission ("SEC") as an open-end
management investment company. The Institutional Shares of each Portfolio
(collectively, the "Shares") are offered by this prospectus in connection with
investment in and payments under variable annuity contracts and variable life
insurance contracts (collectively "variable insurance contracts"), as well as
certain qualified retirement plans. Janus Capital Corporation ("Janus Capital")
serves as investment adviser to each Portfolio. Janus Capital has been in the
investment advisory business for over 27 years and currently manages
approximately $80 billion in assets.
 
The Trust sells and redeems its Shares at net asset value without any sales
charges, commissions or redemption fees. Each variable insurance contract
involves fees and expenses 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 about the Shares that a prospective
purchaser of a variable insurance contract or plan participant should consider
before allocating purchase payments or premiums to the Portfolios. It should be
read carefully in conjunction with the separate account prospectus of the
specific insurance product that accompanies this Prospectus and retained for
future reference. Additional information about the Shares is contained in a
Statement of Additional Information ("SAI") filed with the SEC. The SAI dated
May 1, 1998 is incorporated by reference into this Prospectus. Copies of the SAI
are available upon request and without charge by writing or calling your
insurance company or plan sponsor.
 
THESE SECURITIES HAVE NOT BEEN APPROVED BY THE SEC NOR HAS THE SEC PASSED ON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
THIS PROSPECTUS DOES NOT CONSTITUTE AN OFFER TO SELL SECURITIES IN ANY STATE OR
OTHER JURISDICTION TO ANY PERSON TO WHOM IT IS UNLAWFUL TO MAKE SUCH AN OFFER IN
SUCH STATE OR OTHER JURISDICTION.
 
JANUS ASPEN SERIES PROSPECTUS                                        MAY 1, 1998
<PAGE> 
 
                                    CONTENTS
 
<TABLE>
<S>                                                           <C>
PORTFOLIOS AT A GLANCE
Brief description of the Portfolios.........................    2
EXPENSE INFORMATION
Each Portfolio's annual operating expenses..................    3
Financial Highlights - a summary of financial data..........    4
PERFORMANCE TERMS
An explanation of performance terms.........................    6
THE PORTFOLIOS IN DETAIL
Investment Objectives and Policies..........................    7
General Portfolio Policies..................................    9
Additional Risk Factors.....................................   10
MANAGEMENT OF THE PORTFOLIOS
Investment Adviser and Investment Personnel.................   13
Personal Investing..........................................   14
Management Expenses.........................................   14
Portfolio Transactions......................................   14
Other Service Providers.....................................   14
Other Information...........................................   14
DISTRIBUTIONS AND TAXES
Distributions...............................................   16
Taxes.......................................................   16
SHAREHOLDER'S GUIDE
Purchases...................................................   17
Redemptions.................................................   17
Shareholder Communications..................................   17
APPENDIX A
Glossary of Investment Terms................................   18
</TABLE>
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    1
<PAGE> 
 
                             PORTFOLIOS AT A GLANCE
 
This section is designed to provide you with a brief overview of the Portfolios
and their investment emphasis. A more detailed discussion of the Portfolios'
investment objectives and policies begins on page 7.
 
GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on companies with larger
market capitalizations.
Inception: September 1993
Manager: James P. Craig, III
Assistant Managers: David C. Decker
                    Blaine P. Rollins
 
AGGRESSIVE GROWTH PORTFOLIO
Focus: A nondiversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks, with an emphasis on securities issued by
medium-sized companies.
Inception: September 1993
Manager: James P. Goff
 
INTERNATIONAL GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign issuers.
Inception: May 1994
Managers: Laurence J. Chang
          Helen Young Hayes
 
WORLDWIDE GROWTH PORTFOLIO
Focus: A diversified portfolio that seeks long-term growth of capital by
investing primarily in common stocks of foreign and domestic issuers.
Inception: September 1993
Manager: Helen Young Hayes
Assistant Manager: Laurence J. Chang
 
 2   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                              EXPENSE INFORMATION
 
The following tables and example are designed to assist participants in
qualified plans that invest in the Shares of the Portfolios in understanding the
various costs and expenses that you will bear directly or indirectly as an
investor in the Shares. OWNERS OF VARIABLE INSURANCE CONTRACTS THAT INVEST IN
THE SHARES SHOULD REFER TO THE VARIABLE INSURANCE CONTRACT PROSPECTUS FOR A
DESCRIPTION OF COSTS AND EXPENSES, AS THE TABLES AND EXAMPLE DO NOT REFLECT
DEDUCTIONS AT THE SEPARATE ACCOUNT LEVEL OR CONTRACT LEVEL FOR ANY CHARGES THAT
MAY BE INCURRED UNDER A CONTRACT.
 
    SHAREHOLDER TRANSACTION EXPENSES (applicable to each Portfolio)
 
<TABLE>
         <S>                                                           <C>
         Maximum sales load imposed on purchases                       None
         Maximum sales load imposed on reinvested dividends            None
         Deferred sales charges on redemptions                         None
         Redemption fee                                                None
         Exchange fee                                                  None
</TABLE>
 
ANNUAL OPERATING EXPENSES (after fee reductions)(1)
(expressed as a percentage of average net assets)
 
<TABLE>
<CAPTION>
- -----------------------------------------------------------------------------------------------------------------
                                                             MANAGEMENT         OTHER         TOTAL OPERATING
                                                                 FEE           EXPENSES          EXPENSES
- -----------------------------------------------------------------------------------------------------------------
<S>                                                          <C>           <C>                <C>             
Growth Portfolio                                                0.65%           0.05%              0.70%
Aggressive Growth Portfolio                                     0.73%           0.03%              0.76%
International Growth Portfolio                                  0.67%           0.29%              0.96%
Worldwide Growth Portfolio                                      0.66%           0.08%              0.74%
 ----------------------------------------------------------------------------------------------------------------
</TABLE>
 
(1) Management fees for Growth, Aggressive Growth, International Growth and
    Worldwide Growth Portfolios reflect a reduced fee schedule effective July 1,
    1997. The management fee for each Portfolio reflects the new rate applied to
    net assets as of December 31, 1997. Other expenses are based on the gross
    expenses of the Shares before expense offset arrangements for the fiscal
    year ended December 31, 1997. The information for each Portfolio is net of
    fee reductions from Janus Capital. Fee reductions for the Growth, Aggressive
    Growth, International Growth and Worldwide Growth Portfolios reduce the
    management fee to the level of the corresponding Janus retail fund. Without
    such reductions, the Management Fee, Other Expenses and Total Operating
    Expenses for the Shares would have been 0.74%, 0.04% and 0.78% for Growth
    Portfolio; 0.74%, 0.04%, and 0.78% for Aggressive Growth Portfolio; 0.79%,
    0.29%, and 1.08% for International Growth Portfolio; and 0.72%, 0.09%, and
    0.81% for Worldwide Growth Portfolio, respectively. Janus Capital may modify
    or terminate the fee reductions at any time upon at least 90 days' notice to
    the Trustees.
 
EXAMPLE
 
Assume you invest $1,000, the Shares of the Portfolios return 5% annually and
each Portfolio's expense ratio remains as listed above. The example below shows
the operating expenses that you would indirectly bear as an investor in the
Shares of the Portfolios.
 
<TABLE>
<CAPTION>
- ---------------------------------------------------------------------------------------------------------------
                                                             1 YEAR      3 YEARS      5 YEARS      10 YEARS
- ---------------------------------------------------------------------------------------------------------------
<S>                                                          <C>         <C>          <C>          <C>      
Growth Portfolio                                              $ 7          $22          $39          $ 87
Aggressive Growth Portfolio                                   $ 8          $24          $42          $ 94
International Growth Portfolio                                $10          $31          $53          $118
Worldwide Growth Portfolio                                    $ 8          $24          $41          $ 92
 --------------------------------------------------------------------------------------------------------------
</TABLE>
 
THE EXAMPLE SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR FUTURE RETURNS
OR EXPENSES WHICH MAY BE MORE OR LESS THAN THOSE SHOWN.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    3
<PAGE> 
 
                              FINANCIAL HIGHLIGHTS
 
Unless otherwise stated, the information below is for fiscal periods ending on
December 31st of each year. The accounting firm of Price Waterhouse LLP has
audited the Portfolios' financial statements since their inception. Their report
is included in the Portfolios' Annual Report, which is incorporated by reference
into the SAI. A DETAILED EXPLANATION OF THE FINANCIAL HIGHLIGHTS CAN BE FOUND ON
PAGE 5.
<TABLE>
<CAPTION>
                                                                    GROWTH PORTFOLIO
            INSTITUTIONAL SHARES                1997         1996         1995        1994         1993(1)
- ----------------------------------------------------------------------------------------------------------
<C>  <S>                                      <C>          <C>          <C>          <C>           <C>
 1.  Net asset value, beginning of period       $15.51       $13.45       $10.57      $10.32       $10.00
- ----------------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                         .15          .17          .28         .09          .03
 3.  Net gains or (losses) on securities
     (both realized and unrealized)               3.34         2.29         2.90         .20          .32
- ----------------------------------------------------------------------------------------------------------
 4.  Total from investment operations             3.49         2.46         3.18         .29          .35
- ----------------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)      (.15)        (.17)        (.30)       (.04)        (.03)
 6.  Tax return of capital distributions            --           --           --          --           --
 7.  Distributions (from capital gains)          (.37)        (.23)           --          --           --
- ----------------------------------------------------------------------------------------------------------
 8.  Total distributions                         (.52)        (.40)        (.30)       (.04)        (.03)
- ----------------------------------------------------------------------------------------------------------
 9.  Net asset value, end of period             $18.48       $15.51       $13.45      $10.57       $10.32
- ----------------------------------------------------------------------------------------------------------
10.  Total return*                              22.75%       18.45%       30.17%       2.76%        3.50%
- ----------------------------------------------------------------------------------------------------------
11.  Net assets, end of period (in
     thousands)                               $608,281     $325,789     $126,911     $43,549       $7,482
12.  Average net assets for the period (in
     thousands)                               $477,914     $216,125      $77,344     $26,464       $3,191
13.  Ratio of gross expenses to average net
     assets**                                    0.70%(8)     0.69%(7)     0.78%(6)      N/A          N/A
14.  Ratio of net expenses to average net
     assets**                                    0.69%        0.69%        0.76%       0.88%(3)(5)  0.25%(4)
15.  Ratio of net investment income to
     average net assets**                        0.91%        1.39%        1.24%       1.45%        2.54%
16.  Portfolio turnover rate**                    122%          87%         185%        169%         162%
17.  Average commission rate                   $0.0500      $0.0466          N/A         N/A          N/A
- ----------------------------------------------------------------------------------------------------------
 
<CAPTION>
                     AGGRESSIVE GROWTH PORTFOLIO
       1997         1996         1995        1994         1993(1)
 
<C>  <C>          <C>          <C>          <C>           <C>      
 1.    $18.24       $17.08       $13.62      $11.80       $10.00   
- -------------------------------------------------------------------
                                                                   
 2.        --           --          .24         .11          .01   
 3.                                                                
         2.31         1.36         3.47        1.82         1.80   
- -------------------------------------------------------------------
 4.      2.31         1.36         3.71        1.93         1.81   
- -------------------------------------------------------------------
                                                                   
 5.        --           --        (.25)       (.11)        (.01)   
 6.        --        (.01)           --          --           --   
 7.        --        (.19)           --          --           --   
- -------------------------------------------------------------------
 8.        --        (.20)        (.25)       (.11)        (.01)   
- -------------------------------------------------------------------
 9.    $20.55       $18.24       $17.08      $13.62       $11.80   
- -------------------------------------------------------------------
10.    12.66%        7.95%       27.48%      16.33%       18.05%   
- -------------------------------------------------------------------
11.                                                                
     $508,198     $383,693     $185,911     $41,289       $1,985   
12.                                                                
     $418,464     $290,629     $107,582     $14,152       $1,091   
13.                                                                
        0.76%(8)     0.76%(7)     0.86%(6)      N/A          N/A   
14.                                                                
        0.76%        0.76%        0.84%       1.05%(3)(5)  0.25%(4)
15.                                                                
      (0.10%)      (0.27%)        0.58%       2.18%        0.34%   
16.      130%          88%         155%        259%          31%   
17.   $0.0367      $0.0347          N/A         N/A          N/A   
- -------------------------------------------------------------------
</TABLE>
<TABLE>
<CAPTION>
                                                             INTERNATIONAL GROWTH PORTFOLIO
                INSTITUTIONAL SHARES                    1997         1996        1995       1994(2)
- ---------------------------------------------------------------------------------------------------
<C>  <S>                                              <C>          <C>          <C>         <C>
 1.  Net asset value, beginning of period               $15.72       $11.95       $9.72     $10.00
- ---------------------------------------------------------------------------------------------------
     INCOME FROM INVESTMENT OPERATIONS:
 2.  Net investment income                                 .11          .05         .09      (.09)
 3.  Net gains or (losses) on securities (both
     realized and unrealized)                             2.80         4.06        2.16      (.19)
- ---------------------------------------------------------------------------------------------------
 4.  Total from investment operations                     2.91         4.11        2.25      (.28)
- ---------------------------------------------------------------------------------------------------
     LESS DISTRIBUTIONS:
 5.  Dividends (from net investment income)              (.11)        (.11)       (.02)         --
 6.  Dividends (in excess of net investment income)         --           --          --         --
 7.  Tax return of capital distributions                    --           --          --         --
 8.  Distributions (from capital gains)                  (.01)        (.23)          --         --
 9.  Distributions (in excess of realized gains)         (.03)           --          --         --
- ---------------------------------------------------------------------------------------------------
10.  Total distributions                                (0.15)        (.34)       (.02)         --
- ---------------------------------------------------------------------------------------------------
11.  Net asset value, end of period                     $18.48       $15.72      $11.95      $9.72
- ---------------------------------------------------------------------------------------------------
12.  Total return*                                      18.51%       34.71%      23.15%     (2.80%)
- ---------------------------------------------------------------------------------------------------
13.  Net assets, end of period (in thousands)         $161,091      $27,192      $1,608     $1,353
14.  Average net assets for the period (in
     thousands)                                        $96,164       $7,437      $1,792     $1,421
15.  Ratio of gross expenses to average net assets**     0.96%(8)     1.26%(7)    2.69%(6)     N/A
16.  Ratio of net expenses to average net assets**       0.96%        1.25%       2.50%      2.50%(5)
17.  Ratio of net investment income to average net
     assets**                                            0.70%        0.62%     (0.80%)     (1.30%)
18.  Portfolio turnover rate**                             86%          65%        211%       275%
19.  Average commission rate                           $0.0241      $0.0305         N/A        N/A
- ---------------------------------------------------------------------------------------------------
 
<CAPTION>
                       WORLDWIDE GROWTH PORTFOLIO
        1997          1996         1995        1994         1993(1)
- ---------------------------------------------------------------------------------------------------
<C>  <C>            <C>          <C>          <C>           <C>    
 1.      $19.44       $15.31       $12.07      $11.89       $10.00
- ---------------------------------------------------------------------------------------------------
 
 2.         .16          .16          .11         .04          .02
 3.
           4.14         4.27         3.19         .14         1.89
- ---------------------------------------------------------------------------------------------------
 4.        4.30         4.43         3.30         .18         1.91
- ---------------------------------------------------------------------------------------------------
 
 5.       (.17)        (.17)        (.06)          --        (.01)
 6.       (.02)           --           --          --           --
 7.          --           --           --          --           --
 8.       (.16)        (.13)           --          --        (.01)
 9.          --           --           --          --           --
- ---------------------------------------------------------------------------------------------------
10.       (.35)        (.30)        (.06)          --        (.02)
- ---------------------------------------------------------------------------------------------------
11.      $23.39       $19.44       $15.31      $12.07       $11.89
- ---------------------------------------------------------------------------------------------------
12.      22.15%       29.04%       27.37%       1.53%       19.10%
- ---------------------------------------------------------------------------------------------------
13.  $1,576,548     $582,603     $108,563     $37,728       $4,856
14.
     $1,148,951     $304,111      $59,440     $22,896       $2,200
15.       0.74%(8)     0.80%(7)     0.90%(6)      N/A          N/A
16.       0.74%        0.80%        0.87%       1.18%(3)(5)  0.25%(4)
17.
          0.67%        0.83%        0.95%       0.50%        0.84%
18.         80%          62%         113%        217%          57%
19.     $0.0337      $0.0345          N/A         N/A          N/A
- ---------------------------------------------------------------------------------------------------
</TABLE>
 
 * Total return not annualized for periods of less than one year.
** Annualized for periods of less than one full year.
(1) September 13, 1993 (inception) to December 31, 1993.
(2) May 2, 1994 (inception) to December 31, 1994.
(3) 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.
(4) The ratio was 2.16%, 5.79% and 2.71%, respectively, for Growth, Aggressive
    Growth and Worldwide Growth Portfolios, before waiver of certain fees and/or
    voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
(5) The ratio was 1.23%, 1.14%, 4.67% and 1.49%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(6) The ratio was 0.98%, 0.93%, 3.57% and 1.09%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(7) The ratio was 0.83%, 0.83%, 2.21% and 0.91%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before waiver of certain fees and/or voluntary reduction of adviser's fees
    to the effective rate of the corresponding Janus retail fund.
(8) The ratio was 0.78%, 0.78%, 1.08% and 0.81%, respectively, for Growth,
    Aggressive Growth, International Growth and Worldwide Growth Portfolios,
    before voluntary reduction of adviser's fees to the effective rate of the
    corresponding Janus retail fund.
 
 4   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                               UNDERSTANDING THE
                              FINANCIAL HIGHLIGHTS
 
This section is designed to help you better understand the information
summarized in the Financial Highlights tables. The tables contain important
historical operating information that may be useful in making your investment
decision or understanding how your investment has performed. The Portfolios'
Annual Report contains additional information about each Portfolio's
performance, including a comparison to an appropriate securities index. To
request a copy of the Annual Report, please call or write your insurance
company.
 
NET ASSET VALUE ("NAV") is the value of a single Share of a Portfolio. It is
computed by adding the value of all of a Portfolio's investments and other
assets, subtracting any liabilities and dividing the result by the number of
shares outstanding. The difference between NET ASSET VALUE, BEGINNING OF PERIOD
and NET ASSET VALUE, END OF PERIOD in the Financial Highlights tables represents
the change in value of a Share of a Portfolio over the fiscal period, but not
its total return.
 
NET INVESTMENT INCOME is the per share amount of dividends and interest income
earned on securities held by a Portfolio, less Portfolio expenses. DIVIDENDS
(FROM NET INVESTMENT INCOME) are the per share amount that a Portfolio paid from
net investment income.
 
NET GAINS OR (LOSSES) ON SECURITIES is the per share increase or decrease in
value of the securities a Portfolio holds. A gain (or loss) is realized when
securities are sold. A gain (or loss) is unrealized when securities increase or
decrease in value but are not sold. DISTRIBUTIONS (FROM CAPITAL GAINS) are the
per share amount that a Portfolio paid from net realized gains.
 
TOTAL RETURN is the percentage increase or decrease in the value of an
investment over a stated period of time. Total return includes both changes in
NAV and income. For the purposes of calculating total return, it is assumed that
dividends and distributions are reinvested at the NAV on the day of the
distribution.
 
RATIO OF GROSS EXPENSES TO AVERAGE NET ASSETS is the total of a Portfolio's
operating expenses before expense offset arrangements divided by its average net
assets for the stated period. The Portfolios were not required to disclose the
ratio of gross expenses to average net assets prior to 1995. RATIO OF NET
EXPENSES TO AVERAGE NET ASSETS reflects reductions in a Portfolio's expenses
through the use of brokerage commissions and uninvested cash balances earning
interest or balance credits.
 
RATIO OF NET INVESTMENT INCOME TO AVERAGE NET ASSETS is a Portfolio's net
investment income divided by its average net assets for the stated period.
 
PORTFOLIO TURNOVER RATE is a measure of the amount of a Portfolio's buying and
selling activity. It is computed by dividing total purchases or sales, whichever
is less, by the average monthly market value of a Portfolio's securities.
 
AVERAGE COMMISSION RATE is the total of a Portfolio's agency commission paid on
equity securities trades divided by the number of shares purchased and sold.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    5
<PAGE> 
 
                               PERFORMANCE TERMS
 
This section will help you understand various terms that are commonly used to
describe a Portfolio's performance. You may see references to these terms in our
newsletters or advertisements (or those published by participating insurance
companies) and in media articles. Newsletters and advertisements may include
comparisons of a Portfolio's performance to the performance of other mutual
funds, mutual fund averages or recognized stock market indices. Growth
Portfolio, Aggressive Growth Portfolio, International Growth Portfolio and
Worldwide Growth Portfolio generally measure performance in terms of total
return.
 
CUMULATIVE TOTAL RETURN represents the actual rate of return on an investment
for a specified period. The Financial Highlights tables show total return for a
single fiscal period. Cumulative total return is generally quoted for more than
one year (e.g., the life of a Portfolio). A cumulative total return does not
show interim fluctuations in the value of an investment.
 
AVERAGE ANNUAL TOTAL RETURN represents the average annual percentage change of
an investment over a specified period. It is calculated by taking the cumulative
total return for the stated period and determining what constant annual return
would have produced the same cumulative return. Average annual returns for more
than one year tend to smooth out variations in a Portfolio's return and are not
the same actual annual results.
 
THE PORTFOLIOS IMPOSE NO SALES OR OTHER CHARGES THAT WOULD AFFECT TOTAL RETURN
COMPUTATIONS. TOTAL RETURN FIGURES INCLUDE THE EFFECT OF DEDUCTING EACH
PORTFOLIO'S EXPENSES, BUT MAY NOT INCLUDE CHARGES AND EXPENSES ATTRIBUTABLE TO
ANY PARTICULAR INSURANCE PRODUCT. PORTFOLIO PERFORMANCE FIGURES ARE BASED UPON
HISTORICAL RESULTS AND ARE NOT INTENDED TO INDICATE FUTURE PERFORMANCE.
INVESTMENT RETURNS AND NET ASSET VALUE WILL FLUCTUATE SO THAT SHARES, WHEN
REDEEMED, MAY BE WORTH MORE OR LESS THAN THEIR ORIGINAL COST.
 
 6   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
                            THE PORTFOLIOS IN DETAIL
 
This section takes a closer look at the Portfolios' investment objectives,
policies and the securities in which they invest. Please carefully review the
"Additional Risk Factors" section of this Prospectus for a more detailed
discussion of the risks associated with certain investment techniques. Appendix
A contains a more detailed discussion of investment terms used throughout this
Prospectus. You should carefully consider your investment goals, time horizon
and risk tolerance before choosing a Portfolio.
 
Each Portfolio has an investment objective and policies that are similar to
those of a Janus retail fund, as illustrated in the chart below. Although it is
anticipated that each Portfolio and its corresponding retail fund will hold
similar securities, differences in asset size and cash flow needs as well as the
relative weightings of securities selections may result in differences in
investment performance. 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 contract fees and
expenses.
 
Policies that are noted as "fundamental" cannot be changed without a shareholder
vote. All other policies, including each Portfolio's investment objective, are
not fundamental and may be changed by the Portfolios' Trustees without a
shareholder vote. You will be notified of any such changes that are material. If
there is a material change in a Portfolio's objective or policies, you should
consider whether that Portfolio remains an appropriate investment for your
variable insurance contract or qualified retirement plan.
 
  EACH OF THE PORTFOLIOS HAS A SIMILAR INVESTMENT
  OBJECTIVE AND SIMILAR INVESTMENT POLICIES TO AN EXISTING JANUS RETAIL FUND.
 
Growth Portfolio....................................................Janus Fund
Aggressive Growth Portfolio..............................Janus Enterprise Fund
International Growth Portfolio.............................Janus Overseas Fund
Worldwide Growth Portfolio................................Janus Worldwide Fund
 
GROWTH PORTFOLIO, AGGRESSIVE GROWTH PORTFOLIO, INTERNATIONAL GROWTH PORTFOLIO
AND WORLDWIDE GROWTH PORTFOLIO ARE DESIGNED FOR LONG-TERM INVESTORS WHO SEEK
GROWTH OF CAPITAL ONLY AND WHO CAN TOLERATE THE GREATER RISKS ASSOCIATED WITH
COMMON STOCK INVESTMENTS.
 
GROWTH PORTFOLIOS
Investment Objective:..........................................Growth of Capital
Primary Holdings:..................................................Common Stocks
Shareholder's Investment Horizon:......................................Long-Term
 
GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective by investing primarily in common stocks of
issuers of any size. This Portfolio generally invests in larger, more
established issuers.
 
AGGRESSIVE GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a nondiversified portfolio that pursues its investment objective by normally
investing at least 50% of its equity assets in securities issued by medium-sized
companies. Medium-sized companies are those whose market capitalizations fall
within the range of companies in the S&P MidCap 400 Index (the "MidCap Index").
Companies whose capitalization falls outside this range after the Portfolio's
initial purchase continue to be considered medium-sized companies for the
purpose of this policy. As of December 31, 1997, the MidCap Index included
companies with capitalizations between approximately $213 million and $13.7
billion. The range of the MidCap Index is expected to change on a regular basis.
Subject to the above policy, the Portfolio may also invest in smaller or larger
issuers.
 
INTERNATIONAL GROWTH PORTFOLIO
The investment objective of this Portfolio is long-term growth of capital. It is
a diversified portfolio that pursues its objective primarily through investments
in common stocks of issuers located outside the United States. The Portfolio has
the flexibility to invest on a worldwide basis in companies and other
organizations of any size, regardless of country of organization or place of
principal business activity. The 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 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
The investment objective of this Portfolio is long-term growth of capital in a
manner consistent with the preservation of capital. It is a diversified
portfolio that pursues its objective primarily through investments in common
stocks of foreign and domestic issuers. The Portfolio has the flexibility to
invest on a worldwide basis in companies and other organizations of any size,
regardless of country of organization or place of principal business activity.
Worldwide Growth 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.
 
TYPES OF INVESTMENTS
Each of these Portfolios invests primarily in common stocks of foreign and
domestic companies. However, the percentage of
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    7
<PAGE> 
 
each Portfolio's assets invested in common stocks will vary and each Portfolio
may at times hold substantial positions in cash equivalents or interest bearing
securities. See "General Portfolio Policies" on page 9. Each Portfolio may
invest to a lesser degree in other types of securities including preferred
stocks, warrants, convertible securities and debt securities when its portfolio
manager perceives an opportunity for capital growth from such securities or to
receive a return on idle cash. Some securities that the Portfolios purchase may
be on a when-issued, delayed delivery or forward commitment basis. The
Portfolios may invest up to 25% of their assets in mortgage- and asset-backed
securities, up to 10% of their assets in zero coupon, pay-in-kind and step
coupon securities, and without limit in indexed/structured securities. No Growth
Portfolio will invest 35% or more of its assets in high-yield/high-risk
securities.
 
Although International Growth Portfolio and Worldwide Growth Portfolio are
committed to foreign investing, Growth Portfolio and Aggressive Growth Portfolio
may also invest without limit in foreign equity and debt securities. The
Portfolios may invest directly in foreign securities denominated in a foreign
currency and not publicly traded in the United States. Other ways of investing
in foreign securities include depositary receipts or shares, and passive foreign
investment companies ("PFICs"). These Portfolios may use futures, options and
other derivatives for hedging purposes or for non-hedging purposes such as
seeking to enhance return. See "Additional Risk Factors" on page 10 for a
discussion of the risks associated with foreign investing and derivatives.
 
THE FOLLOWING QUESTIONS ARE DESIGNED TO HELP YOU BETTER UNDERSTAND AN INVESTMENT
IN THE GROWTH PORTFOLIOS.
 
Q:    HOW ARE COMMON STOCKS SELECTED?

A:    Each of these Portfolios invests substantially all of its assets in common
      stocks to the extent its portfolio manager believes that the relevant
market environment favors profitable investing in those securities. Portfolio
managers generally take a "bottom up" approach to building their portfolios. In
other words, they seek to identify individual companies with earnings growth
potential that may not be recognized by the market at large. Although themes may
emerge in any Portfolio, securities are generally selected without regard to any
defined industry sector or other similarly defined selection procedure.
Realization of income is not a significant investment consideration. Any income
realized on these Portfolios' investments will be incidental to their
objectives.
 
Q:    ARE THE SAME CRITERIA USED TO SELECT FOREIGN SECURITIES?

A:    Generally, yes. Portfolio managers seek companies that meet their
      selection criteria, regardless of country of organization or place of
principal business activity. 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. See "Additional Risk Factors" on page 10.
 
Q:    WHAT IS THE MAIN RISK OF INVESTING IN A GROWTH PORTFOLIO?

A:    Since the Growth Portfolios usually invest heavily in common stocks, the
      fundamental risk is that the value of the stocks a Portfolio holds might
decrease. Stock values may fluctuate in response to the activities of an
individual company or in response to general market and economic conditions.
Historically, common stocks have provided greater long-term returns and have
entailed greater short-term risks than other investment choices. Smaller or
newer issuers are more likely to realize more substantial growth as well as
suffer more significant losses than larger or more established issuers.
Investments in such companies can be both more volatile and more speculative.
See "Additional Risk Factors" on page 10.
 
Q:    WHAT IS MEANT BY "MARKET CAPITALIZATION"?

A:    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 which may invest in small to medium sized
companies to a greater degree. Although Growth Portfolio, International Growth
Portfolio and Worldwide Growth Portfolio do not emphasize companies of any
particular size, Portfolios with a larger asset base are more likely to invest
in larger, more-established issuers.
 
Q:    HOW DOES A DIVERSIFIED PORTFOLIO DIFFER FROM A NONDIVERSIFIED PORTFOLIO?

A:    Diversification is a means of reducing risk by investing a Portfolio's
      assets 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 share price of a nondiversified portfolio, its share price
can be expected to fluctuate more than a comparable diversified portfolio.
Aggressive Growth Portfolio is a nondiversified portfolio.
 
Q:    HOW DO THESE PORTFOLIOS TRY TO REDUCE RISK?

A:    Diversification of a Portfolio's assets reduces the effect of any single
      holding on its overall portfolio value. A Portfolio may also use futures,
options and other derivative instruments to protect its portfolio from movements
in securities' prices and interest rates. The Portfolios may use a variety of
currency hedging techniques, including forward currency contracts, to manage
exchange rate risk when
 
 8   JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998
<PAGE> 
 
investing directly in foreign markets. See "Additional Risk Factors" on page 10.
In addition, to the extent that a Portfolio holds a larger cash position, it may
not participate in market declines to the same extent as if the Portfolio
remained more fully invested in common stocks.
 
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 only at the time of purchase of the security. 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's manager believes that market conditions are not favorable for
profitable investing or when the portfolio manager is otherwise unable to locate
favorable investment opportunities, a Portfolio's investments may be hedged to a
greater degree and/or its 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 are a residual - they represent the assets that
remain after a portfolio manager has committed available assets to desirable
investment opportunities. Partly because the portfolio managers act
independently of each other, the cash positions of the Portfolios may vary
significantly. Larger hedged positions and/or larger cash positions may serve as
a means of preserving capital in unfavorable market conditions.
 
Securities that the Portfolios may invest in as means of receiving a return on
idle cash include high-grade commercial paper, certificates of deposit,
repurchase agreements or other short-term debt obligations. The Portfolios may
also invest in money market funds (including funds managed by Janus Capital).
When a Portfolio is hedged or its investments in cash or similar investments
increase. It may not participate in stock or bond market advances or declines to
the same extent that it would if the Portfolio was not hedged or remained more
fully invested in stocks or bonds.
 
DIVERSIFICATION
The Investment Company Act of 1940 (the "1940 Act") classifies investment
companies as either diversified or nondiversified. All of the Portfolios (except
Aggressive Growth Portfolio) qualify as diversified funds under the 1940 Act and
are subject to the following diversification requirements:
 
- - As a fundamental policy, no Portfolio may own more than 10% of the outstanding
  voting shares of any issuer.
 
- - As a fundamental policy, with respect to 50% of the total assets of Aggressive
  Growth Portfolio and 75% of the total assets of the other Portfolios, no
  Portfolio will purchase a security of any issuer (other than cash items and
  U.S. government securities, as defined in the 1940 Act) if such purchase would
  cause a Portfolio's holdings of that issuer to amount to more than 5% of that
  Portfolio's total assets.
 
- - No Portfolio will invest more than 25% of its total assets in a single issuer
  (other than U.S. government securities).
 
- - Aggressive Growth Portfolio reserves the right to become a diversified
  portfolio by limiting the investments in which more than 5% of its total net
  assets are invested.
 
INTERNAL REVENUE SERVICE (IRS) LIMITATIONS
In addition to the diversification requirements stated above, each Portfolio
intends to comply with the diversification requirements currently imposed by the
IRS on separate accounts of insurance companies as a condition of maintaining
the tax-deferred status of variable contracts. More specific information may be
contained in the participating insurance company's separate account prospectus.
 
INDUSTRY CONCENTRATION
As a fundamental policy, no Portfolio will invest 25% or more of its total
assets in any particular industry (excluding U.S. government securities).
 
PORTFOLIO TURNOVER
Each Portfolio generally intends to purchase securities for long-term investment
rather than short-term gains. However, short-term transactions may 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 initial investment
decision. Changes are made in a Portfolio whenever its portfolio manager
believes such changes are desirable. Portfolio turnover rates are generally not
a factor in making buy and sell decisions.
 
To a limited extent, a Portfolio may purchase securities in anticipation of
relatively short-term price gains. A Portfolio may also sell one security and
simultaneously purchase the same or comparable security to take advantage of
short-term differentials in bond yields or securities prices. 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.
 
ILLIQUID INVESTMENTS
Each Portfolio may invest up to 15% of its net assets in illiquid investments,
including restricted securities or private placements that are not deemed to be
liquid by Janus Capital. If illiquid securities exceed 15% of a Portfolio's net
assets after the time of purchase, the Portfolio will take steps to reduce in an
orderly fashion its holdings of illiquid securities. An illiquid investment is a
security or other position that cannot be disposed of quickly in the normal
course of business. Some securities cannot be sold to the U.S. public because of
their terms or because of SEC regulations. Janus Capital will follow guidelines
established by the Trustees of the Trust ("Trustees") in making liquidity
determinations for Rule 144A securities and other securities, including
privately placed commercial paper and municipal lease obligations.
 
JANUS ASPEN SERIES PROSPECTUS                                   MAY 1, 1998    9
<PAGE> 
 
BORROWING AND LENDING
Each Portfolio may borrow money and lend securities or other assets, as follows:
 
- - Each Portfolio may borrow money for temporary or emergency purposes in amounts
  up to 25% of its total assets.
 
- - Each Portfolio may mortgage or pledge securities as collateral for borrowings
  in amounts up to 15% of its net assets.
 
- - As a fundamental policy, each Portfolio may lend securities or other assets
  if, as a result, no more than 25% of its total assets would be lent to other
  parties.
 
Under the terms of an exemptive order received from the SEC, each Portfolio may
borrow money from or lend money to each other and other funds that permit such
transactions and for which Janus Capital serves as investment adviser. All such
borrowing and lending will be subject to the above percentage limits.
 
ADDITIONAL RISK FACTORS
 
FOREIGN SECURITIES
INVESTMENTS IN FOREIGN SECURITIES, INCLUDING THOSE OF FOREIGN GOVERNMENTS, MAY
INVOLVE GREATER RISKS THAN INVESTING IN COMPARABLE DOMESTIC SECURITIES.
 
Securities of some foreign companies and governments may be traded in the United
States, but many foreign securities are traded primarily in foreign markets. The
risks of foreign investing include:
 
- - CURRENCY RISK. A Portfolio may buy the local currency when it buys a foreign
  currency denominated security and sell the local currency when it sells the
  security. 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 though 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 underdeveloped or developing
  countries which may have relatively unstable governments 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. The Portfolios may invest in emerging market countries.
  Emerging market countries involve greater risks such as immature economic
  structures, national policies restricting investments by foreigners, and
  different legal systems.
 
- - REGULATORY RISK. There may be less government supervision of foreign markets.
  Foreign issuers may not be subject to the uniform accounting, auditing and
  financial reporting standards and practices applicable to domestic issuers.
  There may be less publicly available information about foreign issuers than
  domestic issuers.
 
- - MARKET RISK. Foreign securities markets, particularly those of underdeveloped
  or developing 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. There may be limited legal recourse against an
  issuer in the event of a default on a debt instrument.
 
- - TRANSACTION COSTS. Transaction costs of buying and selling foreign securities,
  including brokerage, tax and custody costs, are generally higher than those
  involved in domestic transactions.
 
Foreign securities purchased indirectly (e.g., depositary receipts) are subject
to many of the above risks, including currency risk, because their values depend
on the performance of a foreign security denominated in its home currency.
 
INVESTMENTS IN SMALLER COMPANIES
SMALLER OR NEWER COMPANIES MAY SUFFER MORE SIGNIFICANT LOSSES AS WELL AS REALIZE
MORE SUBSTANTIAL GROWTH THAN LARGER OR MORE ESTABLISHED ISSUERS.
 
Smaller or newer companies may lack depth of management, they may be unable to
generate funds necessary for growth or potential development, or they may 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 be 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 wider
price fluctuations. Investments in such companies tend to be more volatile and
somewhat more speculative.
 
FUTURES, OPTIONS AND OTHER DERIVATIVE INSTRUMENTS
Each Portfolio may enter into futures contracts on securities, financial indices
and foreign currencies and options on such contracts ("futures contracts") and
may invest in options on securities, financial indices and foreign currencies
("options"), forward contracts and interest rate swaps and swap-related products
(collectively "derivative instruments"). The Portfolios intend to use most
derivative instruments primarily to hedge against potential adverse movements in
securities prices, foreign currency markets or interest rates. To a limited
extent, the Portfolios may also use derivative instruments for non-hedging
purposes such as seeking to increase a Portfolio's income or otherwise seeking
to enhance return. Please refer to Appendix A to this Prospectus and the SAI for
a more detailed discussion of these instruments.
 
 10   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
The use of derivative instruments exposes the Portfolios to additional
investment risks and transaction costs. Risks inherent in the use of derivative
instruments include:
 
- - the risk that interest rates, securities prices and currency markets will not
  move in the direction that a portfolio manager anticipates;
 
- - imperfect correlation between the price of derivative instruments and
  movements in the prices of the securities, interest rates or currencies being
  hedged;
 
- - the fact that skills needed to use these strategies are different from those
  needed to select portfolio securities;
 
- - inability to close out certain hedged positions to avoid adverse tax
  consequences;
 
- - the possible absence of a liquid secondary market for any particular
  instrument and possible exchange-imposed price fluctuation limits, either of
  which may make it difficult or impossible to close out a position when
  desired;
 
- - leverage risk, that is, the risk that adverse price movements in an instrument
  can result in a loss substantially greater than a Portfolio's initial
  investment in that instrument (in some cases, the potential loss is
  unlimited); and
 
- - particularly in the case of privately negotiated instruments, the risk that
  the counterparty will fail to perform its obligations, which could leave a
  Portfolio worse off than if it had not entered into the position.
 
Although the Portfolios' managers believe the use of derivative instruments will
benefit the Portfolios, a Portfolio's performance could be worse than if the
Portfolio had not used such instruments
if the portfolio manager's judgment proves incorrect.
 
When a Portfolio invests in a derivative instrument, it may be required to
segregate cash and other liquid assets or certain portfolio securities with its
custodian to "cover" the Portfolio's position. Assets segregated or set aside
generally may not be disposed of so long as the Portfolio maintains the
positions requiring segregation or cover. Segregating assets could diminish the
Portfolio's return due to the opportunity losses of foregoing other potential
investments with the segregated assets.
 
HIGH-YIELD/HIGH-RISK SECURITIES
HIGH-YIELD/HIGH-RISK SECURITIES (OR "JUNK" BONDS) ARE DEBT 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 the ability
of the issuer to meet interest and principal payments (i.e., credit risk) than
is the case for higher quality securities. Conversely, the value of higher
quality securities may be more sensitive to interest rate movements than lower
rated securities. Issuers of high-yield/high-risk securities may not be as
strong financially as those issuing bonds with higher credit ratings.
Investments in such companies are considered to be more speculative than higher
quality investments.
 
Issuers of high-yield/high-risk securities are more vulnerable to real or
perceived economic changes (for instance, an economic downturn or prolonged
period of rising interest rates), political changes or adverse developments
specific to the issuer. Adverse economic, political or other developments may
impair the issuer's ability to service principal and interest obligations, to
meet projected business goals and to obtain additional financing, particularly
if the issuer is highly leveraged. In the event of a default, a Portfolio would
experience a reduction of its income and could expect a decline in the market
value of the defaulted securities.
 
The market for lower quality securities is generally less liquid than the market
for higher quality securities. Adverse publicity and investor perceptions as
well as new or proposed laws may also have a greater negative impact on the
market for lower quality securities. Unrated debt, while not necessarily of
lower quality than rated securities, may not have as broad a market as higher
quality securities. Sovereign debt of foreign governments is generally rated by
country. Because these ratings do not take into account individual factors
relevant to each issue and may not be updated regularly, Janus Capital may treat
such securities as unrated debt.
 
The market prices of high-yield/high-risk securities structured as zero coupon
or pay-in-kind securities are generally affected to a greater extent by interest
rate changes and tend to be more volatile than securities which pay interest
periodically. In addition, zero coupon, pay-in-kind and delayed interest bonds
often do not pay interest until maturity. However, a Portfolio must recognize
imputed interest income and pay dividends to shareholders even though it has
received no cash. In some instances, the Portfolios may have to sell securities
to have sufficient cash to pay the dividends.
 
SHORT SALES
Each Portfolio may engage in "short sales against the box." This technique
involves selling either a security that a Portfolio owns, or a security
equivalent in kind and amount that a Portfolio has the right to obtain, for
delivery at a specified date in the future. A Portfolio may enter into a short
sale against the box to hedge against anticipated declines in the market price
of portfolio securities. If the value of the securities sold short increases
prior to the scheduled delivery date, the Portfolio loses the opportunity to
participate in the gain.
 
SPECIAL SITUATIONS
Each Portfolio may invest in "special situations" from time to time. 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    11
<PAGE> 
 
other extraordinary corporate event, or differences in market supply of and
demand for the security. Investment in special situations may carry an
additional risk of loss in the event that the anticipated development does not
occur or does not attract the expected attention.
 
See Appendix A for risks associated with certain other investments.
 
 12   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
                                 MANAGEMENT OF
                                 THE PORTFOLIOS
 
TRUSTEES
The Trustees oversee the business affairs of the Trust and are responsible for
major decisions relating to each Portfolio's investment objective and policies.
The Trustees delegate the day-to-day management of the Portfolios to the
officers of the Trust and meet at least quarterly to review the Portfolios'
investment policies, performance, expenses and other business affairs.
 
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 has served as investment adviser to Janus Fund since its inception
in 1970 and currently serves as investment adviser to all of the Janus retail
funds, as well as adviser or subadviser to other mutual funds and individual,
corporate, charitable and retirement accounts.
 
Kansas City Southern Industries, Inc. ("KCSI") owns approximately 83% of the
outstanding voting stock of Janus Capital, most of which it acquired in 1984.
KCSI is a publicly traded holding company whose primary subsidiaries are engaged
in transportation, information processing and financial services. Thomas H.
Bailey, President and Chairman of the Board of Janus Capital, owns approximately
12% of its voting stock and, by agreement with KCSI, selects a majority of Janus
Capital's Board.
 
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.
 
INVESTMENT PERSONNEL
PORTFOLIO MANAGERS
LAWRENCE 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. From
1989 to 1993, he was a Project Director for the National Security Archives, a
nonprofit research organization. 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. Mr. Craig previously managed Balanced Portfolio from September
1993 through April 1996 and has co-managed Janus Venture Fund since February 1,
1997. Mr. Craig previously managed Janus Venture Fund from its inception to
December 1993 and Janus Balanced Fund from December 1993 to December 1995. 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
1997. He holds a Bachelor of Arts in Economics from Yale University and is a
Chartered Financial Analyst.
 
HELEN YOUNG HAYES is Executive Vice President and portfolio manager of Worldwide
Growth Portfolio and co-manager of International Growth Portfolio, which she has
managed or co-managed since inception. Ms. Hayes joined Janus Capital in 1987
and has managed or co-managed Janus Worldwide Fund and Janus Overseas Fund since
their inceptions. She holds a Bachelor of Arts in Economics from Yale University
and is a Chartered Financial Analyst.
 
BLAINE P. ROLLINS is Executive Vice President and portfolio manager of Balanced
Portfolio, which he has managed since May 1996, and Equity Income Portfolio,
which he has managed since inception. He is an assistant portfolio manager of
Growth Portfolio. Mr. Rollins joined Janus Capital in 1990 and has managed both
Janus Balanced Fund since January 1996 and Janus Equity Income Fund since June
1996. 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    13
<PAGE> 
 
Business at Duke University and a Bachelor's Degree in Economics and Political
Science from Tufts University. He is a Chartered Financial Analyst.
 
PERSONAL INVESTING
Janus Capital does not permit portfolio managers to purchase and sell securities
for their own accounts, except under the limited exceptions contained in Janus
Capital's policy governing personal investing. Janus Capital's policy requires
investment and other personnel to conduct their personal investment activities
in a manner that Janus Capital believes is not detrimental to the Portfolios or
Janus Capital's other advisory clients. See the SAI for more detailed
information.
 
MANAGEMENT EXPENSES
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):
 
<TABLE>
<CAPTION>
                                 AVERAGE DAILY
                                   NET ASSETS       ANNUAL RATE
        FEE SCHEDULE              OF PORTFOLIO     PERCENTAGE(%)
- -----------------------------------------------------------------
<S>                            <C>                 <C>
Growth Portfolio               First $300 Million    0.75
Aggressive Growth Portfolio    Next $200 Million     0.70
International Growth
  Portfolio                    Over $500 Million     0.65
Worldwide Growth Portfolio
- -----------------------------------------------------------------
</TABLE>
 
 Janus Capital has agreed to reduce each Portfolio's advisory fee to the extent
 that such fee exceeds the effective rate of the Janus retail fund
 corresponding to such Portfolio. Janus Capital may terminate this fee
 reduction at any time upon at least 90 days' notice to the Trustees. The
 effective rate is the advisory fee calculated by the corresponding retail fund
 as of the last day of each calendar quarter (expressed as an annual rate). The
 effective rates of Janus Fund, Janus Enterprise Fund, Janus Overseas Fund and
 Janus Worldwide Fund were .65%, .72%, .66% and .65%, respectively, for the
 quarter ended March 31, 1998.
 
Differences in the actual management fees incurred by the Portfolios are due
primarily to variances in the asset sizes of the corresponding retail funds. As
asset size increases, the annual rate of the management fee rate declines in
accordance with the above schedule. 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. The fee schedule for the Portfolios was
effective July 1, 1997.
 
During the most recent fiscal year the Portfolios paid the following management
fees expressed as a percentage of average net assets: Growth Portfolio .65%;
Aggressive Growth Portfolio .73%; International Growth Portfolio .67%; and
Worldwide Growth Portfolio .66%.
 
PORTFOLIO TRANSACTIONS
Purchases and sales of securities on behalf of each Portfolio are executed by
broker-dealers selected by Janus Capital. Broker-dealers are selected on the
basis of their ability to obtain best price and execution for a Portfolio's
transactions and recognizing brokerage, research and other services provided to
the Portfolio and to Janus Capital. Janus Capital may consider sales of shares
of the Portfolios or other Janus funds by a broker-dealer or the recommendation
of a broker-dealer to its customers that they purchase a Portfolio's shares as a
factor in the selection of broker-dealers to execute portfolio transactions.
Janus Capital may also consider payments made by brokers effecting transactions
for a Portfolio i) to the Portfolio or ii) to other persons on behalf of the
Portfolio for services provided to the Portfolio for which it would be obligated
to pay. The Portfolios' Trustees have authorized Janus Capital to place
portfolio transactions on an agency basis with a broker-dealer affiliated with
Janus Capital. When transactions for a Portfolio are effected with that
broker-dealer, the commissions payable by the Portfolio are credited against
certain Portfolio operating expenses serving to reduce those expenses. The SAI
further explains the selection of broker-dealers.
 
OTHER SERVICE PROVIDERS
The following parties provide the Portfolios with administrative and other
services.
 
CUSTODIAN
State Street Bank and Trust Company
P.O. Box 0351
Boston, Massachusetts 02117-0351
 
TRANSFER AGENT
Janus Service Corporation
P.O. Box 173375
Denver, Colorado 80217-3375
 
Janus Service Corporation is a wholly-owned subsidiary of Janus Capital.
 
OTHER INFORMATION
ORGANIZATION
The Trust is a "mutual fund" that was organized as a Delaware business trust on
May 20, 1993. A mutual fund is an investment vehicle that pools money from
numerous investors and invests the money to achieve a specified objective. The
Trust consists of twelve separate series, four of which are offered by this
Prospectus.
 
Each Portfolio currently offers two classes of Shares, one of which, the
Institutional Shares, are offered pursuant to this Prospectus and are sold under
the name Janus Aspen Series. The Shares offered by this Prospectus are available
only in connection with investment in and payments under variable insurance
contracts as well as certain qualified retirement plans. Retirement Shares of
each Portfolio are offered by separate prospectus and are available only to
participant directed qualified plans using plan service providers that are
compensated for providing distribution and/or recordkeeping and other
administrative services provided to plan participants. Because the expenses of
each class may differ, the performance of each class is expected to differ. If
you would like additional
 
 14   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
information about the Retirement Shares, please call 1-800-525-0020.
 
SHAREHOLDER MEETINGS AND VOTING RIGHTS
The Trust does not intend to hold annual shareholder meetings. However, special
meetings may be called for a specific class or Portfolio or for the Trust as a
whole for purposes such as electing or removing Trustees, terminating or
reorganizing the Trust, changing fundamental policies, or for any other purpose
requiring a shareholder vote under the 1940 Act. Separate votes are taken by
each class or Portfolio only if a matter affects or requires the vote of only
that class or Portfolio or interest of a class or Portfolio in the matter
differs from the interest of the other class or Portfolios of the Trust. As a
shareholder, you are entitled to one vote for each share that you own. An
insurance company issuing a variable contract invested in Shares of a Portfolio
will request voting instructions from variable contract holders. Under current
law, the insurance company must vote all Shares held by the separate account in
proportion to the voting instructions received.
 
CONFLICTS OF INTEREST
Each Portfolio's Shares are available only to variable annuity and variable life
separate accounts of insurance companies that are unaffiliated with Janus
Capital and to certain qualified retirement plans. Retirement Shares of the
Portfolios (offered through a separate prospectus) are available to certain
participant directed qualified plans. Although the Portfolios do not currently
anticipate any disadvantages to policy owners arising out of the fact that 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 anticipated disadvantages or material irreconcilable conflicts that may
arise 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 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 securities at disadvantageous prices. In
addition, the Trustees may refuse to sell shares of any Portfolio to any
separate account 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.
 
MASTER/FEEDER OPTION
The Trust may in the future seek to achieve a Portfolio's investment objective
by investing all of that Portfolio's assets in another investment company having
the same investment objective and substantially the same investment policies and
restrictions as those applicable to that Portfolio. Unless otherwise required by
law, this policy may be implemented by the Trustees without shareholder
approval.
 
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 entered
into a consulting arrangement with one of the foremost experts in Year 2000
compliance to help Janus Capital successfully achieve Year 2000 compliance.
Janus Capital does not anticipate that the move to Year 2000 will have a
material impact on its ability to continue to provide the Portfolios with
service at current levels.
 
THE VALUATION OF SHARES
The NAV of the Shares of a Portfolio is determined at the close of the regular
trading session of the New York Stock Exchange (the "NYSE") (normally 4:00 p.m.,
New York time) each day that the NYSE is open. NAV per Share is determined by
dividing the total value of the securities and other assets, less liabilities,
by the total number of Shares outstanding.
 
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.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    15
<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. ORDINARY 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, IF ANY, ARE PAID TO SHAREHOLDERS AS CAPITAL GAINS
DISTRIBUTIONS. EACH CLASS OF EACH PORTFOLIO MAKES SEMIANNUAL 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 THE SHARES OF A PORTFOLIO WILL BE AUTOMATICALLY
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 in the Shares depends on the
features of the variable insurance contracts purchased from a participating
insurance company. Further information may be found 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 as a foreign tax credit.
If such an 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.
 
 16   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<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. 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.
 
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.
 
All investments in the Portfolios are credited to a participating insurance
company's separate account or a qualified plan immediately upon acceptance of
the investment by a Portfolio. Investments will be processed at the NAV next
determined after an order is received and accepted by a Portfolio.
 
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. Any Portfolio may
discontinue sales of its shares if management believes that a substantial
further increase may adversely affect that Portfolio's ability to achieve its
investment objective. In such event, however, it is anticipated that existing
policy owners and plan participants invested in that Portfolio would be
permitted to continue to authorize investment in such Portfolio and to reinvest
any dividends or capital gains distributions.
 
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. Redemption proceeds will normally be wired to
the participating insurance company the business day following receipt of the
redemption order, but in no event later than seven days after receipt of such
order.
 
SHAREHOLDER COMMUNICATIONS
Shareholders will receive annual and semiannual reports including the financial
statements of the Shares of the Portfolios that they have authorized for
investment. Each report will show the investments owned by each Portfolio and
the market values thereof, as well as other information about the Portfolios and
their operations. The Trust's fiscal year ends December 31.
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    17
<PAGE> 
 
                                   APPENDIX A
 
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 objective 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
under Section 4(2) of the Securities Act of 1933.
 
COMMON STOCK represents a share of ownership in a company and usually carries
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.
 
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 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 STOCK is a class of stock that generally pays dividends at a specified
rate and has 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 to provide cash to satisfy unusually high redemption
requests or for other temporary or emergency purposes.
 
 18   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
RULE 144A SECURITIES are securities that are not registered for sale to the
general public under the Securities Act of 1933, but that may be resold to
certain institutional investors.
 
STANDBY COMMITMENTS are obligations purchased by a Portfolio from a dealer that
give the Portfolio the option to sell a security to the dealer at a specified
price.
 
STEP COUPON BONDS are debt securities that trade at a discount from their face
value and pay coupon interest. The discount from the face value depends on the
time remaining until cash payments begin, prevailing interest rates, liquidity
of the security and the perceived credit quality of the issuer.
 
STRIP BONDS are debt securities that are stripped of their interest (usually by
a financial intermediary) after the securities are issued. The market value of
these securities generally fluctuates more in response to changes in interest
rates than interest-paying securities of comparable maturity.
 
TENDER OPTION BONDS are generally long-term securities that are coupled with an
option to tender the securities to a bank, broker-dealer or other financial
institution at periodic intervals and receive the face value of the bond. This
type of security is commonly used as a means of enhancing the security's
liquidity.
 
U.S. GOVERNMENT SECURITIES include direct obligations of the U.S. government
that are supported by its full faith and credit. Treasury bills have initial
maturities of less than one year, Treasury notes have initial maturities of one
to ten years and Treasury bonds may be issued with any maturity but generally
have maturities of at least ten years. U.S. government securities also include
indirect obligations of the U.S. government that are issued by federal agencies
and government sponsored entities. Unlike Treasury securities, agency securities
generally are not backed by the full faith and credit of the U.S. government.
Some agency securities are supported by the right of the issuer to borrow from
the Treasury, others are supported by the discretionary authority of the U.S.
government to purchase the agency's obligations and others are supported only by
the credit of the sponsoring agency.
 
VARIABLE AND FLOATING RATE SECURITIES have variable or floating rates of
interest and, under certain limited circumstances, may have varying principal
amounts. These securities pay interest at rates that are adjusted periodically
according to a 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
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 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 of comparable 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 non-dollar denominated securities or to
reduce the impact of currency appreciation on purchases of non-dollar
denominated 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
 
JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998    19
<PAGE> 
 
instrument or index. For example, upon reset the interest rate payable on a
security may go down when the underlying index has risen. Certain inverse
floaters may have an interest rate reset mechanism that multiplies the effects
of change in the underlying index. Such mechanism may increase the volatility of
the security's market value.
 
OPTIONS are the right, but not the obligation, to buy or sell a specified amount
of securities or other assets on or before a fixed date at a predetermined
price. The Portfolios may purchase and write put and call options on securities,
securities indices and foreign currencies.
 
 20   JANUS ASPEN SERIES PROSPECTUS                                  MAY 1, 1998
<PAGE> 
 
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<PAGE> 
                               JANUS ASPEN SERIES
                                   PROSPECTUS
                               Growth Portfolio
                           Aggressive Growth Portfolio
                         International Growth Portfolio
                           Worldwide Growth Portfolio



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