JANUS INVESTMENT FUND
497, 1999-02-19
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<PAGE> 
                                                      [JANUS LOGO]
 
                         Janus Fund
 
                                       PROSPECTUS
                                       FEBRUARY 17, 1999
 
                         THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED
                         OR DISAPPROVED OF THESE SECURITIES OR PASSED ON THE
                         ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
                         REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
<PAGE> 
 
    [JANUS LOGO]
<PAGE> 
 
                                                               Table of contents
 
<TABLE>
                <S>                                               <C>
                RISK/RETURN SUMMARY
                   Janus Fund...................................    2
                   Fees and expenses............................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT
                STRATEGIES AND RISKS
                   Investment objective and principal investment
                   strategies...................................    5
                   General portfolio policies...................    6
                   Risks........................................    9
                SHAREHOLDER'S MANUAL
                   Minimum investments..........................   15
                   Types of account ownership...................   15
                   How to open your Janus account...............   17
                   How to purchase shares.......................   18
                   How to exchange shares.......................   20
                   How to redeem shares.........................   22
                   Shareholder services and account policies....   26
                MANAGEMENT OF THE FUND
                   Investment adviser...........................   31
                   Portfolio manager............................   32
                   Assistant portfolio managers.................   32
                OTHER INFORMATION............... ...............   34
                DISTRIBUTIONS AND TAXES
                   Distributions................................   35
                   Taxes........................................   36
                FINANCIAL HIGHLIGHTS.............. .............   38
                GLOSSARY
                   Glossary of investment terms.................   39
</TABLE>
 
                                                            Table of contents  1
<PAGE> 
Risk return summary
 
JANUS FUND
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF JANUS FUND?
 
               The Fund seeks long-term growth of capital in a manner consistent
               with the preservation of capital.
 
               The Fund's Trustees may change this objective without a
               shareholder vote and the Fund will notify you of any changes that
               are material. If there is a material change in the Fund's
               objective or policies, you should consider whether the Fund
               remains an appropriate investment for you. There is no guarantee
               that the Fund will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF JANUS FUND?
 
               The Fund invests primarily in common stocks selected for their
               growth potential. Although the Fund can invest in companies of
               any size, it generally invests in larger, more established
               companies.
 
               The portfolio manager applies a "bottom up" approach in choosing
               investments. In other words, he looks for companies with earnings
               growth potential one at a time. If the portfolio manager is
               unable to find investments with earnings growth potential, a
               significant portion of the Fund's assets may be in cash or
               similar investments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN JANUS FUND?
 
               The biggest risk of investing in this Fund is that its returns
               may vary and you could lose money. If you are considering
               investing in the Fund, remember that it is designed for long-term
               investors who can accept the risks of investing in a portfolio
               with significant common stock holdings. Common stocks tend to be
               more volatile than other investment choices.
 
               The value of the Fund's portfolio may decrease if the value of an
               individual company in the portfolio decreases. The value of the
               Fund's portfolio could also decrease if the stock market goes
               down. If the value of the Fund's portfolio decreases, the Fund's
               net asset value (NAV) will also decrease which means if you sell
               your shares in the Fund you would get back less money.
 
 2 Janus Fund
<PAGE> 
 
               An investment in the Fund is not a bank deposit and is not
               insured or guaranteed by the Federal Deposit Insurance
               Corporation or any other government agency.
 
               The following information illustrates how the Fund's performance
               has varied over time. The bar chart depicts the change in the
               Fund's performance from year-to-year during the periods
               indicated. The table compares the Fund's average annual returns
               for the periods indicated to a broad-based securities market
               index.
 
                JANUS FUND

A BAR CHART showing Annual Total Returns for Janus Fund from 1989 through 1998:

<TABLE>
<CAPTION>
                Annual returns for periods ended 12/31
                <S>      <C>       <C>      <C>     <C>      <C>       <C>      <C>      <C>      <C>
                46.32%   (0.74%)   42.80%   6.87%   10.92%   (1.10%)   29.43%   19.61%   22.72%   38.89%
                 1989     1990      1991    1992     1993     1994      1995     1996     1997     1998

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

                Best Quarter: 4th-1998  28.44%,  Worst Quarter: 3rd-1990  (13.91%)
</TABLE> 
 
<TABLE>
<CAPTION>
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
 
                                                                                Since Inception
                                                  1 year   5 years   10 years      (2/5/70)
                <S>                           <C>          <C>       <C>        <C>           
                Janus Fund                        38.89%   21.15%     20.43%        17.59%
                S&P 500 Index*                    28.74%   24.08%     19.20%        13.83%
                                              --------------------------------------------     
</TABLE>
 
               * The S&P 500 is the Standard & Poor's Composite Index of 500
                 Stocks, a widely recognized, unmanaged index of common stock
                 prices.
 
               The Fund's past performance does not necessarily indicate how it
               will perform in the future.
 
                                                          Risk return summary  3
<PAGE> 
 
FEES AND EXPENSES
 
               SHAREHOLDER FEES, such as sales loads, redemption fees or
               exchange fees, are charged directly to an investor's account. All
               Janus funds are no-load investments, so you will not pay any
               shareholder fees when you buy or sell shares of the Fund.
 
               ANNUAL FUND OPERATING EXPENSES are paid out of the Fund's assets
               and include fees for portfolio management, maintenance of
               shareholder accounts, shareholder servicing, accounting and other
               services. You do not pay these fees directly but, as the example
               below shows, these costs are borne indirectly by all
               shareholders.
 
               This table describes the fees and expenses that you may pay if
               you buy and hold shares of the Fund. It is based upon gross
               expenses (without the effect of expense offset arrangements) for
               the fiscal year ended October 31, 1998.
 
<TABLE>
<CAPTION>
                                                               Janus Fund
   <S>                                                        <C>       
   Management Fee                                                 0.65%
   Other Expenses                                                 0.22%
   Total Annual Fund Operating Expenses                           0.87%
</TABLE>
 
- --------------------------------------------------------------------------------
  EXAMPLE:
  This example is intended to help you compare the cost of investing in the
  Fund with the cost of investing in other mutual funds. The example
  assumes that you invest $10,000 in the Fund for the time periods
  indicated then redeem all of your shares at the end of those periods. The
  example also assumes that your investment has a 5% return each year and
  that the Fund's operating expenses remain the same. Although your actual
  costs may be higher or lower, based on these assumptions your costs would
  be:
 
<TABLE>
<CAPTION>
                                             1 Year    3 Years    5 Years    10 Years
                                             ----------------------------------------
   <S>                                       <C>       <C>        <C>        <C>     
   Janus Fund                                 $89       $278       $482       $1,073
</TABLE>
 
 4 Janus Fund
<PAGE> 
                                                Investment objective, principal 
                                                           investment strategies
                                                           and risks
  
               This section takes a closer look at the investment objective of
               the Fund, its principal investment strategies and certain risks
               of investing in the Fund. Strategies and policies that are noted
               as "fundamental" cannot be changed without a shareholder vote.
 
               Please carefully review the "Risks" section of this Prospectus on
               pages 9-11 for a discussion of risks associated with certain
               investment techniques. We've also included a Glossary with
               descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
               Janus Fund seeks long-term growth of capital in a manner
               consistent with the preservation of capital. It pursues its
               objective by investing primarily in common stocks selected for
               their growth potential. Although the Fund can invest in companies
               of any size, it generally invests in larger, more established
               companies.
 
               The Fund may invest substantially all of its assets in common
               stocks if its portfolio manager believes that common stocks will
               appreciate in value. The portfolio manager generally takes a
               "bottom up" approach to selecting companies. In other words, he
               seeks to identify individual companies with earnings growth
               potential that may not be recognized by the market at large. He
               makes this assessment by looking at companies one at a time,
               regardless of size, country of organization, place of principal
               business activity, or other similar selection criteria.
               Realization of income is not a significant consideration when
               choosing investments for the Fund. Income realized on the Fund's
               investments will be incidental to its objective.
 
               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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE> 
 
               There are no limitations on the countries in which the Fund may
               invest and the Fund may at times have significant foreign
               exposure.
 
GENERAL PORTFOLIO POLICIES
 
               In investing its portfolio assets, the Fund 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. So, for example, if
               the Fund exceeds a limit as a result of market fluctuations or
               the sale of securities, it will not be required to dispose of any
               securities.
 
               CASH POSITION
               When the Fund's portfolio manager believes that market conditions
               are unfavorable for profitable investing, or when he is otherwise
               unable to locate attractive investment opportunities, the Fund's
               cash or similar investments may increase. In other words, the
               Fund does not always stay fully invested in stocks. Cash or
               similar investments generally are a residual - they represent the
               assets that remain after the portfolio manager has committed
               available assets to desirable investment opportunities. However,
               the portfolio manager may also temporarily increase the Fund's
               cash position to protect its assets or maintain liquidity. When
               the Fund's investments in cash or similar investments increase,
               it may not participate in market advances or declines to the same
               extent that it would if the Fund remained more fully invested in
               stocks.
 
               OTHER TYPES OF INVESTMENTS
               The Fund invests primarily in domestic and foreign equity
               securities, which may include preferred stocks, common stocks,
               warrants and securities convertible into common or preferred
               stocks, but it may also invest to a lesser degree in other types
               of securities. These securities (which are described in the
               Glossary) may include:
 
               - debt securities
 
               - indexed/structured securities
 
 6 Janus Fund
<PAGE> 
 
               - high-yield/high-risk securities (less than 35% of the Fund's
                 assets)
 
               - options, futures, forwards and other types of derivatives for
                 hedging purposes or for non-hedging purposes such as seeking to
                 enhance return
 
               - securities purchased on a when-issued, delayed delivery or
                 forward commitment basis
 
               ILLIQUID INVESTMENTS
               The Fund may invest up to 15% of its net assets in illiquid
               investments. An illiquid investment is a security or other
               position that cannot be disposed of quickly in the normal course
               of business. For example, some securities are not registered
               under the U.S. securities laws and cannot be sold to the U.S.
               public because of SEC regulations (these are known as "restricted
               securities"). Under procedures adopted by the Fund's Trustees,
               certain restricted securities may be deemed liquid, and will not
               be counted toward this 15% limit.
 
               FOREIGN SECURITIES
               The Fund may invest without limit in foreign equity and debt
               securities. The Fund may invest directly in foreign securities
               denominated in a foreign currency and not publicly traded in the
               United States. Other ways of investing in foreign securities
               include depositary receipts or shares, and passive foreign
               investment companies.
 
               SPECIAL SITUATIONS
               The Fund may invest in special situations. A special situation
               arises when, in the opinion of the Fund'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. The
 
              Investment objective, principal investment strategies and risks  7
<PAGE> 
 
               Fund's performance could suffer if the anticipated development in
               a "special situation" investment does not occur or does not
               attract the expected attention.
 
               PORTFOLIO TURNOVER
               The Fund generally intends to purchase securities for long-term
               investment although, to a limited extent, the Fund may purchase
               securities in anticipation of relatively short-term price gains.
               Short-term transactions may also result from liquidity needs,
               securities having reached a price or yield objective, changes in
               interest rates or the credit standing of an issuer, or by reason
               of economic or other developments not foreseen at the time of the
               investment decision. The Fund may also sell one security and
               simultaneously purchase the same or a comparable security to take
               advantage of short-term differentials in bond yields or
               securities prices. Changes are made in the Fund's portfolio
               whenever its portfolio manager believes such changes are
               desirable. Portfolio turnover rates are generally not a factor in
               making buy and sell decisions.
 
               Increased portfolio turnover may result in higher costs for
               brokerage commissions, dealer mark-ups and other transaction
               costs and may also result in taxable capital gains. Higher costs
               associated with increased portfolio turnover may offset gains in
               the Fund's performance.
 
 8 Janus Fund
<PAGE> 
 
RISKS
 
               Because the Fund may invest substantially all of its assets in
               common stocks, the main risk is the risk that the value of the
               stocks it holds might decrease in response to the activities of
               an individual company or in response to general market and/or
               economic conditions. If this occurs, the Fund's share price may
               also decrease. The Fund's performance may also be affected by
               risks specific to certain types of investments, such as foreign
               securities, derivative investments, non-investment grade debt
               securities or companies with relatively small market
               capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Fund.
 
1. THE FUND MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
               Many attractive investment opportunities may be smaller, start-up
               companies offering emerging products or services. Smaller or
               newer companies may suffer more significant losses as well as
               realize more substantial growth than larger or more established
               issuers because they may lack depth of management, be unable to
               generate funds necessary for growth or potential development, or
               be developing or marketing new products or services for which
               markets are not yet established and may never become established.
               In addition, such companies may be insignificant factors in their
               industries and may become subject to intense competition from
               larger or more established companies. Securities of smaller or
               newer companies may have more limited trading markets than the
               markets for securities of larger or more established issuers, and
               may be subject to wide price fluctuations. Investments in such
               companies tend to be more volatile and somewhat more speculative.
 
              Investment objective, principal investment strategies and risks  9
<PAGE> 
 
2. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
               The portfolio manager carefully researches each potential
               investment before making an investment decision and, among other
               things, considers Year 2000 readiness when selecting portfolio
               holdings. However, there is no guarantee that the information the
               portfolio manager receives regarding a company's Year 2000
               readiness is completely accurate. If a company has not
               satisfactorily addressed Year 2000 issues, the Fund's performance
               could suffer.
 
3. HOW COULD THE FUND'S INVESTMENTS IN FOREIGN SECURITIES AFFECT ITS
   PERFORMANCE?
 
               The Fund may invest without limit in foreign securities either
               indirectly (e.g., depositary receipts) or directly in foreign
               markets. Investments in foreign securities, including those of
               foreign governments, may involve greater risks than investing in
               domestic securities because the Fund's performance may depend on
               issues other than the performance of a particular company. These
               issues include:
 
               - currency risk
 
               - political and economic risk
 
               - regulatory risk
 
               - market risk
 
               - transaction costs
 
               These risks are described in the SAI.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
               High-yield/high-risk securities (or "junk" bonds) are securities
               rated below investment grade by the primary rating agencies such
               as Standard & Poor's and Moody's. The value of lower quality
               securities generally is more dependent on credit risk, or the
               ability of the issuer to meet interest and principal payments,
               than
 
 10 Janus Fund
<PAGE> 
 
               investment grade debt securities. Issuers of high-yield
               securities may not be as strong financially as those issuing
               bonds with higher credit ratings and are more vulnerable to real
               or perceived economic changes, political changes or adverse
               developments specific to the issuer.
 
               Please refer to the SAI for a description of bond rating
               categories.
 
5. HOW DOES THE FUND TRY TO REDUCE RISK?
 
               The Fund may use futures, options and other derivative
               instruments to "hedge" or protect its portfolio from adverse
               movements in securities prices and interest rates. The Fund may
               also use a variety of currency hedging techniques, including
               forward currency contracts, to manage exchange rate risk. The
               Fund believes the use of these instruments will benefit the Fund.
               However, the Fund's performance could be worse than if the Fund
               had not used such instruments if the portfolio manager's
               judgement proves incorrect. Risks associated with the use of
               derivative instruments are described in the SAI.
 
             Investment objective, principal investment strategies and risks  11
<PAGE> 
 
Janus Fund
<PAGE> 
 
                   Shareholder's
                   Manual
 
                   This section will help you become
                   familiar with the different types
                   of accounts you can establish with
                   Janus. It also explains in detail
                   the wide array of services and
                   features you can establish on your
                   account, as well as account
                   policies and fees that may apply
                   to your account. Account policies
                   (including fees), services and
                   features may be modified or
                   discontinued without shareholder
                   approval or prior notice.
 
                                                    [JANUS LOGO]
<PAGE> 
 
HOW TO GET IN TOUCH WITH JANUS
 
               Janus offers two Investor Service Centers for those individuals
               who would like to conduct their investing in person. Our
               representatives will be happy to assist you at either of the
               following locations: Monday-Friday 7:00 a.m. to 6:00 p.m.
               Mountain time and Saturday 9:00 a.m. to 1:00 p.m. Mountain time.
 
               100 Fillmore Street, Suite 100
               Denver, CO 80206
 
               3773 Cherry Creek North Drive, Suite 101
               Denver, CO 80209
 
 QUICK ADDRESS AND TELEPHONE REFERENCE
- --------------------------------------------------------------------------------
 
<TABLE>
  <S>                                           <C>
  MAILING ADDRESS                               JANUS XPRESSLINE(TM)
  Janus                                         1-888-979-7737
  P.O. Box 173375                               For 24-hour access to account
  Denver, CO 80217-3375                         and fund information,
                                                exchanges, purchases and
  FOR OVERNIGHT CARRIER                         redemptions, automated daily
  Janus                                         quotes on fund share prices,
  Suite 101                                     yields and total refunds.
  3773 Cherry Creek North Drive
  Denver, CO 80209-3811                         TDD
                                                1-800-525-0056
  INVESTOR SERVICE REPRESENTATIVES              A telecommunications device
  If you have any questions while reading       for our hearing- and
  this Prospectus, please call one of our       speech-impaired shareholders.
  Investor Service Representatives at
  1-800-525-3713 Monday-Friday: 8:00            JANUS LITERATURE LINE
  a.m.-8:00 p.m., and Saturday: 10:00           1-800-525-8983
  a.m.-4:00 p.m., New York time.                To request a prospectus,
                                                shareholder reports or
  JANUS INTERNET ADDRESS                        marketing materials 24 hours a
  janus.com                                     day.
</TABLE>
 
 14 Janus Fund
<PAGE> 
 
MINIMUM INVESTMENTS*
- ---------------------------------------------
 
<TABLE>
<S>                                 <C>
To open a new regular account        $2,500

To open a new retirement,
education or UGMA/UTMA account       $  500

To open a new regular account with
an Automatic Investment Program      $  500**

To add to any type of an account     $  100+
</TABLE>
 
 * The Fund reserves the right to
   change the amount of these
   minimums from time to time or
   to waive them in whole or in
   part for certain types of
   accounts.
** An Automatic Investment Program
   requires a $100 minimum
   automatic investment per month
   until the account balance
   reaches $2,500.
 + The minimum subsequent
   investment for IRA UGMA/UTMA
   accounts is $50.
 
TYPES OF ACCOUNT OWNERSHIP
 
               If you are investing in the Fund for the first time, you will
               need to establish an account. You can establish the following
               types of accounts by completing a New Account Application. To
               request an application, call 1-800-525-3713 or visit our Web site
               at janus.com to download an application.
 
               INDIVIDUAL OR JOINT OWNERSHIP
               Individual accounts are owned by one person. Joint accounts have
               two or more owners.
 
               A GIFT OR TRANSFER TO MINOR (UGMA OR UTMA)
               An UGMA/UTMA account is a custodial account managed for the
               benefit of a minor. To open an UGMA or UTMA account, you must
               include the minor's Social Security number on the application.
 
               TRUST
               An established trust can open an account. The names of each
               trustee, the name of the trust and the date of the trust
               agreement must be included on the application.
 
                                                        Shareholder's manual  15
<PAGE> 
 
               BUSINESS ACCOUNTS
               Corporations and partnerships may also open an account. The
               application must be signed by an authorized officer of the
               corporation or a general partner of the partnership.
 
TAX-DEFERRED ACCOUNTS
 
               If you are eligible, you may set up one or more tax-deferred
               accounts. A tax-deferred account allows you to shelter your
               investment income and capital gains from current income taxes. A
               contribution to certain of these plans may also be tax
               deductible. Tax-deferred accounts include retirement plans
               described below and the Education IRA. Distributions from these
               plans are generally subject to income tax and may be subject to
               an additional tax if withdrawn prior to age 59 1/2 or used for a
               nonqualifying purpose. Investors should consult their tax adviser
               or legal counsel before selecting a tax-deferred account.
 
               Investors Fiduciary Trust Company serves as custodian for the
               tax-deferred accounts offered by the Fund. You will be charged an
               annual account maintenance fee of $12 for each taxpayer
               identification number no matter how many tax-deferred accounts
               you have with Janus. You may pay the fee by check or have it
               automatically deducted from your account (usually in December).
               The custodian reserves the right to change the amount of this fee
               or to waive it in whole or in part for certain types of accounts.
 
               The following plans require a special application. For an
               application and more details about our Retirement Plans, call
               1-800-525-3713.
 
               TRADITIONAL AND ROTH INDIVIDUAL RETIREMENT ACCOUNTS
               Both types of IRAs allow most individuals with earned income to
               contribute up to the lesser of $2,000 ($4,000 for most married
               couples) or 100% of compensation annually. Please refer to the
               Janus IRA booklet for more complete information regarding the
               different types of IRAs.
 
 16 Janus Fund
<PAGE> 
 
               EDUCATION IRA
               This plan allows individuals, subject to certain income
               limitations, to contribute up to $500 annually on behalf of any
               child under the age of 18. Please refer to the Janus IRA booklet
               for more complete information regarding the Education IRA.
 
               SIMPLIFIED EMPLOYEE PENSION PLAN
               This plan allows small business owners (including sole
               proprietors) to make tax-deductible contributions for themselves
               and any eligible employee(s). A SEP requires an IRA (a SEP-IRA)
               to be set up for each SEP participant.
 
               PROFIT SHARING OR MONEY PURCHASE PENSION PLAN
               These plans are open to corporations, partnerships and sole
               proprietors to benefit their employees and themselves.
 
               SECTION 403(b)(7) PLAN
               Employees of educational organizations or other qualifying, tax-
               exempt organizations may be eligible to participate in a Section
               403(b)(7) Plan.
 
HOW TO OPEN YOUR JANUS ACCOUNT
 
               Complete and sign the appropriate application. Please be sure to
               provide your Social Security or taxpayer identification number on
               the application and make your check payable to Janus. The Fund is
               available only to U.S. citizens or residents, and your
               application will be returned to you if you do not meet these
               criteria. Send all items to one of the addresses listed in the
               "Quick Address and Telephone Reference" on page 14.
 
                                                        Shareholder's manual  17
<PAGE> 
 
HOW TO PURCHASE SHARES
 
               PAYING FOR SHARES
 
               When you purchase shares, your request will be processed at the
               next NAV calculated after your order is received and accepted.
               Please note the following:
 
               - Cash, credit cards, third party checks and credit card checks
                 will not be accepted.
 
               - All purchases must be made in U.S. dollars.
 
               - Checks must be drawn on a U.S. bank and made payable to Janus.
 
               - If a check does not clear your bank, the Fund reserves the
                 right to cancel the purchase.
 
               - If the Fund is unable to debit your predesignated bank account
                 on the day of purchase, it may make additional attempts or
                 cancel the purchase.
 
               - The Fund reserves the right to reject any specific purchase
                 request.
 
               If your purchase is cancelled you will be responsible for any
               losses or fees imposed by your bank and losses that may be
               incurred as a result of any decline in the value of the cancelled
               purchase. The Fund (or its agents) has the authority to redeem
               shares in your account(s) to cover any such losses due to
               fluctuations in share price. Any profit on such cancellation will
               accrue to the Fund.
 
               ONCE YOU HAVE OPENED YOUR JANUS ACCOUNT, THE MINIMUM AMOUNT FOR
               AN ADDITIONAL INVESTMENT IS $100 ($50 FOR IRAs OR UGMA/UTMA
               ACCOUNTS). You may add to your account at any time through any of
               the following options:
 
               BY MAIL
 
               Complete the remittance slip attached at the bottom of your
               confirmation statement. If you are making a purchase into a
               retirement account, please indicate whether the purchase is a
 
 18 Janus Fund
<PAGE> 
 
               rollover or a current or prior year contribution. Send your check
               made payable to Janus and remittance slip or written instructions
               to one of the addresses listed previously. You may also request a
               booklet of remittance slips for non-retirement accounts.
 
               BY TELEPHONE
 
               This service allows you to purchase additional shares quickly and
               conveniently through an electronic transfer of money. To purchase
               shares by telephone, call an Investor Service Representative at
               1-800-525-3713 during normal business hours or call the Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. When you make an additional purchase by telephone, Janus
               will automatically debit your predesignated bank account for the
               desired amount. To establish the telephone purchase option on
               your new account, complete the "Telephone Purchase of Shares
               Option" section on the application and attach a "voided" check or
               deposit slip from your bank account. If your account is already
               established, call 1-800-525-3713 to request the appropriate form.
               This option will become effective ten business days after the
               form is received.
 
               BY WIRE
 
               Purchases may also be made by wiring money from your bank account
               to your Janus account. Call 1-800-525-3713 to receive wiring
               instructions.
 
               BY INTERNET
 
               You must pre-establish the "Telephone Purchase of Shares Option"
               to make a purchase on our Web site at janus.com. If you have
               questions, please call 1-800-975-9932 to speak to a Janus
               representative.
 
               AUTOMATIC INVESTMENT PROGRAMS
 
               Janus offers several automatic investment programs to help you
               achieve your financial goals as simply and conveniently as
 
                                                        Shareholder's manual  19
<PAGE> 
 
               possible. You may open a new account with a $500 initial purchase
               and $100 automatic subsequent investments.
 
               AUTOMATIC MONTHLY INVESTMENT PROGRAM
               You select the day each month that your money ($100 minimum) will
               be electronically transferred from your bank account to your Fund
               account. To establish this option, complete the "Automatic
               Monthly Investment Program" section on the application and attach
               a "voided" check from your bank account. If your Fund account is
               already established, call 1-800-525-3713 to request the
               appropriate form.
 
               PAYROLL DEDUCTION
               If your employer can initiate an automatic payroll deduction, you
               may have all or a portion of your paycheck ($100 minimum)
               invested directly into your Fund account. To obtain information
               on establishing this option, call 1-800-525-3713.
 
               SYSTEMATIC EXCHANGE
               With a Systematic Exchange you determine the amount of money
               ($100 minimum) you would like automatically exchanged from one
               Janus account to another on any day of the month. For more
               information on how to establish this option, call 1-800-525-3713.
 
HOW TO EXCHANGE SHARES
 
               On any business day, you may exchange all or a portion of your
               shares into any other available Janus fund.
 
               IN WRITING
 
               To request an exchange in writing, please follow the instructions
               for written requests on page 24.
 
               BY TELEPHONE
 
               All accounts are automatically eligible for the telephone
               exchange option. To exchange shares by telephone, call an
               Investor Service Representative at 1-800-525-3713 during normal
               business hours
 
 20 Janus Fund
<PAGE> 
 
               or call the Janus XpressLine, 1-888-979-7737, for access to this
               option 24 hours a day.
 
               BY SYSTEMATIC EXCHANGE
 
               As noted above, you may establish a Systematic Exchange for as
               little as $100 per month on established accounts. You may
               establish a new account with a $500 initial purchase and
               subsequent $100 systematic exchanges. If the balance in the
               account you are exchanging from falls below the systematic
               exchange amount, all remaining shares will be exchanged and the
               program will be discontinued.
 
               BY INTERNET
 
               Exchanges may also be made on our Web site at janus.com.
 
               EXCHANGE POLICIES
 
               - Except for Systematic Exchanges, new accounts established by
                 exchange must be opened with $2,500 or the total account value
                 if the value of the account you are exchanging from is less
                 than $2,500.
 
               - Exchanges between existing accounts must meet the $100
                 subsequent investment requirement.
 
               - You may make four exchanges out of the Fund during a calendar
                 year (exclusive of Systematic Exchanges). Exchanges in excess
                 of this limit may be subject to an exchange fee or may result
                 in termination of the exchange privilege.
 
               - The Fund reserves the right to reject any exchange request and
                 to modify or terminate the exchange privilege at any time. For
                 example, the Fund may reject exchanges from accounts engaged in
                 or known to engage in trading in excess of the limit above
                 (including market timing transactions).
 
               - Exchanges between accounts will be accepted only if the
                 registrations are identical.
 
                                                        Shareholder's manual  21
<PAGE> 
 
               - If the shares you are exchanging are held in certificate form,
                 you must return the certificate to the Fund prior to making any
                 exchanges.
 
               - Be sure to read the prospectus for the fund into which you are
                 exchanging.
 
               - An exchange represents the sale of shares from one fund and the
                 purchase of shares of another fund, which may produce a taxable
                 gain or loss in a non-tax deferred account.
 
HOW TO REDEEM SHARES
 
               On any business day, you may redeem all or a portion of your
               shares. If the shares are held in certificate form, the
               certificate must be returned with or before your redemption
               request. Your transaction will be processed at the next NAV
               calculated after your order is received and accepted. The
               redemption may be suspended for 10 days following an address
               change unless a signature guarantee is provided.
 
               IN WRITING
 
               To request a redemption in writing, please follow the
               instructions for written requests noted on page 24.
 
               BY TELEPHONE
 
               Most accounts have the telephone redemption option, unless this
               option was specifically declined on the application or in
               writing. This option enables you to request redemptions daily
               from your account by calling 1-800-525-3713 by the close of the
               regular trading session of the New York Stock Exchange ("NYSE")
               normally 4:00 p.m. New York time. You may also use Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. (There is a daily limit of $100,000 per account for
               redemptions payable by check.)
 
 22 Janus Fund
<PAGE> 
 
               BY INTERNET
 
               Redemptions may also be made on our Web site at janus.com.
 
               SYSTEMATIC REDEMPTION OPTION
 
               The Systematic Redemption Option allows you to redeem a specific
               dollar amount from your Fund account on a regular basis. For more
               information or to request the appropriate form, please call
               1-800-525-3713.
 
PAYMENT OF REDEMPTION PROCEEDS
 
               BY CHECK
               Redemption proceeds will be sent to the shareholder(s) of record
               at the address of record within seven days after receipt of a
               valid redemption request.
 
               BY ELECTRONIC TRANSFER
               If you have established the electronic redemption option, your
               redemption proceeds can be electronically transferred to your
               predesignated bank account on the next bank business day after
               receipt of your redemption request (wire transfer) or the second
               bank business day after receipt of your redemption request (ACH
               transfer). Wire transfers will be charged an $8 fee per wire and
               your bank may charge an additional fee to receive the wire. ACH
               transfers are made free of charge. Wire redemptions are not
               available for retirement accounts.
 
               If you would like to establish the electronic redemption option
               on an existing account, please call 1-800-525-3713 to request the
               appropriate form.
 
               IF THE SHARES BEING REDEEMED WERE PURCHASED BY CHECK, TELEPHONE,
               ON OUR WEB SITE, OR THROUGH THE AUTOMATIC MONTHLY INVESTMENT
               PROGRAM, THE FUND MAY DELAY THE PAYMENT OF YOUR REDEMPTION
               PROCEEDS FOR UP TO 15 DAYS FROM THE DAY OF PURCHASE TO ALLOW THE
               PURCHASE TO CLEAR. Unless you provide alternate instructions,
               your proceeds will be invested in Janus
 
                                                        Shareholder's manual  23
<PAGE> 
 
               Money Market Fund - Investor Shares during the 15 day hold
               period.
 
WRITTEN INSTRUCTIONS
 
               To redeem all or part of your shares in writing, your request
               should be sent to one of the addresses listed on page 14 and must
               include the following information:
 
               - the name of the Fund
 
               - the account number
 
               - the amount of money or number of shares being redeemed or
                 exchanged
 
               - the name(s) on the account registration
 
               - the signature(s) of all registered account owners
 
               - your daytime telephone number
 
               SIGNATURE REQUIREMENTS BASED ON ACCOUNT TYPE
 
               INDIVIDUAL, JOINT TENANTS, TENANTS IN COMMON
               Written instructions must be signed by each shareholder, exactly
               as the names appear in the account registration.
 
               UGMA OR UTMA
               Written instructions must be signed by the custodian in his/her
               capacity as it appears in the account registration.
 
               SOLE PROPRIETOR, GENERAL PARTNER
               Written instructions must be signed by an authorized individual
               in his/her capacity as it appears on the account registration.
 
               CORPORATION, ASSOCIATION
               Written instructions must be signed by the person(s) authorized
               to act on the account. In addition, a certified copy of the
               corporate resolution authorizing the signer to act must accompany
               the request.
 
 24 Janus Fund
<PAGE> 
 
               TRUST
               Written instructions must be signed by the trustee(s). If the
               name(s) of the current trustee(s) does not appear in the account
               registration, a certificate of incumbency dated within 60 days
               must also be submitted.
 
               IRA
               Written instructions must be signed by the account owner. If you
               do not want federal income tax withheld from your redemption, you
               must state that you elect not to have such withholding apply. In
               addition, your instructions must state whether the distribution
               is normal (after age 59 1/2) or premature (before age 59 1/2)
               and, if premature, whether any exceptions such as death or
               disability apply with regard to the 10% additional tax on early
               distributions.
 
SIGNATURE GUARANTEE
 
               In addition to the signature requirements, A SIGNATURE GUARANTEE
               IS ALSO REQUIRED if any of the following is applicable:
 
               - You request a redemption by check that exceeds $100,000.
 
               - You would like the check made payable to anyone other than the
                 shareholder(s) of record.
 
               - You would like the check mailed to an address which has been
                 changed within 10 days of the redemption request.
 
               - You would like the check mailed to an address other than the
                 address of record.
 
               THE FUND RESERVES THE RIGHT TO REQUIRE A SIGNATURE GUARANTEE
               UNDER OTHER CIRCUMSTANCES OR TO REJECT OR DELAY A REDEMPTION ON
               CERTAIN LEGAL GROUNDS. FOR MORE INFORMATION PERTAINING TO
               SIGNATURE GUARANTEES, PLEASE CALL 1-800-525-3713.
 
               HOW TO OBTAIN A SIGNATURE GUARANTEE
 
               A signature guarantee assures that a signature is genuine. The
               signature guarantee protects shareholders from unauthorized
               account transfers. The following financial institutions may
               guaran-
 
                                                        Shareholder's manual  25
<PAGE> 
 
               tee signatures: banks, savings and loan associations, trust
               companies, credit unions, broker-dealers, and member firms of a
               national securities exchange. Call your financial institution to
               see if they have the ability to guarantee a signature. A
               signature guarantee cannot be provided by a notary public.
 
               If you live outside the United States, a foreign bank properly
               authorized to do business in your country of residence or a U.S.
               consulate may be able to authenticate your signature.
 
PRICING OF FUND SHARES
 
               All purchases, redemptions and exchanges will be processed at the
               NAV next calculated after your request is received and accepted
               by the Fund (or the Fund's agent or authorized designee). The
               Fund's NAV is calculated at the close of the regular trading
               session of the NYSE (normally 4:00 p.m. New York time) each day
               that the NYSE is open. The NAV of Fund shares is not determined
               on days the NYSE is closed (generally, New Year's Day, Martin
               Luther King Day, Presidents' Day, Good Friday, Memorial Day,
               Independence Day, Labor Day, Thanksgiving and Christmas). In
               order to receive a day's price, your order must be received by
               the close of the regular trading session of the NYSE. Securities
               are valued at market value or, if a market quotation is not
               readily available, at their fair value determined in good faith
               under procedures established by and under the supervision of the
               Trustees. Short-term instruments maturing within 60 days are
               valued at amortized cost, which approximates market value. See
               the SAI for more detailed information.
 
SHAREHOLDER SERVICES AND ACCOUNT POLICIES
 
               JANUS XPRESSLINE(TM)
 
               Janus XpressLine, our electronic telephone service, offers you
               24-hour access by TouchTone(TM) telephone to obtain information
               on account balances, Fund performance or dividends. You can also
               make exchanges, purchases and redemptions in existing accounts,
               request literature about any Janus fund, or order
 
 26 Janus Fund
<PAGE> 
 
               duplicate statements. Janus XpressLine is accessed by calling
               1-888-979-7737. Calls are limited to five minutes.
 
               JANUS WEB SITE
 
               Janus maintains a Web site located at JANUS.COM. You can
               purchase, exchange and redeem shares and access information such
               as your account balance and the Fund's NAV through the Web site.
               In order to engage in transactions on our Web site, you must
               authorize us to transmit account information online and accept
               online instructions (see janus.com and follow the procedures
               accordingly). You may also need to have bank account information,
               wire instructions or other options established on your account.
               The Fund and its agents will not be responsible for any losses
               resulting from unauthorized transactions on our Web site when
               procedures designed for engaging in such transactions are
               followed. If you have questions, please call 1-800-975-9932 to
               speak to a Janus representative.
 
               ACCOUNT MINIMUMS
 
               Due to the proportionately higher costs of maintaining small
               accounts, Janus reserves the right to deduct a $10 minimum
               balance fee (or the value of the account if less than $10) from
               accounts with values below the minimums described on page 15 or
               to close such accounts. This policy will apply to accounts
               participating in the Automatic Monthly Investment Program only if
               your account balance does not reach the required minimum initial
               investment or falls below such minimum and you have discontinued
               monthly investments. This policy does not apply to accounts that
               fall below the minimums solely as a result of market value
               fluctuations. It is expected that, for purposes of this policy,
               accounts will be valued in September, and the $10 fee will be
               assessed on the second Friday of September of each year. You will
               receive notice before we charge the $10 fee or close your account
               so that you may increase your account balance to the required
               minimum.
 
                                                        Shareholder's manual  27
<PAGE> 
 
               TRANSACTIONS THROUGH PROCESSING ORGANIZATIONS
 
               You may purchase or sell Fund shares through a broker-dealer,
               bank or other financial institution, or an organization that
               provides recordkeeping and consulting services to 401(k) plans or
               other employee benefit plans (a "Processing Organization").
               Processing Organizations may charge you a fee for this service
               and may require different minimum initial and subsequent
               investments than the Fund. Processing Organizations may also
               impose other charges or restrictions different from those
               applicable to shareholders who invest in the Fund directly. A
               Processing Organization, rather than its customer, may be the
               shareholder of record of your shares. The Fund is not responsible
               for the failure of any Processing Organization to carry out its
               obligations to its customers. Certain Processing Organizations
               may receive compensation from Janus Capital or its affiliates and
               certain Processing Organizations may receive compensation from
               the Fund for shareholder recordkeeping and similar services.
 
               TAXPAYER IDENTIFICATION NUMBER
 
               On your application or other appropriate form, you will be asked
               to certify that your Social Security or taxpayer identification
               number is correct and that you are not subject to backup
               withholding for failing to report income to the IRS. If you are
               subject to the 31% backup withholding or you did not certify your
               taxpayer identification number, the IRS requires the Fund to
               withhold 31% of any dividends paid and redemption or exchange
               proceeds. In addition to the 31% backup withholding, you may be
               subject to a $50 fee to reimburse the Fund for any penalty that
               the IRS may impose.
 
               SHARE CERTIFICATES
 
               Most shareholders choose not to hold their shares in certificate
               form because account transactions such as exchanges and
               redemptions cannot be completed until the certificate has been
               returned to the Fund. The Fund will issue share certificates upon
               written request only and reserves the right to charge a fee for
               this service.
 
 28 Janus Fund
<PAGE> 
 
               Share certificates will not be issued until the shares have been
               held for at least 15 days and will not be issued for accounts
               that do not meet the minimum investment requirements. Share
               certificates cannot be issued for retirement accounts. In
               addition, if the certificate is lost, there may be a replacement
               charge.
 
               INVOLUNTARY REDEMPTIONS
 
               The Fund reserves the right to close an account if the
               shareholder is deemed to engage in activities which are illegal
               or otherwise believed to be detrimental to the Fund.
 
               TELEPHONE TRANSACTIONS
 
               You may initiate many transactions by telephone. The Fund and its
               agents will not be responsible for any losses resulting from
               unauthorized transactions when procedures designed to verify the
               identity of the caller are followed.
 
               It may be difficult to reach an Investor Service Representative
               by telephone during periods of unusual market activity. If you
               are unable to reach a representative by telephone, please
               consider sending written instructions, stopping by a Service
               Center, calling the Janus XpressLine or visiting our Web site.
 
               TEMPORARY SUSPENSION OF SERVICES
 
               The Fund or its agents may, in case of emergency, temporarily
               suspend telephone transactions or other shareholder services.
 
               ADDRESS CHANGES
 
               To change the address on your account, call 1-800-525-3713 or
               send a written request signed by all account owners. Include the
               name of the Fund, the account number(s), the name(s) on the
               account and both the old and new addresses. Certain options may
               be suspended for 10 days following an address change unless a
               signature guarantee is provided.
 
                                                        Shareholder's manual  29
<PAGE> 
 
               REGISTRATION CHANGES
 
               To change the name on an account, the shares are generally
               transferred to a new account. In some cases, legal documentation
               may be required. For more information call 1-800-525-3713.
 
               STATEMENTS AND REPORTS
 
               Investors participating in an automatic investment program will
               receive quarterly confirmations of all transactions. Dividend
               information will be distributed annually. In addition, the Fund
               will send you an immediate transaction confirmation statement
               after every non-systematic transaction.
 
               The Fund produces financial reports, which include a list of the
               Fund's portfolio holdings, semiannually and updates its
               prospectus annually. To reduce expenses, the Fund may choose to
               mail only one report or prospectus to your household, even if
               more than one person in the household has a Fund account. Please
               call 1-800-525-3713 if you would like to receive additional
               reports or prospectuses. The Fund reserves the right to charge a
               fee for additional statement and report requests.
 
 30 Janus Fund
<PAGE> 
                                                          Management of the fund
 
INVESTMENT ADVISER
 
               Janus Capital Corporation, 100 Fillmore Street, Denver, Colorado
               80206-4928, is the investment adviser to the Fund and is
               responsible for the day-to-day management of its investment
               portfolio and other business affairs.
 
               Janus Capital began serving as investment adviser to Janus Fund
               in 1970 and currently serves as investment adviser to all of the
               Janus funds, acts as sub-adviser for a number of private-label
               mutual funds and provides separate account advisory services for
               institutional accounts.
 
               Janus Capital furnishes continuous advice and recommendations
               concerning the Fund's investments. Janus Capital also furnishes
               certain administrative, compliance and accounting services for
               the Fund, and may be reimbursed by the Fund 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 Fund and pays the salaries, fees and expenses of
               all Fund officers and those Trustees who are affiliated with
               Janus Capital.
 
               The Fund pays Janus Capital a management fee which is calculated
               daily and paid monthly. The advisory agreement with the Fund
               spells out the management fee and other expenses that the Fund
               must pay.
 
               The Fund incurs 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 Annual Fund Operating Expenses
               table on page 4 lists the actual management fee and total
               operating expenses of the Fund for the most recent fiscal year.
 
                                                      Management of the fund  31
<PAGE> 
 
PORTFOLIO MANAGER
 
JAMES P. CRAIG, III
- --------------------------------------------------------------------------------
                   is Chief Investment Officer of Janus Capital. He is
                   Executive Vice President and portfolio manager of the
                   Fund, which he has managed since 1986. He is also
                   Executive Vice President and a co-manager of Janus Venture
                   Fund, which he has managed since February 1, 1997. Mr.
                   Craig previously managed 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.
 
ASSISTANT PORTFOLIO MANAGERS
 
DAVID C. DECKER
- --------------------------------------------------------------------------------
                   is an assistant portfolio manager of the Fund. He is
                   Executive Vice President and portfolio manager of Janus
                   Special Situations Fund, which he has managed since
                   inception. He joined Janus Capital in 1992 as a research
                   analyst and focused on companies in the automotive and
                   defense industries prior to managing Janus Special
                   Situations Fund. He obtained his Master of Business
                   Administration in Finance from the Fuqua School of
                   Business at Duke University and a Bachelor of Arts in
                   Economics and Political Science from Tufts University. Mr.
                   Decker received the Chartered Financial Analyst
                   designation.
 
 32 Janus Fund
<PAGE> 
 
KAREN L. REIDY
- --------------------------------------------------------------------------------
                   is an assistant portfolio manager of the Fund. Prior to
                   joining Janus Capital in 1995, she worked for Price
                   Waterhouse in the Mergers and Acquisitions area,
                   performing corporate due diligence, and as an audit
                   senior, analyzing financials for corporate clients. She
                   received an undergraduate degree in Accounting from the
                   University of Colorado. She is seeking the Chartered
                   Financial Analyst designation.
 
BLAINE P. ROLLINS
- --------------------------------------------------------------------------------
                   is an assistant portfolio manager of the Fund. He is
                   Executive Vice President and portfolio manager of Janus
                   Balanced Fund, which he has managed since January 1996,
                   and Janus Equity Income Fund, which he has managed since
                   inception. Mr. Rollins joined Janus Capital in 1990 and
                   gained experience as a fixed-income trader and equity
                   research analyst prior to managing Janus Balanced Fund. He
                   holds a Bachelor of Science in Finance from the University
                   of Colorado and received the Chartered Financial Analyst
                   designation.
 
                                                      Management of the fund  33
<PAGE> 
Other information
 
               SIZE OF THE FUND
 
               Although there is no present intention to do so, the Fund may
               discontinue sales of its shares if management and the Trustees
               believe that continued sales may adversely affect the Fund's
               ability to achieve its investment objective. If sales of the Fund
               are discontinued, it is expected that existing shareholders of
               the Fund would be permitted to continue to purchase shares and to
               reinvest any dividends or capital gains distributions, absent
               highly unusual circumstances.
 
               YEAR 2000
 
               Preparing for Year 2000 is a high priority for Janus Capital,
               which has established a dedicated group to address this issue.
               Janus Capital has devoted considerable internal resources and has
               engaged one of the foremost experts in the field to help achieve
               Year 2000 readiness. Janus Capital does not anticipate that Year
               2000-related issues will have a material impact on its ability to
               continue to provide the Fund with service at current levels;
               however, Janus Capital cannot make any assurances that the steps
               it has taken to ensure Year 2000 readiness will be successful. In
               addition, there can be no assurance that Year 2000 issues will
               not affect the companies in which the Fund invests or worldwide
               markets and economies.
 
 34 Janus Fund
<PAGE> 
                                                         Distributions and taxes
 
DISTRIBUTIONS
 
               To avoid taxation of the Fund, the Internal Revenue Code requires
               the Fund to distribute net income and any net capital gains
               realized on its investments annually. The Fund's income from
               dividends and interest and any net realized short-term gains are
               paid to shareholders as ordinary income dividends. Net realized
               long-term gains are paid to shareholders as capital gains
               distributions. Dividends and capital gains distributions are
               normally declared and paid in December.
 
               HOW DISTRIBUTIONS AFFECT THE FUND'S NAV
 
               Distributions are paid to shareholders as of the record date of
               the distribution of the Fund, regardless of how long the shares
               have been held. Dividends and capital gains awaiting distribution
               are included in the Fund's daily NAV. The share price of the Fund
               drops by the amount of the distribution, net of any subsequent
               market fluctuations. As an example, assume that on December 31,
               the Fund declared a dividend in the amount of $0.25 per share. If
               the Fund's share price was $10.00 on December 30, the Fund's
               share price on December 31 would be $9.75, barring market
               fluctuations. Shareholders should be aware that distributions
               from a taxable mutual fund are not value-enhancing and may create
               income tax obligations.
 
               "BUYING A DIVIDEND"
 
               If you purchase shares of the Fund just before the distribution,
               you will pay the full price for the shares and receive a portion
               of the purchase price back as a taxable distribution. This is
               referred to as "buying a dividend." In the above example, if you
               bought shares on December 30, you would have paid $10.00 per
               share. On December 31, the Fund would pay you $0.25 per share as
               a dividend and your shares would now be worth $9.75 per share.
               Unless your account is set up as a tax-deferred account,
               dividends paid to you would be included in your gross income for
               tax purposes, even though you may not have participated in the
 
                                                     Distributions and taxes  35
<PAGE> 
 
               increase in NAV of the Fund, whether or not you reinvested the
               dividends.
 
DISTRIBUTION OPTIONS
 
               When you open an account, you must specify on your application
               how you want to receive your distributions. You may change your
               distribution option at any time by writing the Fund at one of the
               addresses on page 14 or calling 1-800-525-3713. The Fund offers
               the following options:
 
               1. REINVESTMENT OPTION. You may reinvest your income dividends
                  and capital gains distributions in additional shares. This
                  option is assigned automatically if no other choice is made.
 
               2. CASH OPTION. You may receive your income dividends and capital
                  gains distributions in cash.
 
               3. REINVEST AND CASH OPTION. You may receive either your income
                  dividends or capital gains distributions in cash and reinvest
                  the other in additional shares.
 
               4. REDIRECT OPTION. You may direct your dividends or capital
                  gains to purchase shares of another Janus fund.
 
               The Fund reserves the right to reinvest into your account
               undeliverable and uncashed dividend and distribution checks that
               remain outstanding for six months in shares of the Fund at the
               NAV next computed after the check is cancelled. Subsequent
               distributions may also be reinvested.
 
TAXES
 
               As with any investment, you should consider the tax consequences
               of investing in the Fund. Any time you sell or exchange shares of
               a Fund in a taxable account, it is considered a taxable event.
               Depending on the purchase price and the sale price, you may have
               a gain or loss on the transaction. Any tax liabilities generated
               by your transactions are your responsibility.
 
               The following discussion does not apply to tax-deferred accounts,
               nor is it a complete analysis of the federal tax implications of
               investing in the Fund. You may wish to consult your own tax
 
 36 Janus Fund
<PAGE> 
 
               adviser. Additionally, state or local taxes may apply to your
               investment, depending upon the laws of your state of residence.
 
               TAXES ON DISTRIBUTIONS
 
               Dividends and distributions by the Fund are subject to federal
               income tax, regardless of whether the distribution is made in
               cash or reinvested in additional shares of the Fund.
               Distributions may be taxable at different rates depending on the
               length of time the Fund holds a security. In certain states, a
               portion of the dividends and distributions (depending on the
               source of the Fund's income) may be exempt from state and local
               taxes. Information regarding the tax status of income dividends
               and capital gains distributions will be mailed to shareholders on
               or before January 31st of each year. Account tax information will
               also be sent to the IRS.
 
               TAXATION OF THE FUND
 
               Dividends, interest, and some capital gains received by the Fund
               on foreign securities may be subject to tax withholding or other
               foreign taxes. The Fund 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 Fund.
 
               The Fund does not expect to pay federal income or excise taxes
               because it intends to meet certain requirements of the Internal
               Revenue Code. It is important that the Fund meet these
               requirements so that any earnings on your investment will not be
               taxed twice.
 
                                                     Distributions and taxes  37
<PAGE> 
Financial highlights
 
               The financial highlights table is intended to help you understand
               the Fund's financial performance for the past 5 years through
               October 31st of each fiscal year shown. Items 1 through 9 reflect
               financial results for a single Fund share. The total returns in
               the table represent the rate that an investor would have earned
               (or lost) on an investment in the Fund (assuming reinvestment of
               all dividends and distributions). This information has been
               audited by PricewaterhouseCoopers LLP, whose report, along with
               the Fund's financial statements, are included in the Annual
               Report, which is available upon request and incorporated by
               reference into the SAI.
 
 
<TABLE>
<CAPTION>

                                   JANUS FUND
- ----------------------------------------------------------------------------------------
                                                   Periods ending October 31st
                                           1998      1997      1996      1995      1994
<S>                                       <C>       <C>       <C>       <C>       <C>
  1. NET ASSET VALUE, BEGINNING OF
     PERIOD                                $29.36    $26.65    $23.37    $19.62   $20.81
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                 (0.02)      0.15      0.31      0.16     0.17
  3. Net gains or losses on securities
     (both realized and unrealized)          3.70      5.69      4.23      3.99   (0.03)
  4. Total from investment operations        3.68      5.84      4.54      4.15     0.14
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment
     income)                               (0.23)    (0.21)    (0.13)    (0.01)   (0.39)
  6. Dividends (in excess of net
     investment income)                        --        --        --        --       --
  7. Distributions (from capital gains)    (4.84)    (2.92)    (1.13)    (0.39)   (0.94)
  8. Total distributions                   (5.07)    (3.13)    (1.26)    (0.40)   (1.33)
  9. NET ASSET VALUE, END OF PERIOD        $27.97    $29.36    $26.65    $23.37   $19.62
 10. Total return                          15.12%    24.18%    20.31%    21.62%    0.75%
 11. Net assets, end of period (in
     millions)                            $20,721   $19,029   $15,313   $11,963   $9,647
 12. Average net assets for the period
     (in millions)                        $20,777   $17,515   $13,753   $10,560   $9,339
 13. Ratio of gross expenses to average
     net assets                             0.87%     0.87%     0.86%     0.87%      N/A
 14. Ratio of net expenses to average
     net assets                             0.86%     0.86%     0.85%     0.86%    0.91%
 15. Ratio of net investment
     income/(loss) to average net assets       --     0.85%     0.91%     1.25%    1.12%
 16. Portfolio turnover rate                  70%      132%      104%      118%     139%
- ----------------------------------------------------------------------------------------
</TABLE>
 
 38 Janus Fund
<PAGE> 
                                                    Glossary of investment terms
 
               This glossary provides a more detailed description of some of the
               types of securities and other instruments in which the Fund may
               invest. The Fund may invest in these instruments to the extent
               permitted by its investment objective and policies. The Fund 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
               Fund may purchase commercial paper issued in private placements
               under Section 4(2) of the Securities Act of 1933.
 
               COMMON STOCKS are equity securities representing shares of
               ownership in a company, and usually carry voting rights and earns
               dividends. Unlike preferred stock, dividends on common stocks are
               not fixed but are declared at the discretion of the issuer's
               board of directors.
 
               CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a
               fixed dividend or interest payment and are convertible into
               common stock at a specified price or conversion ratio.
 
               DEBT SECURITIES are equity securities representing money borrowed
               that must be repaid at a later date. Such securities have
               specific maturities and usually a specific rate of interest or an
               original purchase discount.
 
               DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
               corporation that entitle the holder to dividends and capital
               gains on the underlying security. Receipts include those issued
               by domestic banks (American Depositary Receipts), foreign banks
 
                                                Glossary of investment terms  39

<PAGE> 
 
               (Global or European Depositary Receipts) and broker-dealers
               (depositary shares).
 
               FIXED-INCOME SECURITIES are securities that pay a specified rate
               of return. The term generally includes short- and long-term
               government, corporate and municipal obligations that pay a
               specified rate of interest or coupons for a specified period of
               time, and preferred stock, which pays fixed dividends. Coupon and
               dividend rates may be fixed for the life of the issue or, in the
               case of adjustable and floating rate securities, for a shorter
               period.
 
               HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated
               below investment grade by the primary rating agencies (e.g., BB
               or lower by Standard & Poor's and Ba or lower by Moody's). Other
               terms commonly used to describe such securities include "lower
               rated bonds," "noninvestment grade bonds" and "junk bonds."
 
               MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
               mortgages or other debt. These securities are generally pass-
               through securities, which means that principal and interest
               payments on the underlying securities (less servicing fees) are
               passed through to shareholders on a pro rata basis. These
               securities involve prepayment risk, which is the risk that the
               underlying mortgages or other debt may be refinanced or paid off
               prior to their maturities during periods of declining interest
               rates. In that case, the portfolio managers 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 Fund
               must pay if these investments are profitable, the Fund may make
               various elections permitted by the tax laws. These elections
               could require that the Fund recognize taxable income, which in
               turn
 
 40 Janus Fund
<PAGE> 
 
               must be distributed, before the securities are sold and before
               cash is received to pay the distributions.
 
               PREFERRED STOCKS are equity securities that generally pay
               dividends at a specified rate and have preference over common
               stock in the payment of dividends and liquidation. Preferred
               stock generally does not carry voting rights.
 
               REPURCHASE AGREEMENTS involve the purchase of a security by the
               Fund and a simultaneous agreement by the seller (generally a bank
               or dealer) to repurchase the security from the Fund 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 Fund 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 Fund to another party (generally a bank or dealer) in return
               for cash and an agreement by the Fund to buy the security back at
               a specified price and time. This technique will be used primarily
               to provide cash to satisfy unusually heavy redemption requests,
               or for other temporary or emergency purposes.
 
               U.S. GOVERNMENT SECURITIES include direct obligations of the U.S.
               government that are supported by its full faith and credit.
               Treasury bills have initial maturities of less than one year,
               Treasury notes have initial maturities of one to ten years and
               Treasury bonds may be issued with any maturity but generally have
               maturities of at least ten years. U.S. government securities also
               include indirect obligations of the U.S. government that are
               issued by federal agencies and government sponsored entities.
               Unlike Treasury securities, agency securities generally are not
               backed by the full faith and credit of the U.S. government. Some
               agency securities are supported by the right of the issuer to
               borrow from the Treasury, others are supported by the
               discretionary authority of the U.S. government to purchase the
               agency's obligations and others are supported only by the credit
               of the sponsoring agency.
 
                                                Glossary of investment terms  41
<PAGE> 
 
               WARRANTS are securities, typically issued with preferred stocks
               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
               Fund does not earn interest on such securities until settlement
               and bears the risk of market value fluctuations in between the
               purchase and settlement dates. New issues of stocks and bonds,
               private placements and U.S. government securities may be sold in
               this manner.
 
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
               Fund may enter into forward currency contracts to hedge against
               declines in the value of securities denominated in, or whose
               value is tied to, a currency other than the U.S. dollar or to
               reduce the impact of currency appreciation on purchases of such
               securities. It may also enter into forward contracts to purchase
               or sell securities or other financial indices.
 
               FUTURES CONTRACTS are contracts that obligate the buyer to
               receive and the seller to deliver an instrument or money at a
               specified price on a specified date. The Fund 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 Fund
               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.
 
 42 Janus Fund
<PAGE> 
 
               INDEXED/STRUCTURED SECURITIES are typically short- to
               intermediate-term debt securities whose value at maturity or
               interest rate is linked to currencies, interest rates, equity
               securities, indices, commodity prices or other financial
               indicators. Such securities may be positively or negatively
               indexed (i.e., their value may increase or decrease if the
               reference index or instrument appreciates). Indexed/structured
               securities may have return characteristics similar to direct
               investments in the underlying instrument and may be more volatile
               than the underlying instrument. The Fund bears the market risk of
               an investment in the underlying instrument, as well as the credit
               risk of the issuer.
 
               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 Fund may purchase and
               write put and call options on securities, securities indices and
               foreign currencies.
 
                                                Glossary of investment terms  43
<PAGE> 
 
                       This page intentionally left blank
 
 44
<PAGE> 
 
                       This page intentionally left blank
<PAGE> 
 
[JANUS LOGO]
          1-800-525-3713
          P.O. Box 173375
          Denver, Colorado 80217-3375
          janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting Janus at 1-800-525-3713 or visiting our Web
site at janus.com. In the Fund's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Fund's performance during its last fiscal
year. Other information is also available from financial
intermediaries that sell shares of the Fund.
 
The Statement of Additional Information provides detailed
information about the Fund and is incorporated into this Prospectus
by reference. You may review the Fund's Statement of Additional
Information at the Public Reference Room of the SEC or get text only
copies for a fee, by writing to or calling the Public Reference
Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You may obtain
the Statement of Additional Information for free from the SEC's Web
site at http://www.sec.gov.
 
              Investment Company Act File No. 811-1879
 
  3176
<PAGE> 
 
                                                     [JANUS LOGO]
 
                         Janus Mercury Fund
 
                                       PROSPECTUS
 
                                       FEBRUARY 17, 1999
 
                         THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED
                         OR DISAPPROVED OF THESE SECURITIES OR PASSED ON THE
                         ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
                         REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
<PAGE> 
 
    [JANUS LOGO]
<PAGE> 
 
                                                               Table of contents
 
<TABLE>
                <S>                                               <C>
                RISK/RETURN SUMMARY
                   Janus Mercury Fund...........................    2
                   Fees and expenses............................    4
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT
                STRATEGIES AND RISKS
                   Investment objective and principal investment
                   strategies...................................    5
                   General portfolio policies...................    6
                   Risks........................................    9
                SHAREHOLDER'S MANUAL
                   Minimum investments..........................   15
                   Types of account ownership...................   15
                   How to open your Janus account...............   17
                   How to purchase shares.......................   18
                   How to exchange shares.......................   20
                   How to redeem shares.........................   22
                   Shareholder services and account policies....   26
                MANAGEMENT OF THE FUND
                   Investment adviser...........................   31
                   Portfolio manager............................   32
                OTHER INFORMATION............... ...............   33
                DISTRIBUTIONS AND TAXES
                   Distributions................................   34
                   Taxes........................................   35
                FINANCIAL HIGHLIGHTS.............. .............   37
                GLOSSARY
                   Glossary of investment terms.................   38
 
</TABLE>
 
                                                            Table of contents  1
<PAGE> 
Risk return summary
 
JANUS MERCURY FUND
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF JANUS MERCURY FUND?
 
               The Fund seeks long-term growth of capital.
 
               The Fund's Trustees may change this objective without a
               shareholder vote and the Fund will notify you of any changes that
               are material. If there is a material change in the Fund's
               objective or policies, you should consider whether the Fund
               remains an appropriate investment for you. There is no guarantee
               that the Fund will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF JANUS MERCURY FUND?
 
               The Fund invests primarily in common stocks selected for their
               growth potential. The Fund may invest in companies of any size,
               from larger, well-established companies to smaller, emerging
               growth companies.
 
               The portfolio manager applies a "bottom up" approach in choosing
               investments. In other words, he looks for companies with earnings
               growth potential one at a time. If the portfolio manager is
               unable to find investments with earnings growth potential, a
               significant portion of the Fund's assets may be in cash or
               similar investments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN JANUS MERCURY FUND?
 
               The biggest risk of investing in this Fund is that its returns
               may vary and you could lose money. If you are considering
               investing in the Fund, remember that it is designed for long-term
               investors who can accept the risks of investing in a portfolio
               with significant common stock holdings. Common stocks tend to be
               more volatile than other investment choices.
 
               The value of the Fund's portfolio may decrease if the value of an
               individual company in the portfolio decreases. The value of the
               Fund's portfolio could also decrease if the stock market goes
               down. If the value of the Fund's portfolio decreases, the Fund's
               net asset value (NAV) will also decrease which means if you sell
               your shares in the Fund you would get back less money.
 
 2 Janus Mercury Fund
<PAGE> 
 
               An investment in the Fund is not a bank deposit and is not
               insured or guaranteed by the Federal Deposit Insurance
               Corporation or any other government agency.
 
               The following information illustrates how the Fund's performance
               has varied over time. The bar chart depicts the change in the
               Fund's performance from year-to-year during the periods
               indicated. The table compares the Fund's average annual returns
               for the periods indicated to a broad-based securities market
               index.
 
                JANUS MERCURY FUND

A BAR CHART showing Annual Total Returns for Janus Mercury Fund from 1994 
through 1998:
 
                Annual returns for period ended 12/31

                 15.86%  33.01%  17.67%  11.86%  58.41%
                  1994    1995    1996    1997    1998

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

                Best Quarter: 4th-1998 30.04%; Worst Quarter: 3rd-1998 (6.94%)
 
 
<TABLE>
<CAPTION>
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
                                                                             Since Inception
                                                          1 year   5 years      (5/3/93)
                <S>                                       <C>      <C>       <C>           
                Janus Mercury Fund                        58.41%   26.30%        26.86%
                S&P 500 Index*                            28.74%   24.08%        22.64%
                                                      --------------------------------
</TABLE>
 
                * The S&P 500 is the Standard & Poor's Composite Index of 500
                  Stocks, a widely recognized, unmanaged index of common stock
                  prices.
 
                The Fund's past performance does not necessarily indicate how it
                will perform in the future.
 
                                                          Risk return summary  3
<PAGE> 
 
FEES AND EXPENSES
 
               SHAREHOLDER FEES, such as sales loads, redemption fees or
               exchange fees, are charged directly to an investor's account. All
               Janus funds are no-load investments, so you will not pay any
               shareholder fees when you buy or sell shares of the Fund.
 
               ANNUAL FUND OPERATING EXPENSES are paid out of the Fund's assets
               and include fees for portfolio management, maintenance of
               shareholder accounts, shareholder servicing, accounting and other
               services. You do not pay these fees directly but, as the example
               below shows, these costs are borne indirectly by all
               shareholders.
 
               This table describes the fees and expenses that you may pay if
               you buy and hold shares of the Fund. It is based upon gross
               expenses (without the effect of expense offset arrangements) for
               the fiscal year ended October 31, 1998.
 
<TABLE>
<CAPTION>
                                                             Janus Mercury Fund
   <S>                                                       <C>           
   Management Fee                                                   0.67%
   Other Expenses                                                   0.30%
   Total Annual Fund Operating Expenses                             0.97%
</TABLE>
 
- --------------------------------------------------------------------------------
  EXAMPLE:
  This example is intended to help you compare the cost of investing in the
  Fund with the cost of investing in other mutual funds. The example
  assumes that you invest $10,000 in the Fund for the time periods
  indicated then redeem all of your shares at the end of those periods. The
  example also assumes that your investment has a 5% return each year and
  that the Fund's operating expenses remain the same. Although your actual
  costs may be higher or lower, based on these assumptions your costs would
  be:
 
<TABLE>
<CAPTION>
                                             1 Year    3 Years    5 Years    10 Years
                                             ----------------------------------------
   <S>                                       <C>       <C>        <C>        <C>
   Janus Mercury Fund                         $99       $309       $536       $1,190
</TABLE>
 
 4 Janus Mercury Fund
<PAGE> 
                                              Investment objective, principal
                                                         investment strategies
                                                         and risks
 
               This section takes a closer look at the investment objective of
               the Fund, its principal investment strategies and certain risks
               of investing in the Fund. Strategies and policies that are noted
               as "fundamental" cannot be changed without a shareholder vote.
 
               Please carefully review the "Risks" section of this Prospectus on
               pages 9-11 for a discussion of risks associated with certain
               investment techniques. We've also included a Glossary with
               descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
               Janus Mercury Fund seeks long-term growth of capital. It pursues
               its objective by investing primarily in common stocks selected
               for their growth potential. The Fund may invest in companies of
               any size, from larger, well-established companies to smaller,
               emerging growth companies.
 
               The Fund may invest substantially all of its assets in common
               stocks if its portfolio manager believes that common stocks will
               appreciate in value. The portfolio manager generally takes a
               "bottom up" approach to selecting companies. In other words, he
               seeks to identify individual companies with earnings growth
               potential that may not be recognized by the market at large. He
               makes this assessment by looking at companies one at a time,
               regardless of size, country of organization, place of principal
               business activity, or other similar selection criteria.
               Realization of income is not a significant consideration when
               choosing investments for the Fund. Income realized on the Fund's
               investments will be incidental to its objective.
 
               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.
 
              Investment objective, principal investment strategies and risks  5
<PAGE> 
 
               There are no limitations on the countries in which the Fund may
               invest and the Fund may at times have significant foreign
               exposure.
 
GENERAL PORTFOLIO POLICIES
 
               In investing its portfolio assets, the Fund 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. So, for example, if
               the Fund exceeds a limit as a result of market fluctuations or
               the sale of securities, it will not be required to dispose of any
               securities.
 
               CASH POSITION
               When the Fund's portfolio manager believes that market conditions
               are unfavorable for profitable investing, or when he is otherwise
               unable to locate attractive investment opportunities, the Fund's
               cash or similar investments may increase. In other words, the
               Fund does not always stay fully invested in stocks. Cash or
               similar investments generally are a residual - they represent the
               assets that remain after the portfolio manager has committed
               available assets to desirable investment opportunities. However,
               the portfolio manager may also temporarily increase the Fund's
               cash position to protect its assets or maintain liquidity. When
               the Fund's investments in cash or similar investments increase,
               it may not participate in market advances or declines to the same
               extent that it would if the Fund remained more fully invested in
               stocks.
 
               OTHER TYPES OF INVESTMENTS
               The Fund invests primarily in domestic and foreign equity
               securities, which may include preferred stocks, common stocks,
               warrants and securities convertible into common or preferred
               stocks, but it may also invest to a lesser degree in other types
               of securities. These securities (which are described in the
               Glossary) may include:
 
               - debt securities
 
               - indexed/structured securities
 
 6 Janus Mercury Fund
<PAGE> 
 
               - high-yield/high-risk securities (less than 35% of the Fund's
                 assets)
 
               - options, futures, forwards and other types of derivatives for
                 hedging purposes or for non-hedging purposes such as seeking to
                 enhance return
 
               - securities purchased on a when-issued, delayed delivery or
                 forward commitment basis
 
               ILLIQUID INVESTMENTS
               The Fund may invest up to 15% of its net assets in illiquid
               investments. An illiquid investment is a security or other
               position that cannot be disposed of quickly in the normal course
               of business. For example, some securities are not registered
               under the U.S. securities laws and cannot be sold to the U.S.
               public because of SEC regulations (these are known as "restricted
               securities"). Under procedures adopted by the Fund's Trustees,
               certain restricted securities may be deemed liquid, and will not
               be counted toward this 15% limit.
 
               FOREIGN SECURITIES
               The Fund may invest without limit in foreign equity and debt
               securities. The Fund may invest directly in foreign securities
               denominated in a foreign currency and not publicly traded in the
               United States. Other ways of investing in foreign securities
               include depositary receipts or shares, and passive foreign
               investment companies.
 
               SPECIAL SITUATIONS
               The Fund may invest in special situations. A special situation
               arises when, in the opinion of the Fund'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. The
 
              Investment objective, principal investment strategies and risks  7
<PAGE> 
 
               Fund's performance could suffer if the anticipated development in
               a "special situation" investment does not occur or does not
               attract the expected attention.
 
               PORTFOLIO TURNOVER
               The Fund generally intends to purchase securities for long-term
               investment although, to a limited extent, the Fund may purchase
               securities in anticipation of relatively short-term price gains.
               Short-term transactions may also result from liquidity needs,
               securities having reached a price or yield objective, changes in
               interest rates or the credit standing of an issuer, or by reason
               of economic or other developments not foreseen at the time of the
               investment decision. The Fund may also sell one security and
               simultaneously purchase the same or a comparable security to take
               advantage of short-term differentials in bond yields or
               securities prices. Changes are made in the Fund's portfolio
               whenever its portfolio manager believes such changes are
               desirable. Portfolio turnover rates are generally not a factor in
               making buy and sell decisions.
 
               Increased portfolio turnover may result in higher costs for
               brokerage commissions, dealer mark-ups and other transaction
               costs and may also result in taxable capital gains. Higher costs
               associated with increased portfolio turnover may offset gains in
               the Fund's performance.
 
 8 Janus Mercury Fund
<PAGE> 
 
RISKS
 
               Because the Fund may invest substantially all of its assets in
               common stocks, the main risk is the risk that the value of the
               stocks it holds might decrease in response to the activities of
               an individual company or in response to general market and/or
               economic conditions. If this occurs, the Fund's share price may
               also decrease. The Fund's performance may also be affected by
               risks specific to certain types of investments, such as foreign
               securities, derivative investments, non-investment grade debt
               securities or companies with relatively small market
               capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Fund.
 
1. THE FUND MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
               Many attractive investment opportunities may be smaller, start-up
               companies offering emerging products or services. Smaller or
               newer companies may suffer more significant losses as well as
               realize more substantial growth than larger or more established
               issuers because they may lack depth of management, be unable to
               generate funds necessary for growth or potential development, or
               be developing or marketing new products or services for which
               markets are not yet established and may never become established.
               In addition, such companies may be insignificant factors in their
               industries and may become subject to intense competition from
               larger or more established companies. Securities of smaller or
               newer companies may have more limited trading markets than the
               markets for securities of larger or more established issuers, and
               may be subject to wide price fluctuations. Investments in such
               companies tend to be more volatile and somewhat more speculative.
 
2. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
               The portfolio manager carefully researches each potential
               investment before making an investment decision and, among
 
              Investment objective, principal investment strategies and risks  9
<PAGE> 
 
               other things, considers Year 2000 readiness when selecting
               portfolio holdings. However, there is no guarantee that the
               information the portfolio manager receives regarding a company's
               Year 2000 readiness is completely accurate. If a company has not
               satisfactorily addressed Year 2000 issues, the Fund's performance
               could suffer.
 
3. HOW COULD THE FUND'S INVESTMENTS IN FOREIGN SECURITIES AFFECT ITS
   PERFORMANCE?
 
               The Fund may invest without limit in foreign securities either
               indirectly (e.g., depositary receipts) or directly in foreign
               markets. Investments in foreign securities, including those of
               foreign governments, may involve greater risks than investing in
               domestic securities because the Fund's performance may depend on
               issues other than the performance of a particular company. These
               issues include:
 
               - currency risk
 
               - political and economic risk
 
               - regulatory risk
 
               - market risk
 
               - transaction costs
 
               These risks are described in the SAI.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
               High-yield/high-risk securities (or "junk" bonds) are securities
               rated below investment grade by the primary rating agencies such
               as Standard & Poor's and Moody's. The value of lower quality
               securities generally is more dependent on credit risk, or the
               ability of the issuer to meet interest and principal payments,
               than investment grade debt securities. Issuers of high-yield
               securities may not be as strong financially as those issuing
               bonds with higher credit ratings and are more vulnerable to real
               or perceived economic changes, political changes or adverse
               developments specific to the issuer.
 
 10 Janus Mercury Fund
<PAGE> 
 
               Please refer to the SAI for a description of bond rating
               categories.
 
5. HOW DOES THE FUND TRY TO REDUCE RISK?
 
               The Fund may use futures, options and other derivative
               instruments to "hedge" or protect its portfolio from adverse
               movements in securities prices and interest rates. The Fund may
               also use a variety of currency hedging techniques, including
               forward currency contracts, to manage exchange rate risk. The
               Fund believes the use of these instruments will benefit the Fund.
               However, the Fund's performance could be worse than if the Fund
               had not used such instruments if the portfolio manager's
               judgement proves incorrect. Risks associated with the use of
               derivative instruments are described in the SAI.
 
             Investment objective, principal investment strategies and risks  11
<PAGE> 
 
Janus Mercury Fund
<PAGE> 
 
                   Shareholder's
                   Manual
 
                   This section will help you become
                   familiar with the different types
                   of accounts you can establish with
                   Janus. It also explains in detail
                   the wide array of services and
                   features you can establish on your
                   account, as well as account
                   policies and fees that may apply
                   to your account. Account policies
                   (including fees), services and
                   features may be modified or
                   discontinued without shareholder
                   approval or prior notice.
 
                                                    [JANUS LOGO]
<PAGE> 
 
HOW TO GET IN TOUCH WITH JANUS
 
               Janus offers two Investor Service Centers for those individuals
               who would like to conduct their investing in person. Our
               representatives will be happy to assist you at either of the
               following locations: Monday-Friday 7:00 a.m. to 6:00 p.m.
               Mountain time and Saturday 9:00 a.m. to 1:00 p.m. Mountain time.
 
               100 Fillmore Street, Suite 100
               Denver, CO 80206
 
               3773 Cherry Creek North Drive, Suite 101
               Denver, CO 80209
 
 QUICK ADDRESS AND TELEPHONE REFERENCE
- --------------------------------------------------------------------------------
 
<TABLE>
  <S>                                           <C>
  MAILING ADDRESS                               JANUS XPRESSLINE(TM)
  Janus                                         1-888-979-7737
  P.O. Box 173375                               For 24-hour access to account
  Denver, CO 80217-3375                         and fund information,
                                                exchanges, purchases and
  FOR OVERNIGHT CARRIER                         redemptions, automated daily
  Janus                                         quotes on fund share prices,
  Suite 101                                     yields and total returns.
  3773 Cherry Creek North Drive
  Denver, CO 80209-3811                         TDD
                                                1-800-525-0056
  INVESTOR SERVICE REPRESENTATIVES              A telecommunications device
  If you have any questions while reading       for our hearing- and
  this Prospectus, please call one of our       speech-impaired shareholders.
  Investor Service Representatives at
  1-800-525-3713 Monday-Friday: 8:00            JANUS LITERATURE LINE
  a.m.-8:00 p.m., and Saturday: 10:00           1-800-525-8983
  a.m.-4:00 p.m., New York time.                To request a prospectus,
                                                shareholder reports or
  JANUS INTERNET ADDRESS                        marketing materials 24 hours a
  janus.com                                     day.
</TABLE>
 
 14 Janus Mercury Fund
<PAGE> 
 
MINIMUM INVESTMENTS*
- ---------------------------------------------
 
<TABLE>
<S>                                 <C>
To open a new regular account        $2,500

To open a new retirement,
education or UGMA/UTMA account       $  500

To open a new regular account with
an Automatic Investment Program      $  500**

To add to any type of an account     $  100+
</TABLE>
 
 * The Fund reserves the right to
   change the amount of these
   minimums from time to time or
   to waive them in whole or in
   part for certain types of
   accounts.
** An Automatic Investment Program
   requires a $100 minimum
   automatic investment per month
   until the account balance
   reaches $2,500.
 + The minimum subsequent
   investment for IRA UGMA/UTMA
   accounts is $50.
 
TYPES OF ACCOUNT OWNERSHIP
 
               If you are investing in the Fund for the first time, you will
               need to establish an account. You can establish the following
               types of accounts by completing a New Account Application. To
               request an application, call 1-800-525-3713 or visit our Web site
               at janus.com to download an application.
 
               INDIVIDUAL OR JOINT OWNERSHIP
               Individual accounts are owned by one person. Joint accounts have
               two or more owners.
 
               A GIFT OR TRANSFER TO MINOR (UGMA OR UTMA)
               An UGMA/ UTMA account is a custodial account managed for the
               benefit of a minor. To open an UGMA or UTMA account, you must
               include the minor's Social Security number on the application.
 
               TRUST
               An established trust can open an account. The names of each
               trustee, the name of the trust and the date of the trust
               agreement must be included on the application.
 
                                                        Shareholder's manual  15
<PAGE> 
 
               BUSINESS ACCOUNTS
               Corporations and partnerships may also open an account. The
               application must be signed by an authorized officer of the
               corporation or a general partner of the partnership.
 
TAX-DEFERRED ACCOUNTS
 
               If you are eligible, you may set up one or more tax-deferred
               accounts. A tax-deferred account allows you to shelter your
               investment income and capital gains from current income taxes. A
               contribution to certain of these plans may also be tax
               deductible. Tax-deferred accounts include retirement plans
               described below and the Education IRA. Distributions from these
               plans are generally subject to income tax and may be subject to
               an additional tax if withdrawn prior to age 59 1/2 or used for a
               nonqualifying purpose. Investors should consult their tax adviser
               or legal counsel before selecting a tax-deferred account.
 
               Investors Fiduciary Trust Company serves as custodian for the
               tax-deferred accounts offered by the Fund. You will be charged an
               annual account maintenance fee of $12 for each taxpayer
               identification number no matter how many tax-deferred accounts
               you have with Janus. You may pay the fee by check or have it
               automatically deducted from your account (usually in December).
               The custodian reserves the right to change the amount of this fee
               or to waive it in whole or in part for certain types of accounts.
 
               The following plans require a special application. For an
               application and more details about our Retirement Plans, call
               1-800-525-3713.
 
               TRADITIONAL AND ROTH INDIVIDUAL RETIREMENT ACCOUNTS
               Both types of IRAs allow most individuals with earned income to
               contribute up to the lesser of $2,000 ($4,000 for most married
               couples) or 100% of compensation annually. Please refer to the
               Janus IRA booklet for more complete information regarding the
               different types of IRAs.
 
 16 Janus Mercury Fund
<PAGE> 
 
               EDUCATION IRA
               This plan allows individuals, subject to certain income
               limitations, to contribute up to $500 annually on behalf of any
               child under the age of 18. Please refer to the Janus IRA booklet
               for more complete information regarding the Education IRA.
 
               SIMPLIFIED EMPLOYEE PENSION PLAN
               This plan allows small business owners (including sole
               proprietors) to make tax-deductible contributions for themselves
               and any eligible employee(s). A SEP requires an IRA (a SEP-IRA)
               to be set up for each SEP participant.
 
               PROFIT SHARING OR MONEY PURCHASE PENSION PLAN
               These plans are open to corporations, partnerships and sole
               proprietors to benefit their employees and themselves.
 
               SECTION 403(b)(7) PLAN
               Employees of educational organizations or other qualifying, tax-
               exempt organizations may be eligible to participate in a Section
               403(b)(7) Plan.
 
HOW TO OPEN YOUR JANUS ACCOUNT
 
               Complete and sign the appropriate application. Please be sure to
               provide your Social Security or taxpayer identification number on
               the application and make your check payable to Janus. The Fund is
               available only to U.S. citizens or residents, and your
               application will be returned to you if you do not meet these
               criteria. Send all items to one of addresses listed in the "Quick
               Address and Telephone Reference" on page 14.
 
                                                        Shareholder's manual  17
<PAGE> 
 
HOW TO PURCHASE SHARES
 
               PAYING FOR SHARES
 
               When you purchase shares, your request will be processed at the
               next NAV calculated after your order is received and accepted.
               Please note the following:
 
               - Cash, credit cards, third party checks and credit card checks
                 will not be accepted.
 
               - All purchases must be made in U.S. dollars.
 
               - Checks must be drawn on a U.S. bank and made payable to Janus.
 
               - If a check does not clear your bank, the Fund reserves the
                 right to cancel the purchase.
 
               - If the Fund is unable to debit your predesignated bank account
                 on the day of purchase, it may make additional attempts or
                 cancel the purchase.
 
               - The Fund reserves the right to reject any specific purchase
                 request.
 
               If your purchase is cancelled you will be responsible for any
               losses or fees imposed by your bank and losses that may be
               incurred as a result of any decline in the value of the cancelled
               purchase. The Fund (or its agents) has the authority to redeem
               shares in your account(s) to cover any such losses due to
               fluctuations in share price. Any profit on such cancellation will
               accrue to the Fund.
 
               ONCE YOU HAVE OPENED YOUR JANUS ACCOUNT, THE MINIMUM AMOUNT FOR
               AN ADDITIONAL INVESTMENT IS $100 ($50 FOR IRAs OR UGMA/UTMA
               ACCOUNTS). You may add to your account at any time through any of
               the following options:
 
               BY MAIL
 
               Complete the remittance slip attached at the bottom of your
               confirmation statement. If you are making a purchase into a
               retirement account, please indicate whether the purchase is a
 
 18 Janus Mercury Fund
<PAGE> 
 
               rollover or a current or prior year contribution. Send your check
               made payable to Janus and remittance slip or written instructions
               to one of the addresses listed previously. You may also request a
               booklet of remittance slips for non-retirement accounts.
 
               BY TELEPHONE
 
               This service allows you to purchase additional shares quickly and
               conveniently through an electronic transfer of money. To purchase
               shares by telephone, call an Investor Service Representative at
               1-800-525-3713 during normal business hours or call the Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. When you make an additional purchase by telephone, Janus
               will automatically debit your predesignated bank account for the
               desired amount. To establish the telephone purchase option on
               your new account, complete the "Telephone Purchase of Shares
               Option" section on the application and attach a "voided" check or
               deposit slip from your bank account. If your account is already
               established, call 1-800-525-3713 to request the appropriate form.
               This option will become effective ten business days after the
               form is received.
 
               BY WIRE
 
               Purchases may also be made by wiring money from your bank account
               to your Janus account. Call 1-800-525-3713 to receive wiring
               instructions.
 
               BY INTERNET
 
               You must pre-establish the "Telephone Purchase of Shares Option"
               to make a purchase on our Web site at janus.com. If you have
               questions, please call 1-800-975-9932 to speak to a Janus
               representative.
 
               AUTOMATIC INVESTMENT PROGRAMS
 
               Janus offers several automatic investment programs to help you
               achieve your financial goals as simply and conveniently as
 
                                                        Shareholder's manual  19
<PAGE> 
 
               possible. You may open a new account with a $500 initial purchase
               and $100 automatic subsequent investments.
 
               AUTOMATIC MONTHLY INVESTMENT PROGRAM
               You select the day each month that your money ($100 minimum) will
               be electronically transferred from your bank account to your Fund
               account. To establish this option, complete the "Automatic
               Monthly Investment Program" section on the application and attach
               a "voided" check from your bank account. If your Fund account is
               already established, call 1-800-525-3713 to request the
               appropriate form.
 
               PAYROLL DEDUCTION
               If your employer can initiate an automatic payroll deduction, you
               may have all or a portion of your paycheck ($100 minimum)
               invested directly into your Fund account. To obtain information
               on establishing this option, call 1-800-525-3713.
 
               SYSTEMATIC EXCHANGE
               With a Systematic Exchange you determine the amount of money
               ($100 minimum) you would like automatically exchanged from one
               Janus account to another on any day of the month. For more
               information on how to establish this option, call 1-800-525-3713.
 
HOW TO EXCHANGE SHARES
 
               On any business day, you may exchange all or a portion of your
               shares into any other available Janus fund.
 
               IN WRITING
 
               To request an exchange in writing, please follow the instructions
               for written requests on page 24.
 
               BY TELEPHONE
 
               All accounts are automatically eligible for the telephone
               exchange option. To exchange shares by telephone, call an
               Investor Service Representative at 1-800-525-3713 during normal
               business hours
 
 20 Janus Mercury Fund
<PAGE> 
 
               or call the Janus XpressLine, 1-888-979-7737, for access to this
               option 24 hours a day.
 
               BY SYSTEMATIC EXCHANGE
 
               As noted above, you may establish a Systematic Exchange for as
               little as $100 per month on established accounts. You may
               establish a new account with a $500 initial purchase and
               subsequent $100 systematic exchanges. If the balance in the
               account you are exchanging from falls below the systematic
               exchange amount, all remaining shares will be exchanged and the
               program will be discontinued.
 
               BY INTERNET
 
               Exchanges may also be made on our Web site at janus.com.
 
               EXCHANGE POLICIES
 
               - Except for Systematic Exchanges, new accounts established by
                 exchange must be opened with $2,500 or the total account value
                 if the value of the account you are exchanging from is less
                 than $2,500.
 
               - Exchanges between existing accounts must meet the $100
                 subsequent investment requirement.
 
               - You may make four exchanges out of the Fund during a calendar
                 year (exclusive of Systematic Exchanges). Exchanges in excess
                 of this limit may be subject to an exchange fee or may result
                 in termination of the exchange privilege.
 
               - The Fund reserves the right to reject any exchange request and
                 to modify or terminate the exchange privilege at any time. For
                 example, the Fund may reject exchanges from accounts engaged in
                 or known to engage in trading in excess of the limit above
                 (including market timing transactions).
 
               - Exchanges between accounts will be accepted only if the
                 registrations are identical.
 
                                                        Shareholder's manual  21
<PAGE> 
 
               - If the shares you are exchanging are held in certificate form,
                 you must return the certificate to the Fund prior to making any
                 exchanges.
 
               - Be sure to read the prospectus for the fund into which you are
                 exchanging.
 
               - An exchange represents the sale of shares from one fund and the
                 purchase of shares of another fund, which may produce a taxable
                 gain or loss in a non-tax deferred account.
 
HOW TO REDEEM SHARES
 
               On any business day, you may redeem all or a portion of your
               shares. If the shares are held in certificate form, the
               certificate must be returned with or before your redemption
               request. Your transaction will be processed at the next NAV
               calculated after your order is received and accepted. The
               redemption may be suspended for 10 days following an address
               change unless a signature guarantee is provided.
 
               IN WRITING
 
               To request a redemption in writing, please follow the
               instructions for written requests noted on page 24.
 
               BY TELEPHONE
 
               Most accounts have the telephone redemption option, unless this
               option was specifically declined on the application or in
               writing. This option enables you to request redemptions daily
               from your account by calling 1-800-525-3713 by the close of the
               regular trading session of the New York Stock Exchange ("NYSE")
               normally 4:00 p.m. New York time. You may also use Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. (There is a daily limit of $100,000 per account for
               redemptions payable by check.)
 
 22 Janus Mercury Fund
<PAGE> 
 
               BY INTERNET
 
               Redemptions may also be made on our Web site at janus.com.
 
               SYSTEMATIC REDEMPTION OPTION
 
               The Systematic Redemption Option allows you to redeem a specific
               dollar amount from your Fund account on a regular basis. For more
               information or to request the appropriate form, please call
               1-800-525-3713.
 
PAYMENT OF REDEMPTION PROCEEDS
 
               BY CHECK
               Redemption proceeds will be sent to the shareholder(s) of record
               at the address of record within seven days after receipt of a
               valid redemption request.
 
               BY ELECTRONIC TRANSFER
               If you have established the electronic redemption option, your
               redemption proceeds can be electronically transferred to your
               predesignated bank account on the next bank business day after
               receipt of your redemption request (wire transfer) or the second
               bank business day after receipt of your redemption request (ACH
               transfer). Wire transfers will be charged an $8 fee per wire and
               your bank may charge an additional fee to receive the wire. ACH
               transfers are made free of charge. Wire redemptions are not
               available for retirement accounts.
 
               If you would like to establish the electronic redemption option
               on an existing account, please call 1-800-525-3713 to request the
               appropriate form.
 
               IF THE SHARES BEING REDEEMED WERE PURCHASED BY CHECK, TELEPHONE,
               ON OUR WEB SITE, OR THROUGH THE AUTOMATIC MONTHLY INVESTMENT
               PROGRAM, THE FUND MAY DELAY THE PAYMENT OF YOUR REDEMPTION
               PROCEEDS FOR UP TO 15 DAYS FROM THE DAY OF PURCHASE TO ALLOW THE
               PURCHASE TO CLEAR. Unless you provide alternate instructions,
               your proceeds will be invested in Janus
 
                                                        Shareholder's manual  23
<PAGE> 
 
               Money Market Fund - Investor Shares during the 15 day hold
               period.
 
WRITTEN INSTRUCTIONS
 
               To redeem all or part of your shares in writing, your request
               should be sent to one of the addresses listed on page 14 and must
               include the following information:
 
               - the name of the Fund
 
               - the account number
 
               - the amount of money or number of shares being redeemed or
                 exchanged
 
               - the name(s) on the account registration
 
               - the signature(s) of all registered account owners
 
               - your daytime telephone number
 
               SIGNATURE REQUIREMENTS BASED ON ACCOUNT TYPE
 
               INDIVIDUAL, JOINT TENANTS, TENANTS IN COMMON
               Written instructions must be signed by each shareholder, exactly
               as the names appear in the account registration.
 
               UGMA OR UTMA
               Written instructions must be signed by the custodian in his/her
               capacity as it appears in the account registration.
 
               SOLE PROPRIETOR, GENERAL PARTNER
               Written instructions must be signed by an authorized individual
               in his/her capacity as it appears on the account registration.
 
               CORPORATION, ASSOCIATION
               Written instructions must be signed by the person(s) authorized
               to act on the account. In addition, a certified copy of the
               corporate resolution authorizing the signer to act must accompany
               the request.
 
 24 Janus Mercury Fund
<PAGE> 
 
               TRUST
               Written instructions must be signed by the trustee(s). If the
               name(s) of the current trustee(s) does not appear in the account
               registration, a certificate of incumbency dated within 60 days
               must also be submitted.
 
               IRA
               Written instructions must be signed by the account owner. If you
               do not want federal income tax withheld from your redemption, you
               must state that you elect not to have such withholding apply. In
               addition, your instructions must state whether the distribution
               is normal (after age 59 1/2) or premature (before age 59 1/2)
               and, if premature, whether any exceptions such as death or
               disability apply with regard to the 10% additional tax on early
               distributions.
 
SIGNATURE GUARANTEE
 
               In addition to the signature requirements, A SIGNATURE GUARANTEE
               IS ALSO REQUIRED if any of the following is applicable:
 
               - You request a redemption by check that exceeds $100,000.
 
               - You would like the check made payable to anyone other than the
                 shareholder(s) of record.
 
               - You would like the check mailed to an address which has been
                 changed within 10 days of the redemption request.
 
               - You would like the check mailed to an address other than the
                 address of record.
 
               THE FUND RESERVES THE RIGHT TO REQUIRE A SIGNATURE GUARANTEE
               UNDER OTHER CIRCUMSTANCES OR TO REJECT OR DELAY A REDEMPTION ON
               CERTAIN LEGAL GROUNDS. FOR MORE INFORMATION PERTAINING TO
               SIGNATURE GUARANTEES, PLEASE CALL 1-800-525-3713.
 
               HOW TO OBTAIN A SIGNATURE GUARANTEE
 
               A signature guarantee assures that a signature is genuine. The
               signature guarantee protects shareholders from unauthorized
               account transfers. The following financial institutions may
               guaran-
 
                                                        Shareholder's manual  25
<PAGE> 
 
               tee signatures: banks, savings and loan associations, trust
               companies, credit unions, broker-dealers, and member firms of a
               national securities exchange. Call your financial institution to
               see if they have the ability to guarantee a signature. A
               signature guarantee cannot be provided by a notary public.
 
               If you live outside the United States, a foreign bank properly
               authorized to do business in your country of residence or a U.S.
               consulate may be able to authenticate your signature.
 
PRICING OF FUND SHARES
 
               All purchases, redemptions and exchanges will be processed at the
               NAV next calculated after your request is received and accepted
               by the Fund (or the Fund's agent or authorized designee). The
               Fund's NAV is calculated at the close of the regular trading
               session of the NYSE (normally 4:00 p.m. New York time) each day
               that the NYSE is open. The NAV of Fund shares is not determined
               on days the NYSE is closed (generally, New Year's Day, Martin
               Luther King Day, Presidents' Day, Good Friday, Memorial Day,
               Independence Day, Labor Day, Thanksgiving and Christmas). In
               order to receive a day's price, your order must be received by
               the close of the regular trading session of the NYSE. Securities
               are valued at market value or, if a market quotation is not
               readily available, at their fair value determined in good faith
               under procedures established by and under the supervision of the
               Trustees. Short-term instruments maturing within 60 days are
               valued at amortized cost, which approximates market value. See
               the SAI for more detailed information.
 
SHAREHOLDER SERVICES AND ACCOUNT POLICIES
 
               JANUS XPRESSLINE(TM)
 
               Janus XpressLine, our electronic telephone service, offers you
               24-hour access by TouchTone(TM) telephone to obtain information
               on account balances, Fund performance or dividends. You can also
               make exchanges, purchases and redemptions in existing accounts,
               request literature about any Janus fund, or order duplicate
 
 26 Janus Mercury Fund
<PAGE> 
 
               statements. Janus XpressLine is accessed by calling
               1-888-979-7737. Calls are limited to five minutes.
 
               JANUS WEB SITE
 
               Janus maintains a Web site located at janus.com. You can
               purchase, exchange and redeem shares and access information such
               as your account balance and the Fund's NAV through the Web site.
               In order to engage in transactions on our Web site, you must
               authorize us to transmit account information online and accept
               online instructions (see janus.com and follow the procedures
               accordingly). You may also need to have bank account information,
               wire instructions or other options established on your account.
               The Fund and its agents will not be responsible for any losses
               resulting from unauthorized transactions on our Web site when
               procedures designed for engaging in such transactions are
               followed. If you have questions, please call 1-800-975-9932 to
               speak to a Janus representative.
 
               ACCOUNT MINIMUMS
 
               Due to the proportionately higher costs of maintaining small
               accounts, Janus reserves the right to deduct a $10 minimum
               balance fee (or the value of the account if less than $10) from
               accounts with values below the minimums described on page 15 or
               to close such accounts. This policy will apply to accounts
               participating in the Automatic Monthly Investment Program only if
               your account balance does not reach the required minimum initial
               investment or falls below such minimum and you have discontinued
               monthly investments. This policy does not apply to accounts that
               fall below the minimums solely as a result of market value
               fluctuations. It is expected that, for purposes of this policy,
               accounts will be valued in September, and the $10 fee will be
               assessed on the second Friday of September of each year. You will
               receive notice before we charge the $10 fee or close your account
               so that you may increase your account balance to the required
               minimum.
 
                                                        Shareholder's manual  27
<PAGE> 
 
               TRANSACTIONS THROUGH PROCESSING ORGANIZATIONS
 
               You may purchase or sell Fund shares through a broker-dealer,
               bank or other financial institution, or an organization that
               provides recordkeeping and consulting services to 401(k) plans or
               other employee benefit plans (a "Processing Organization").
               Processing Organizations may charge you a fee for this service
               and may require different minimum initial and subsequent
               investments than the Fund. Processing Organizations may also
               impose other charges or restrictions different from those
               applicable to shareholders who invest in the Fund directly. A
               Processing Organization, rather than its customer, may be the
               shareholder of record of your shares. The Fund is not responsible
               for the failure of any Processing Organization to carry out its
               obligations to its customers. Certain Processing Organizations
               may receive compensation from Janus Capital or its affiliates and
               certain Processing Organizations may receive compensation from
               the Fund for shareholder recordkeeping and similar services.
 
               TAXPAYER IDENTIFICATION NUMBER
 
               On your application or other appropriate form, you will be asked
               to certify that your Social Security or taxpayer identification
               number is correct and that you are not subject to backup
               withholding for failing to report income to the IRS. If you are
               subject to the 31% backup withholding or you did not certify your
               taxpayer identification number, the IRS requires the Fund to
               withhold 31% of any dividends paid and redemption or exchange
               proceeds. In addition to the 31% backup withholding, you may be
               subject to a $50 fee to reimburse the Fund for any penalty that
               the IRS may impose.
 
               SHARE CERTIFICATES
 
               Most shareholders choose not to hold their shares in certificate
               form because account transactions such as exchanges and
               redemptions cannot be completed until the certificate has been
               returned to the Fund. The Fund will issue share certificates upon
               written request only and reserves the right to charge a fee for
               this service.
 
 28 Janus Mercury Fund
<PAGE> 
 
               Share certificates will not be issued until the shares have been
               held for at least 15 days and will not be issued for accounts
               that do not meet the minimum investment requirements. Share
               certificates cannot be issued for retirement accounts. In
               addition, if the certificate is lost, there may be a replacement
               charge.
 
               INVOLUNTARY REDEMPTIONS
 
               The Fund reserves the right to close an account if the
               shareholder is deemed to engage in activities which are illegal
               or otherwise believed to be detrimental to the Fund.
 
               TELEPHONE TRANSACTIONS
 
               You may initiate many transactions by telephone. The Fund and its
               agents will not be responsible for any losses resulting from
               unauthorized transactions when procedures designed to verify the
               identity of the caller are followed.
 
               It may be difficult to reach an Investor Service Representative
               by telephone during periods of unusual market activity. If you
               are unable to reach a representative by telephone, please
               consider sending written instructions, stopping by a Service
               Center, calling the Janus XpressLine or visiting our Web site.
 
               TEMPORARY SUSPENSION OF SERVICES
 
               The Fund or its agents may, in case of emergency, temporarily
               suspend telephone transactions or other shareholder services.
 
               ADDRESS CHANGES
 
               To change the address on your account, call 1-800-525-3713 or
               send a written request signed by all account owners. Include the
               name of the Fund, the account number(s), the name(s) on the
               account and both the old and new addresses. Certain options may
               be suspended for 10 days following an address change unless a
               signature guarantee is provided.
 
                                                        Shareholder's manual  29
<PAGE> 
 
               REGISTRATION CHANGES
 
               To change the name on an account, the shares are generally
               transferred to a new account. In some cases, legal documentation
               may be required. For more information call 1-800-525-3713.
 
               STATEMENTS AND REPORTS
 
               Investors participating in an automatic investment program will
               receive quarterly confirmations of all transactions. Dividend
               information will be distributed annually. In addition, the Fund
               will send you an immediate transaction confirmation statement
               after every non-systematic transaction.
 
               The Fund produces financial reports, which include a list of the
               Fund's portfolio holdings, semiannually and updates its
               prospectus annually. To reduce expenses, the Fund may choose to
               mail only one report or prospectus to your household, even if
               more than one person in the household has a Fund account. Please
               call 1-800-525-3713 if you would like to receive additional
               reports or prospectuses. The Fund reserves the right to charge a
               fee for additional statement and report requests.
 
 30 Janus Mercury Fund
<PAGE> 
                                                          Management of the fund
 
INVESTMENT ADVISER
 
               Janus Capital Corporation, 100 Fillmore Street, Denver, Colorado
               80206-4928, is the investment adviser to the Fund and is
               responsible for the day-to-day management of its investment
               portfolio and other business affairs.
 
               Janus Capital began serving as investment adviser to Janus Fund
               in 1970 and currently serves as investment adviser to all of the
               Janus funds, acts as sub-adviser for a number of private-label
               mutual funds and provides separate account advisory services for
               institutional accounts.
 
               Janus Capital furnishes continuous advice and recommendations
               concerning the Fund's investments. Janus Capital also furnishes
               certain administrative, compliance and accounting services for
               the Fund, and may be reimbursed by the Fund 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 Fund and pays the salaries, fees and expenses of
               all Fund officers and those Trustees who are affiliated with
               Janus Capital.
 
               The Fund pays Janus Capital a management fee which is calculated
               daily and paid monthly. The advisory agreement with the Fund
               spells out the management fee and other expenses that the Fund
               must pay.
 
               The Fund incurs 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 Annual Fund Operating Expenses
               table on page 4 lists the actual management fee and total
               operating expenses of the Fund for the most recent fiscal year.
 
                                                      Management of the fund  31
<PAGE> 
 
PORTFOLIO MANAGER
 
WARREN B. LAMMERT
- --------------------------------------------------------------------------------
                   is Executive Vice President and portfolio manager of the
                   Fund. Mr. Lammert joined Janus Capital in 1987 and has
                   managed the Fund since its inception. He previously co-
                   managed Janus Venture Fund from December 1993 to December
                   1996. He holds a Bachelor of Arts in Economics from Yale
                   University and a Master of Science in Economic History
                   from the London School of Economics. Mr. Lammert received
                   the Chartered Financial Analyst designation.
 
 32 Janus Mercury Fund
<PAGE> 
                                                               Other information
 
               SIZE OF THE FUND
 
               Although there is no present intention to do so, the Fund may
               discontinue sales of its shares if management and the Trustees
               believe that continued sales may adversely affect the Fund's
               ability to achieve its investment objective. If sales of the Fund
               are discontinued, it is expected that existing shareholders of
               the Fund would be permitted to continue to purchase shares and to
               reinvest any dividends or capital gains distributions, absent
               highly unusual circumstances.
 
               YEAR 2000
 
               Preparing for Year 2000 is a high priority for Janus Capital,
               which has established a dedicated group to address this issue.
               Janus Capital has devoted considerable internal resources and has
               engaged one of the foremost experts in the field to help achieve
               Year 2000 readiness. Janus Capital does not anticipate that Year
               2000-related issues will have a material impact on its ability to
               continue to provide the Fund with service at current levels;
               however, Janus Capital cannot make any assurances that the steps
               it has taken to ensure Year 2000 readiness will be successful. In
               addition, there can be no assurance that Year 2000 issues will
               not affect the companies in which the Fund invests or worldwide
               markets and economies.
 
                                                           Other information  33
<PAGE> 
Distributions and taxes
 
DISTRIBUTIONS
 
               To avoid taxation of the Fund, the Internal Revenue Code requires
               the Fund to distribute net income and any net capital gains
               realized on its investments annually. The Fund's income from
               dividends and interest and any net realized short-term gains are
               paid to shareholders as ordinary income dividends. Net realized
               long-term gains are paid to shareholders as capital gains
               distributions. Dividends and capital gains distributions are
               normally declared and paid in December.
 
               HOW DISTRIBUTIONS AFFECT THE FUND'S NAV
 
               Distributions are paid to shareholders as of the record date of
               the distribution of the Fund, regardless of how long the shares
               have been held. Dividends and capital gains awaiting distribution
               are included in the Fund's daily NAV. The share price of the Fund
               drops by the amount of the distribution, net of any subsequent
               market fluctuations. As an example, assume that on December 31,
               the Fund declared a dividend in the amount of $0.25 per share. If
               the Fund's share price was $10.00 on December 30, the Fund's
               share price on December 31 would be $9.75, barring market
               fluctuations. Shareholders should be aware that distributions
               from a taxable mutual fund are not value-enhancing and may create
               income tax obligations.
 
               "BUYING A DIVIDEND"
 
               If you purchase shares of the Fund just before the distribution,
               you will pay the full price for the shares and receive a portion
               of the purchase price back as a taxable distribution. This is
               referred to as "buying a dividend." In the above example, if you
               bought shares on December 30, you would have paid $10.00 per
               share. On December 31, the Fund would pay you $0.25 per share as
               a dividend and your shares would now be worth $9.75 per share.
               Unless your account is set up as a tax-deferred account,
               dividends paid to you would be included in your gross income for
               tax purposes, even though you may not have participated in the
 
 34 Janus Mercury Fund
<PAGE> 
 
               increase in NAV of the Fund, whether or not you reinvested the
               dividends.
 
DISTRIBUTION OPTIONS
 
               When you open an account, you must specify on your application
               how you want to receive your distributions. You may change your
               distribution option at any time by writing the Fund at one of the
               addresses on page 14 or calling 1-800-525-3713. The Fund offers
               the following options:
 
               1. REINVESTMENT OPTION. You may reinvest your income dividends
                  and capital gains distributions in additional shares. This
                  option is assigned automatically if no other choice is made.
 
               2. CASH OPTION. You may receive your income dividends and capital
                  gains distributions in cash.
 
               3. REINVEST AND CASH OPTION. You may receive either your income
                  dividends or capital gains distributions in cash and reinvest
                  the other in additional shares.
 
               4. REDIRECT OPTION. You may direct your dividends or capital
                  gains to purchase shares of another Janus fund.
 
               The Fund reserves the right to reinvest into your account
               undeliverable and uncashed dividend and distribution checks that
               remain outstanding for six months in shares of the Fund at the
               NAV next computed after the check is cancelled. Subsequent
               distributions may also be reinvested.
 
TAXES
 
               As with any investment, you should consider the tax consequences
               of investing in the Fund. Any time you sell or exchange shares of
               a Fund in a taxable account, it is considered a taxable event.
               Depending on the purchase price and the sale price, you may have
               a gain or loss on the transaction. Any tax liabilities generated
               by your transactions are your responsibility.
 
               The following discussion does not apply to tax-deferred accounts,
               nor is it a complete analysis of the federal tax implications of
               investing in the Fund. You may wish to consult your own tax
 
                                                     Distributions and taxes  35
<PAGE> 
 
               adviser. Additionally, state or local taxes may apply to your
               investment, depending upon the laws of your state of residence.
 
               TAXES ON DISTRIBUTIONS
 
               Dividends and distributions by the Fund are subject to federal
               income tax, regardless of whether the distribution is made in
               cash or reinvested in additional shares of the Fund.
               Distributions may be taxable at different rates depending on the
               length of time the Fund holds a security. In certain states, a
               portion of the dividends and distributions (depending on the
               source of the Fund's income) may be exempt from state and local
               taxes. Information regarding the tax status of income dividends
               and capital gains distributions will be mailed to shareholders on
               or before January 31st of each year. Account tax information will
               also be sent to the IRS.
 
               TAXATION OF THE FUND
 
               Dividends, interest, and some capital gains received by the Fund
               on foreign securities may be subject to tax withholding or other
               foreign taxes. The Fund 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 Fund.
 
               The Fund does not expect to pay federal income or excise taxes
               because it intends to meet certain requirements of the Internal
               Revenue Code. It is important that the Fund meet these
               requirements so that any earnings on your investment will not be
               taxed twice.
 
 36 Janus Mercury Fund
<PAGE> 
                                                            Financial highlights
 
               The financial highlights table is intended to help you understand
               the Fund's financial performance for the past 5 years through
               October 31st of each fiscal year shown. Items 1 through 9 reflect
               financial results for a single Fund share. The total returns in
               the table represent the rate that an investor would have earned
               (or lost) on an investment in the Fund (assuming reinvestment of
               all dividends and distributions). This information has been
               audited by PricewaterhouseCoopers LLP, whose report, along with
               the Fund's financial statements, are included in the Annual
               Report, which is available upon request and incorporated by
               reference into the SAI.
 
<TABLE>
<CAPTION>                        JANUS MERCURY FUND
- -------------------------------------------------------------------------------------
                               

                                                  Periods ending October 31st
                                           1998     1997     1996     1995     1994
<S>                                       <C>      <C>      <C>      <C>      <C>
  1. NET ASSET VALUE, BEGINNING OF
     PERIOD                               $18.65   $18.20   $17.38   $14.12    $11.70
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                (0.01)   (0.01)     0.14     0.16      0.02
  3. Net gains or (losses) on securities
     (both realized and unrealized)         4.07     2.82     2.74     3.37      2.40
  4. Total from investment operations       4.06     2.81     2.88     3.53      2.42
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment
     income)                                  --   (0.08)       --   (0.16)        --
  6. Dividends (in excess of net
     investment income)                   (0.04)       --       --       --        --
  7. Distributions (from capital gains)   (1.90)   (2.28)   (2.06)   (0.11)        --
  8. Total distributions                  (1.94)   (2.36)   (2.06)   (0.27)        --
  9. NET ASSET VALUE, END OF PERIOD       $20.77   $18.65   $18.20   $17.38    $14.12
 10. Total return                         24.75%   17.07%   18.18%   25.53%    20.68%
 11. Net assets, end of period (in
     millions)                            $2,368   $1,971   $2,002   $1,521      $596
 12. Average net assets for the period
     (in millions)                        $2,103   $2,046   $1,839   $1,116      $258
 13. Ratio of gross expenses to average
     net assets                            0.97%    0.98%    1.02%    1.14%       N/A
 14. Ratio of net expenses to average
     net assets                            0.94%    0.96%    1.00%    1.12%     1.33%
 15. Ratio of net investment
     income/(loss) to average net assets  (0.33%)   0.21%    0.45%    0.50%     0.25%
 16. Portfolio turnover rate                105%     157%     177%     201%      283%
- -------------------------------------------------------------------------------------
</TABLE>
 
                                                        Financial highlights  37
<PAGE> 
Glossary of investment terms
 
               This glossary provides a more detailed description of some of the
               types of securities and other instruments in which the Fund may
               invest. The Fund may invest in these instruments to the extent
               permitted by its investment objective and policies. The Fund 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
               Fund may purchase commercial paper issued in private placements
               under Section 4(2) of the Securities Act of 1933.
 
               COMMON STOCKS are equity securities representing shares of
               ownership in a company, and usually carry voting rights and earns
               dividends. Unlike preferred stock, dividends on common stocks are
               not fixed but are declared at the discretion of the issuer's
               board of directors.
 
               CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a
               fixed dividend or interest payment and are convertible into
               common stock at a specified price or conversion ratio.
 
               DEBT SECURITIES are equity securities representing money borrowed
               that must be repaid at a later date. Such securities have
               specific maturities and usually a specific rate of interest or an
               original purchase discount.
 
               DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
               corporation that entitle the holder to dividends and capital
               gains on the underlying security. Receipts include those issued
               by domestic banks (American Depositary Receipts), foreign banks
 
 38 Janus Mercury Fund
<PAGE> 
 
               (Global or European Depositary Receipts) and broker-dealers
               (depositary shares).
 
               FIXED-INCOME SECURITIES are securities that pay a specified rate
               of return. The term generally includes short- and long-term
               government, corporate and municipal obligations that pay a
               specified rate of interest or coupons for a specified period of
               time, and preferred stock, which pays fixed dividends. Coupon and
               dividend rates may be fixed for the life of the issue or, in the
               case of adjustable and floating rate securities, for a shorter
               period.
 
               HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated
               below investment grade by the primary rating agencies (e.g., BB
               or lower by Standard & Poor's and Ba or lower by Moody's). Other
               terms commonly used to describe such securities include "lower
               rated bonds," "noninvestment grade bonds" and "junk bonds."
 
               MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
               mortgages or other debt. These securities are generally pass-
               through securities, which means that principal and interest
               payments on the underlying securities (less servicing fees) are
               passed through to shareholders on a pro rata basis. These
               securities involve prepayment risk, which is the risk that the
               underlying mortgages or other debt may be refinanced or paid off
               prior to their maturities during periods of declining interest
               rates. In that case, the portfolio managers 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 Fund
               must pay if these investments are profitable, the Fund may make
               various elections permitted by the tax laws. These elections
               could require that the Fund recognize taxable income, which in
               turn
 
                                                Glossary of investment terms  39
<PAGE> 
 
               must be distributed, before the securities are sold and before
               cash is received to pay the distributions.
 
               PREFERRED STOCKS are equity securities that generally pay
               dividends at a specified rate and have preference over common
               stock in the payment of dividends and liquidation. Preferred
               stock generally does not carry voting rights.
 
               REPURCHASE AGREEMENTS involve the purchase of a security by the
               Fund and a simultaneous agreement by the seller (generally a bank
               or dealer) to repurchase the security from the Fund 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 Fund 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 Fund to another party (generally a bank or dealer) in return
               for cash and an agreement by the Fund to buy the security back at
               a specified price and time. This technique will be used primarily
               to provide cash to satisfy unusually heavy redemption requests,
               or for other temporary or emergency purposes.
 
               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.
 
 40 Janus Mercury Fund
<PAGE> 
 
               WARRANTS are securities, typically issued with preferred stocks
               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
               Fund does not earn interest on such securities until settlement
               and bears the risk of market value fluctuations in between the
               purchase and settlement dates. New issues of stocks and bonds,
               private placements and U.S. government securities may be sold in
               this manner.
 
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
               Fund may enter into forward currency contracts to hedge against
               declines in the value of securities denominated in, or whose
               value is tied to, a currency other than the U.S. dollar or to
               reduce the impact of currency appreciation on purchases of such
               securities. It may also enter into forward contracts to purchase
               or sell securities or other financial indices.
 
               FUTURES CONTRACTS are contracts that obligate the buyer to
               receive and the seller to deliver an instrument or money at a
               specified price on a specified date. The Fund 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 Fund
               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.
 
                                                Glossary of investment terms  41
<PAGE> 
 
               INDEXED/STRUCTURED SECURITIES are typically short- to
               intermediate-term debt securities whose value at maturity or
               interest rate is linked to currencies, interest rates, equity
               securities, indices, commodity prices or other financial
               indicators. Such securities may be positively or negatively
               indexed (i.e., their value may increase or decrease if the
               reference index or instrument appreciates). Indexed/structured
               securities may have return characteristics similar to direct
               investments in the underlying instrument and may be more volatile
               than the underlying instrument. The Fund bears the market risk of
               an investment in the underlying instrument, as well as the credit
               risk of the issuer.
 
               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 Fund may purchase and
               write put and call options on securities, securities indices and
               foreign currencies.
 
 42 Janus Mercury Fund
<PAGE> 
 
                       This page intentionally left blank
 
                                                                              43
<PAGE> 
 
                       This page intentionally left blank
 
 44
<PAGE> 
 
                       This page intentionally left blank
<PAGE> 
 
[JANUS LOGO]
          1-800-525-3713
          P.O. Box 173375
          Denver, Colorado 80217-3375
          janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting Janus at 1-800-525-3713 or visiting our Web
site at janus.com. In the Fund's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Fund's performance during its last fiscal
year. Other information is also available from financial
intermediaries that sell shares of the Fund.
 
The Statement of Additional Information provides detailed
information about the Fund and is incorporated into this Prospectus
by reference. You may review the Fund's Statement of Additional
Information at the Public Reference Room of the SEC or get text only
copies for a fee, by writing to or calling the Public Reference
Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You may obtain
the Statement of Additional Information for free from the SEC's Web
site at http://www.sec.gov.
 
              Investment Company Act File No. 811-1879
 
  3178
<PAGE> 
 
                                                     [JANUS LOGO]
 
                         Janus Worldwide Fund
 
                                       PROSPECTUS
                                       FEBRUARY 17, 1999
 
                         THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED
                         OR DISAPPROVED OF THESE SECURITIES OR PASSED ON THE
                         ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
                         REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
<PAGE> 
 
    [JANUS LOGO]
<PAGE> 
 
                                                               Table of contents
 
<TABLE>
                <S>                                               <C>
                RISK/RETURN SUMMARY
                   Janus Worldwide Fund.........................    2
                   Fees and expenses............................    5
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT
                STRATEGIES AND RISKS
                   Investment objective and
                   principal investment strategies..............    6
                   General portfolio policies...................    7
                   Risks........................................   10
                SHAREHOLDER'S MANUAL
                   Minimum investments..........................   17
                   Types of account ownership...................   17
                   How to open your Janus account...............   19
                   How to purchase shares.......................   20
                   How to exchange shares.......................   22
                   How to redeem shares.........................   24
                   Shareholder services and account policies....   29
                MANAGEMENT OF THE FUND
                   Investment adviser...........................   33
                   Portfolio manager............................   34
                   Assistant portfolio manager..................   34
                OTHER INFORMATION............... ...............   35
                DISTRIBUTIONS AND TAXES
                   Distributions................................   36
                   Taxes........................................   37
                FINANCIAL HIGHLIGHTS.............. .............   39
                GLOSSARY
                   Glossary of investment terms.................   40
 
</TABLE>
 
                                                            Table of contents  1
<PAGE> 
Risk return summary
 
JANUS WORLDWIDE FUND
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF JANUS WORLDWIDE FUND?
 
               The Fund seeks long-term growth of capital in a manner consistent
               with the preservation of capital.
 
               The Fund's Trustees may change this objective without a
               shareholder vote and the Fund will notify you of any changes that
               are material. If there is a material change in the Fund's
               objective or policies, you should consider whether the Fund
               remains an appropriate investment for you. There is no guarantee
               that the Fund will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF JANUS WORLDWIDE FUND?
 
               The Fund invests primarily in common stocks of companies of any
               size throughout the world. The Fund normally invests in issuers
               from at least five different countries, including the United
               States. The Fund may at times invest in fewer than five countries
               or even a single country.
 
               The portfolio manager applies a "bottom up" approach in choosing
               investments. In other words, she looks for companies with
               earnings growth potential one at a time. If the portfolio manager
               is unable to find investments with earnings growth potential, a
               significant portion of the Fund's assets may be in cash or
               similar investments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN JANUS WORLDWIDE FUND?
 
               The biggest risk of investing in this Fund is that its returns
               may vary and you could lose money. If you are considering
               investing in the Fund, remember that it is designed for long-term
               investors who can accept the risks of investing in a portfolio
               with significant foreign common stock holdings. Common stocks
               tend to be more volatile than other investment choices.
 
               The value of the Fund's portfolio may decrease if the value of an
               individual company in the portfolio decreases. The value of the
               Fund's portfolio could also decrease if the stock market goes
               down. If the value of the Fund's portfolio decreases, the Fund's
               net asset value (NAV) will also decrease which means if you sell
 
 2 Janus Worldwide Fund
<PAGE> 
 
               your shares in the Fund you would get back less money. The Fund
               may have significant exposure to foreign markets. As a result,
               its returns and NAV may be affected to a large degree by
               fluctuations in currency exchange rates or political or economic
               conditions in a particular country.
 
               An investment in the Fund is not a bank deposit and is not
               insured or guaranteed by the Federal Deposit Insurance
               Corporation or any other government agency.
 
                                                          Risk return summary  3
<PAGE> 
 
               The following information illustrates how the Fund's performance
               has varied over time. The bar chart depicts the change in the
               Fund's performance from year-to-year during the periods
               indicated. The table compares the Fund's average annual returns
               for the periods indicated to a broad-based securities market
               index.
 
               JANUS WORLDWIDE FUND
 
A BAR CHART showing Annual Total Returns for Janus Worldwide Fund from 1992
through 1998:

               Annual returns for periods ended 12/31

               9.01%    28.41%    3.61%     21.90%    26.40%   20.48%   25.87%
               1992      1993     1994       1995      1996     1997     1998

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

               Best Quarter:  4th-1998 20.24%; Worst Quarter: 3rd-1998 (16.10%)
 
 
<TABLE>
<CAPTION>
                                   Average annual total return for periods ended 12/31/98
                                   ------------------------------------------------------

                                                                                  Since
                                                                                Inception
                                                          1 year   5 years      (5/15/91)
                <S>                                       <C>      <C>       <C>             
                Janus Worldwide Fund                      25.87%   19.34%        20.72%
                Morgan Stanley International
                  Worldwide Index*                        24.34%   15.68%        13.22%
                                                      -----------------------------------
</TABLE>
 
                * The Morgan Stanley International Worldwide Index is a market
                  capitalization weighted index composed of countries
                  representative of the market structure of 47 developed and
                  emerging markets.
 
               The Fund's past performance does not necessarily indicate how it
               will perform in the future.
 
 4 Janus Worldwide Fund
<PAGE> 
 
FEES AND EXPENSES
 
               SHAREHOLDER FEES, such as sales loads, redemption fees or
               exchange fees, are charged directly to an investor's account. All
               Janus funds are no-load investments, so you will not pay any
               shareholder fees when you buy or sell shares of the Fund.
 
               ANNUAL FUND OPERATING EXPENSES are paid out of the Fund's assets
               and include fees for portfolio management, maintenance of
               shareholder accounts, shareholder servicing, accounting and other
               services. You do not pay these fees directly but, as the example
               below shows, these costs are borne indirectly by all
               shareholders.
 
               This table describes the fees and expenses that you may pay if
               you buy and hold shares of the Fund. It is based upon gross
               expenses (without the effect of expense offset arrangements) for
               the fiscal year ended October 31, 1998.
 
<TABLE>
<CAPTION>
                                                          Janus Worldwide Fund
   <S>                                                       <C>                           
   Management Fee                                                 0.65%
   Other Expenses                                                 0.27%
   Total Annual Fund Operating Expenses                           0.92%
</TABLE>
 
- --------------------------------------------------------------------------------
  EXAMPLE:
  This example is intended to help you compare the cost of investing in the
  Fund with the cost of investing in other mutual funds. The example
  assumes that you invest $10,000 in the Fund for the time periods
  indicated then redeem all of your shares at the end of those periods. The
  example also assumes that your investment has a 5% return each year and
  that the Fund's operating expenses remain the same. Although your actual
  costs may be higher or lower, based on these assumptions your costs would
  be:
 
<TABLE>
<CAPTION>
                                             1 Year     3 Years    5 Years    10 Years
                                             -----------------------------------------
   <S>                                       <C>        <C>        <C>        <C>         
   Janus Worldwide Fund                        $94       $293       $509       $1,131
</TABLE>
 
                                                          Risk return summary  5
<PAGE> 
Investment objective, principal 
           investment strategies 
           and risks
 
               This section takes a closer look at the investment objective of
               the Fund, its principal investment strategies and certain risks
               of investing in the Fund. Strategies and policies that are noted
               as "fundamental" cannot be changed without a shareholder vote.
 
               Please carefully review the "Risks" section of this Prospectus on
               pages 10-13 for a discussion of risks associated with certain
               investment techniques. We've also included a Glossary with
               descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
               Janus Worldwide Fund seeks long-term growth of capital in a
               manner consistent with the preservation of capital. It pursues
               its objective by investing primarily in common stocks of
               companies of any size throughout the world. The Fund normally
               invests in issuers from at least five different countries,
               including the United States. The Fund may at times invest in
               fewer than five countries or even a single country.
 
               The Fund may invest substantially all of its assets in common
               stocks of foreign issuers if its portfolio manager believes that
               common stocks will appreciate in value. The portfolio manager
               generally takes a "bottom up" approach to selecting companies. In
               other words, she seeks to identify individual companies with
               earnings growth potential that may not be recognized by the
               market at large. She makes this assessment by looking at
               companies one at a time, regardless of size, country of
               organization, place of principal business activity, or other
               similar selection criteria. Realization of income is not a
               significant consideration when choosing investments for the Fund.
               Income realized on the Fund's investments will be incidental to
               its objective.
 
               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
 
 6 Janus Worldwide Fund
<PAGE> 
 
               for economic growth among countries, regions or geographic areas
               may warrant greater consideration in selecting foreign
               securities.
 
GENERAL PORTFOLIO POLICIES
 
               In investing its portfolio assets, the Fund will follow the
               general policies listed below. The percentage limitations
               included in these policies and elsewhere in this Prospectus apply
               at the time of purchase of the security. So, for example, if the
               Fund exceeds a limit as a result of market fluctuations or the
               sale of securities, it will not be required to dispose of any
               securities.
 
               CASH POSITION
               When the Fund's portfolio manager believes that market conditions
               are unfavorable for profitable investing, or when she is
               otherwise unable to locate attractive investment opportunities,
               the Fund's cash or similar investments may increase. In other
               words, the Fund does not always stay fully invested in stocks.
               Cash or similar investments generally are a residual - they
               represent the assets that remain after the portfolio manager has
               committed available assets to desirable investment opportunities.
               However, the portfolio manager may also temporarily increase the
               Fund's cash position to protect its assets or maintain liquidity.
               When the Fund's investments in cash or similar investments
               increase, it may not participate in market advances or declines
               to the same extent that it would if the Fund remained more fully
               invested in stocks.
 
               OTHER TYPES OF INVESTMENTS
               The Fund invests primarily in domestic and foreign equity
               securities, which may include preferred stocks, common stocks,
               warrants and securities convertible into common or preferred
               stocks, but it may also invest to a lesser degree in other types
               of securities. These securities (which are described in the
               Glossary) may include:
 
               - debt securities
 
               - indexed/structured securities
 
              Investment objective, principal investment strategies and risks  7
<PAGE> 
 
               - high-yield/high-risk securities (less than 35% of the Fund's
                 assets)
 
               - options, futures, forwards and other types of derivatives for
                 hedging purposes or for non-hedging purposes such as seeking to
                 enhance return
 
               - securities purchased on a when-issued, delayed delivery or
                 forward commitment basis
 
               ILLIQUID INVESTMENTS
               The Fund may invest up to 15% of its net assets in illiquid
               investments. An illiquid investment is a security or other
               position that cannot be disposed of quickly in the normal course
               of business. For example, some securities are not registered
               under the U.S. securities laws and cannot be sold to the U.S.
               public because of SEC regulations (these are known as "restricted
               securities"). Under procedures adopted by the Fund's Trustees,
               certain restricted securities may be deemed liquid, and will not
               be counted toward this 15% limit.
 
               FOREIGN SECURITIES
               The Fund may invest without limit in foreign equity and debt
               securities. The Fund may invest directly in foreign securities
               denominated in a foreign currency and not publicly traded in the
               United States. Other ways of investing in foreign securities
               include depositary receipts or shares, and passive foreign
               investment companies.
 
               SPECIAL SITUATIONS
               The Fund may invest in special situations. A special situation
               arises when, in the opinion of the Fund'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. The
 
 8 Janus Worldwide Fund
<PAGE> 
 
               Fund's performance could suffer if the anticipated development in
               a "special situation" investment does not occur or does not
               attract the expected attention.
 
               PORTFOLIO TURNOVER
               The Fund generally intends to purchase securities for long-term
               investment although, to a limited extent, the Fund may purchase
               securities in anticipation of relatively short-term price gains.
               Short-term transactions may also result from liquidity needs,
               securities having reached a price or yield objective, changes in
               interest rates or the credit standing of an issuer, or by reason
               of economic or other developments not foreseen at the time of the
               investment decision. The Fund may also sell one security and
               simultaneously purchase the same or a comparable security to take
               advantage of short-term differentials in bond yields or
               securities prices. Changes are made in the Fund's portfolio
               whenever its portfolio manager believes such changes are
               desirable. Portfolio turnover rates are generally not a factor in
               making buy and sell decisions.
 
               Increased portfolio turnover may result in higher costs for
               brokerage commissions, dealer mark-ups and other transaction
               costs and may also result in taxable capital gains. Higher costs
               associated with increased portfolio turnover may offset gains in
               the Fund's performance.
 
              Investment objective, principal investment strategies and risks  9
<PAGE> 
 
RISKS
 
               Because the Fund may invest substantially all of its assets in
               common stocks, the main risk is the risk that the value of the
               stocks it holds might decrease in response to the activities of
               an individual company or in response to general market and/or
               economic conditions. If this occurs, the Fund's share price may
               also decrease. The Fund's performance may also be affected by
               risks specific to certain types of investments, such as foreign
               securities, derivative investments, non-investment grade debt
               securities or companies with relatively small market
               capitalizations.
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Fund.
 
1. HOW COULD THE FUND'S INVESTMENTS IN FOREIGN SECURITIES AFFECT ITS
   PERFORMANCE?
 
               The Fund may invest without limit in foreign securities either
               indirectly (e.g., depositary receipts) or directly in foreign
               markets. Investments in foreign securities, including those of
               foreign governments, may involve greater risks than investing in
               domestic securities because the Fund's performance may depend on
               issues other than the performance of a particular company. These
               issues include:
 
               - CURRENCY RISK. As long as the Fund holds a foreign security,
                 its value will be affected by the value of the local currency
                 relative to the U.S. dollar. When the Fund sells a foreign
                 denominated security, its value may be worth less in U.S.
                 dollars even if the security increases in value in its home
                 country. U.S. dollar denominated securities of foreign issuers
                 may also be affected by currency risk.
 
               - POLITICAL AND ECONOMIC RISK. Foreign investments may be subject
                 to heightened political and economic risks, particularly in
                 emerging markets which may have relatively unstable
                 governments, immature economic structures, national policies
                 restricting investments by foreigners, different legal systems,
                 and economies based on only a few industries. In some
                 countries,
 
 10 Janus Worldwide Fund
<PAGE> 
 
                 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 Fund's assets from that
                 country.
 
               - REGULATORY RISK. There may be less government supervision of
                 foreign markets. As a result, foreign issuers may not be
                 subject to the uniform accounting, auditing and financial
                 reporting standards and practices applicable to domestic
                 issuers and there may be less publicly available information
                 about foreign issuers.
 
               - MARKET RISK. Foreign securities markets, particularly those of
                 emerging market countries, may be less liquid and more volatile
                 than domestic markets. Certain markets may require payment for
                 securities before delivery and delays may be encountered in
                 settling securities transactions. In some foreign markets,
                 there may not be protection against failure by other parties to
                 complete transactions.
 
               - TRANSACTION COSTS. Costs of buying, selling and holding foreign
                 securities, including brokerage, tax and custody costs, may be
                 higher than those involved in domestic transactions.
 
2. THE FUND MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
               Many attractive investment opportunities may be smaller, start-up
               companies offering emerging products or services. Smaller or
               newer companies may suffer more significant losses as well as
               realize more substantial growth than larger or more established
               issuers because they may lack depth of management, be unable to
               generate funds necessary for growth or potential development, or
               be developing or marketing new products or services for which
               markets are not yet established and may never become established.
               In addition, such companies may be insignificant factors in their
               industries and may become subject to intense competition from
               larger or more established companies. Securities of smaller or
               newer companies may have more limited trading markets than the
               markets for securities of larger or more established issuers, and
               may be subject to wide price fluctuations. Investments in such
 
             Investment objective, principal investment strategies and risks  11
<PAGE> 
 
               companies tend to be more volatile and somewhat more speculative.
 
3. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
               The portfolio manager carefully researches each potential
               investment before making an investment decision and, among other
               things, considers Year 2000 readiness when selecting portfolio
               holdings. However, there is no guarantee that the information the
               portfolio manager receives regarding a company's Year 2000
               readiness is completely accurate. If a company has not
               satisfactorily addressed Year 2000 issues, the Fund's performance
               could suffer.
 
4. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
               High-yield/high-risk securities (or "junk" bonds) are securities
               rated below investment grade by the primary rating agencies such
               as Standard & Poor's and Moody's. The value of lower quality
               securities generally is more dependent on credit risk, or the
               ability of the issuer to meet interest and principal payments,
               than investment grade debt securities. Issuers of high-yield
               securities may not be as strong financially as those issuing
               bonds with higher credit ratings and are more vulnerable to real
               or perceived economic changes, political changes or adverse
               developments specific to the issuer.
 
               Please refer to the SAI for a description of bond rating
               categories.
 
5. HOW DOES THE FUND TRY TO REDUCE RISK?
 
               The Fund may use futures, options and other derivative
               instruments to "hedge" or protect its portfolio from adverse
               movements in securities prices and interest rates. The Fund may
               also use a variety of currency hedging techniques, including
               forward currency contracts, to manage exchange rate risk. The
               Fund believes the use of these instruments will benefit the Fund.
               However, the Fund's performance could be worse than if the Fund
               had not used such instruments if the portfolio manager's
               judgement proves
 
 12 Janus Worldwide Fund
<PAGE> 
 
               incorrect. Risks associated with the use of derivative
               instruments are described in the SAI.
 
             Investment objective, principal investment strategies and risks  13
<PAGE> 
 
Janus Worldwide Fund
 
<PAGE> 
 
                   Shareholder's
                   Manual
 
                   This section will help you become
                   familiar with the different types
                   of accounts you can establish with
                   Janus. It also explains in detail
                   the wide array of services and
                   features you can establish on your
                   account, as well as account
                   policies and fees that may apply
                   to your account. Account policies
                   (including fees), services and
                   features may be modified or
                   discontinued without shareholder
                   approval or prior notice.

                                                    [JANUS LOGO]
<PAGE> 
 
HOW TO GET IN TOUCH WITH JANUS
 
               Janus offers two Investor Service Centers for those individuals
               who would like to conduct their investing in person. Our
               representatives will be happy to assist you at either of the
               following locations: Monday-Friday 7:00 a.m. to 6:00 p.m.
               Mountain time and Saturday 9:00 a.m. to 1:00 p.m. Mountain time.
 
               100 Fillmore Street, Suite 100
               Denver, CO 80206
 
               3773 Cherry Creek North Drive, Suite 101
               Denver, CO 80209
 
 QUICK ADDRESS AND TELEPHONE REFERENCE
- --------------------------------------------------------------------------------
 
<TABLE>
  <S>                                           <C>
  MAILING ADDRESS                               JANUS XPRESSLINE(TM)
  Janus                                         1-888-979-7737
  P.O. Box 173375                               For 24-hour access to account
  Denver, CO 80217-3375                         and fund information,
                                                exchanges, purchases and
  FOR OVERNIGHT CARRIER                         redemptions, automated daily
  Janus                                         quotes on fund share prices,
  Suite 101                                     yields and total returns.
  3773 Cherry Creek North Drive
  Denver, CO 80209-3811                         TDD
                                                1-800-525-0056
  INVESTOR SERVICE REPRESENTATIVES              A telecommunications device
  If you have any questions while reading       for our hearing- and
  this Prospectus, please call one of our       speech-impaired shareholders.
  Investor Service Representatives at
  1-800-525-3713 Monday-Friday: 8:00            JANUS LITERATURE LINE
  a.m.-8:00 p.m., and Saturday: 10:00           1-800-525-8983
  a.m.-4:00 p.m., New York time.                To request a prospectus,
                                                shareholder reports or
  JANUS INTERNET ADDRESS                        marketing materials 24 hours a
  janus.com                                     day.
</TABLE>
 
 16 Janus Worldwide Fund
<PAGE> 
 
MINIMUM INVESTMENTS*
- ---------------------------------------------
 
<TABLE>
<S>                                 <C>
To open a new regular account        $2,500

To open a new retirement,
education or UGMA/UTMA account       $  500

To open a new regular account with
an Automatic Investment Program      $  500**

To add to any type of an account     $  100+
</TABLE>
 
 * The Fund reserves the right to
   change the amount of these
   minimums from time to time or
   to waive them in whole or in
   part for certain types of
   accounts.
** An Automatic Investment Program
   requires a $100 minimum
   automatic investment per month
   until the account balance
   reaches $2,500.
 + The minimum subsequent
   investment for IRA UGMA/UTMA
   accounts is $50.
 
TYPES OF ACCOUNT OWNERSHIP
 
               If you are investing in the Fund for the first time, you will
               need to establish an account. You can establish the following
               types of accounts by completing a New Account Application. To
               request an application, call 1-800-525-3713 or visit our Web site
               at janus.com to download an application.
 
               INDIVIDUAL OR JOINT OWNERSHIP
               Individual accounts are owned by one person. Joint accounts have
               two or more owners.
 
               A GIFT OR TRANSFER TO MINOR (UGMA OR UTMA)
               An UGMA/UTMA account is a custodial account managed for the
               benefit of a minor. To open an UGMA or UTMA account, you must
               include the minor's Social Security number on the application.
 
               TRUST
               An established trust can open an account. The names of each
               trustee, the name of the trust and the date of the trust
               agreement must be included on the application.
 
                                                        Shareholder's manual  17
<PAGE> 
 
               BUSINESS ACCOUNTS
               Corporations and partnerships may also open an account. The
               application must be signed by an authorized officer of the
               corporation or a general partner of the partnership.
 
TAX-DEFERRED ACCOUNTS
 
               If you are eligible, you may set up one or more tax-deferred
               accounts. A tax-deferred account allows you to shelter your
               investment income and capital gains from current income taxes. A
               contribution to certain of these plans may also be tax
               deductible. Tax-deferred accounts include retirement plans
               described below and the Education IRA. Distributions from these
               plans are generally subject to income tax and may be subject to
               an additional tax if withdrawn prior to age 59 1/2 or used for a
               nonqualifying purpose. Investors should consult their tax adviser
               or legal counsel before selecting a tax-deferred account.
 
               Investors Fiduciary Trust Company serves as custodian for the
               tax-deferred accounts offered by the Fund. You will be charged an
               annual account maintenance fee of $12 for each taxpayer
               identification number no matter how many tax-deferred accounts
               you have with Janus. You may pay the fee by check or have it
               automatically deducted from your account (usually in December).
               The custodian reserves the right to change the amount of this fee
               or to waive it in whole or in part for certain types of accounts.
 
               The following plans require a special application. For an
               application and more details about our Retirement Plans, call
               1-800-525-3713.
 
               TRADITIONAL AND ROTH INDIVIDUAL RETIREMENT ACCOUNTS
               Both types of IRAs allow most individuals with earned income to
               contribute up to the lesser of $2,000 ($4,000 for most married
               couples) or 100% of compensation annually. Please refer to the
               Janus IRA booklet for more complete information regarding IRAs.
 
 18 Janus Worldwide Fund
<PAGE> 
 
               EDUCATION IRA
               This plan allows individuals, subject to certain income
               limitations, to contribute up to $500 annually on behalf of any
               child under the age of 18. Please refer to the Janus IRA booklet
               for more complete information regarding the Education IRA.
 
               SIMPLIFIED EMPLOYEE PENSION PLAN
               This plan allows small business owners (including sole
               proprietors) to make tax-deductible contributions for themselves
               and any eligible employee(s). A SEP requires an IRA (a SEP-IRA)
               to be set up for each SEP participant.
 
               PROFIT SHARING OR MONEY PURCHASE PENSION PLAN
               These plans are open to corporations, partnerships and sole
               proprietors to benefit their employees and themselves.
 
               SECTION 403(b)(7) PLAN
               Employees of educational organizations or other qualifying, tax-
               exempt organizations may be eligible to participate in a Section
               403(b)(7) Plan.
 
HOW TO OPEN YOUR JANUS ACCOUNT
 
               Complete and sign the appropriate application. Please be sure to
               provide your Social Security or taxpayer identification number on
               the application and make your check payable to Janus. The Fund is
               available only to U.S. citizens or residents, and your
               application will be returned if you do not meet these criteria.
               Send all items to one of the addresses listed in the "Quick
               Address and Telephone Reference" on page 16.
 
                                                        Shareholder's manual  19
<PAGE> 
 
HOW TO PURCHASE SHARES
 
               PAYING FOR SHARES
 
               When you purchase shares, your request will be processed at the
               next NAV calculated after your order is received and accepted.
               Please note the following:
 
               - Cash, credit cards, third party checks and credit card checks
                 will not be accepted.
 
               - All purchases must be made in U.S. dollars.
 
               - Checks must be drawn on a U.S. bank and made payable to Janus.
 
               - If a check does not clear your bank, the Fund reserves the
                 right to cancel the purchase.
 
               - If the Fund is unable to debit your predesignated bank account
                 on the day of purchase, it may make additional attempts or
                 cancel the purchase.
 
               - The Fund reserves the right to reject any specific purchase
                 request.
 
               If your purchase is cancelled, you will be responsible for any
               losses or fees imposed by your bank and losses that may be
               incurred as a result of any decline in the value of the cancelled
               purchase. The Fund (or its agents) has the authority to redeem
               shares in your account(s) to cover any such losses due to
               fluctuations in share price. Any profit on such cancellation will
               accrue to the Fund.
 
               ONCE YOU HAVE OPENED YOUR JANUS ACCOUNT, THE MINIMUM AMOUNT FOR
               AN ADDITIONAL INVESTMENT IS $100 ($50 FOR IRAs OR UGMA/UTMA
               ACCOUNTS). You may add to your account at any time through any of
               the following options:
 
               BY MAIL
 
               Complete the remittance slip attached at the bottom of your
               confirmation statement. If you are making a purchase into a
 
 20 Janus Worldwide Fund
<PAGE> 
 
               retirement account, please indicate whether the purchase is a
               rollover or a current or prior year contribution. Send your check
               made payable to Janus and remittance slip or written instructions
               to one of the addresses listed previously. You may also request a
               booklet of remittance slips for non-retirement accounts.
 
               BY TELEPHONE
 
               This service allows you to purchase additional shares quickly and
               conveniently through an electronic transfer of money. To purchase
               shares by telephone, call an Investor Service Representative at
               1-800-525-3713 during normal business hours or call the Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. When you make an additional purchase by telephone, Janus
               will automatically debit your predesignated bank account for the
               desired amount. To establish the telephone purchase option on
               your new account, complete the "Telephone Purchase of Shares
               Option" section on the application and attach a "voided" check or
               deposit slip from your bank account. If your account is already
               established, call 1-800-525-3713 to request the appropriate form.
               This option will become effective ten days after the form is
               received.
 
               BY WIRE
 
               Purchases may also be made by wiring money from your bank account
               to your Janus account. Call 1-800-525-3713 to receive wiring
               instructions.
 
               BY INTERNET
 
               You must pre-establish the "Telephone Purchase of Shares Option"
               to make a purchase on our Web site at janus.com. If you have
               questions, please call 1-800-975-9932 to speak to a Janus
               representative.
 
                                                        Shareholder's manual  21
<PAGE> 
 
               AUTOMATIC INVESTMENT PROGRAMS
 
               Janus offers several automatic investment programs to help
               investors achieve their financial goals as simply and
               conveniently as possible. You may open a new account with a $500
               initial purchase and $100 automatic subsequent investments.
 
               AUTOMATIC MONTHLY INVESTMENT PROGRAM
               You select the day each month that your money ($100 minimum) will
               be electronically transferred from your bank account to your Fund
               account. To establish this option, complete the "Automatic
               Monthly Investment Program" section on the application and attach
               a "voided" check from your bank account. If your Fund account is
               already established, call 1-800-525-3713 to request the
               appropriate form.
 
               PAYROLL DEDUCTION
               If your employer can initiate an automatic payroll deduction, you
               may have all or a portion of your paycheck ($100 minimum)
               invested directly into your Fund account. To obtain information
               on establishing this option, call 1-800-525-3713.
 
               BY SYSTEMATIC EXCHANGE
               With a Systematic Exchange you determine the amount of money
               ($100 minimum) you would like automatically exchanged from one
               Janus account to another on any day of the month. For more
               information on how to establish this option, call 1-800-525-3713.
 
HOW TO EXCHANGE SHARES
 
               On any business day, you may exchange all or a portion of your
               shares into any other available Janus fund.
 
               IN WRITING
 
               To request an exchange in writing, please follow the instructions
               for written requests on page 26.
 
 22 Janus Worldwide Fund
<PAGE> 
 
               BY TELEPHONE
 
               All accounts are automatically eligible for the telephone
               exchange option. To exchange shares by telephone, call an
               Investor Service Representative at 1-800-525-3713 during normal
               business hours or call the Janus XpressLine, 1-888-979-7737, for
               access to this option 24 hours a day.
 
               BY SYSTEMATIC EXCHANGE
 
               As noted above, you may establish a Systematic Exchange for as
               little as a $100 subsequent purchase per month on established
               accounts. You may establish a new account with a $500 initial
               purchase and subsequent $100 systematic exchanges. If the balance
               in the account you are exchanging from falls below the systematic
               exchange amount, all remaining shares will be exchanged and the
               program will be discontinued.
 
               BY INTERNET
 
               Exchanges may also be made on our Web site at janus.com.
 
               EXCHANGE POLICIES
 
               - Except for Systematic Exchanges, new accounts established by
                 exchange must be opened with $2,500 or the total account value
                 if the value of the account you are exchanging from is less
                 than $2,500.
 
               - Exchanges between existing accounts must meet the $100
                 subsequent investment requirement.
 
               - You may make four exchanges out of the Fund during a calendar
                 year (exclusive of Systematic Exchanges). Exchanges in excess
                 of this limit may be subject to an exchange fee or may result
                 in termination of the exchange privilege.
 
               - The Fund reserves the right to reject any exchange request and
                 to modify or terminate the exchange privilege at any time. For
                 example, the Fund may reject exchanges from accounts engaged
 
                                                        Shareholder's manual  23
<PAGE> 
 
                 in or known to engage in trading in excess of the limit above
                 (including market timing transactions).
 
               - Exchanges between accounts will be accepted only if the
                 registrations are identical.
 
               - If the shares you are exchanging are held in certificate form,
                 you must return the certificate to your Fund prior to making
                 any exchanges.
 
               - Be sure that you read the prospectus for the fund into which
                 you are exchanging.
 
               - An exchange represents the sale of shares from one fund and the
                 purchase of shares of another fund, which may produce a taxable
                 gain or loss in a non-tax deferred account.
 
HOW TO REDEEM SHARES
 
               On any business day, you may redeem all or a portion of your
               shares. If the shares are held in certificate form, the
               certificate must be returned with or before your redemption
               request. Your transaction will be processed at the next NAV
               calculated after your order is received and accepted. The
               redemption may be suspended for 10 days following an address
               change unless a signature guarantee is provided.
 
               IN WRITING
 
               To request a redemption in writing, please follow the
               instructions for written requests noted on page 26.
 
               BY TELEPHONE
 
               Most accounts have the telephone redemption option, unless this
               option was specifically declined on the application or in
               writing. This option enables you to request redemptions daily
               from your account by calling 1-800-525-3713 by the close of the
               regular trading session of the New York Stock Exchange ("NYSE"),
               normally 4:00 p.m., New York time. You may also use Janus
               XpressLine 1-888-979-7737, for access to this option 24 hours a
 
 24 Janus Worldwide Fund
<PAGE> 
 
               day. (There is a daily limit of $100,000 per account for
               redemptions payable by check).
 
               BY INTERNET
 
               Redemptions may also be made on our Web site at janus.com.
 
               SYSTEMATIC REDEMPTION OPTION
 
               The Systematic Redemption Option allows you to redeem a specific
               dollar amount from your account on a regular basis. For more
               information or to request the appropriate form, please call
               1-800-525-3713.
 
PAYMENT OF REDEMPTION PROCEEDS
 
               BY CHECK
               Redemption proceeds will be sent to the shareholder(s) of record
               at the address of record within seven days after receipt of a
               valid redemption request.
 
               BY ELECTRONIC TRANSFER
               If you have established the electronic redemption option, your
               redemption proceeds can be electronically transferred to your
               predesignated bank account on the next bank business day after
               receipt of your redemption request (wire transfer) or the second
               bank business day after receipt of your redemption request (ACH
               transfer). Wire transfers will be charged an $8 fee per wire and
               your bank may charge an additional fee to receive the wire. ACH
               transfers are made free of charge. Wire redemptions are not
               available for retirement accounts.
 
               If you would like to establish the electronic redemption option
               on an existing account, please call 1-800-525-3713 to request the
               appropriate form.
 
               IF THE SHARES BEING REDEEMED WERE PURCHASED BY CHECK, TELEPHONE
               ON OUR WEB SITE, OR THROUGH THE AUTOMATIC MONTHLY INVESTMENT
               PROGRAM, THE FUND MAY DELAY THE PAYMENT OF YOUR REDEMPTION
               PROCEEDS FOR UP TO 15 DAYS FROM THE DAY OF
 
                                                        Shareholder's manual  25
<PAGE> 
 
               PURCHASE TO ALLOW THE PURCHASE TO CLEAR. Unless you provide
               alternate instructions, your proceeds will be invested in Janus
               Money Market Fund - Investor Shares during the 15 day hold
               period.
 
WRITTEN INSTRUCTIONS
 
               To redeem or exchange all or part of your shares in writing, your
               request should be sent to one of the addresses listed on page 16
               and must include the following information:
 
               - the name of the Fund
 
               - the account number
 
               - the amount of money or number of shares being redeemed or
                 exchanged
 
               - the name(s) on the account
 
               - the signature(s) of all registered account owners
 
               - your daytime telephone number
 
               SIGNATURE REQUIREMENTS BASED ON ACCOUNT TYPE
 
               INDIVIDUAL, JOINT TENANTS, TENANTS IN COMMON
               Written instructions must be signed by each shareholder, exactly
               as the names appear in the account registration.
 
               UGMA OR UTMA
               Written instructions must be signed by the custodian in his/her
               capacity as it appears in the account registration.
 
               SOLE PROPRIETOR, GENERAL PARTNER
               Written instructions must be signed by an authorized individual
               in his/her capacity as it appears on the account registration.
 
               CORPORATION, ASSOCIATION
               Written instructions must be signed by the person(s) authorized
               to act on the account. In addition, a certified copy of the
               corporate
 
 26 Janus Worldwide Fund
<PAGE> 
 
               resolution authorizing the signer to act must accompany the
               request.
 
               TRUST
               Written instructions must be signed by the trustee(s). If the
               name(s) of the current trustee(s) does not appear in the account
               registration, a certificate of incumbency dated within 60 days
               must also be submitted.
 
               IRA
               Written instructions must be signed by the account owner. If you
               do not want federal income tax withheld from your redemption, you
               must state that you elect not to have such withholding apply. In
               addition, your instructions must state whether the distribution
               is normal (after age 59 1/2) or premature (before age 59 1/2)
               and, if premature, whether any exceptions such as death or
               disability apply with regard to the 10% additional tax on early
               distributions.
 
SIGNATURE GUARANTEE
 
               In addition to the signature requirements, A SIGNATURE GUARANTEE
               IS ALSO REQUIRED if any of the following is applicable:
 
               - You request a redemption by check that exceeds $100,000.
 
               - You would like the check made payable to anyone other than the
                 shareholder(s) of record.
 
               - You would like the check mailed to an address which has been
                 changed within 10 days of the redemption request.
 
               - You would like the check mailed to an address other than the
                 address of record.
 
               THE FUND RESERVES THE RIGHT TO REQUIRE A SIGNATURE GUARANTEE
               UNDER OTHER CIRCUMSTANCES OR TO REJECT OR DELAY A REDEMPTION ON
               CERTAIN LEGAL GROUNDS. FOR MORE INFORMATION PERTAINING TO
               SIGNATURE GUARANTEES, PLEASE CALL 1-800-525-3713.
 
                                                        Shareholder's manual  27
<PAGE> 
 
               HOW TO OBTAIN A SIGNATURE GUARANTEE
 
               A signature guarantee assures that a signature is genuine. The
               signature guarantee protects shareholders from unauthorized
               account transfers. The following financial institutions may
               guarantee signatures: banks, savings and loan associations, trust
               companies, credit unions, broker-dealers and member firms of a
               national securities exchange. Call your financial institution to
               see if they have the ability to guarantee a signature. A
               signature guarantee cannot be provided by a notary public.
 
               If you live outside the United States, a foreign bank properly
               authorized to do business in your country of residence or a U.S.
               consulate may be able to authenticate your signature.
 
PRICING OF FUND SHARES
 
               All purchases, redemptions and exchanges will be processed at the
               NAV next calculated after your request is received and accepted
               by the Fund (or the Fund's agent or authorized designee). The
               Fund's NAV is calculated at the close of the regular trading
               session of the NYSE (normally 4:00 p.m. New York time) each day
               that the NYSE is open. The NAV of Fund shares is not determined
               on days the NYSE is closed (generally, New Year's Day, Martin
               Luther King Day, Presidents' Day, Good Friday, Memorial Day,
               Independence Day, Labor Day, Thanksgiving and Christmas). In
               order to receive a day's price, your order must be received by
               the close of the regular trading session of the NYSE. The Fund's
               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. See the SAI for more detailed
               information.
 
 28 Janus Worldwide Fund
<PAGE> 
 
SHAREHOLDER SERVICES AND ACCOUNT POLICIES
 
               JANUS XPRESSLINE(TM)
 
               Janus XpressLine, our electronic telephone service, offers you
               24-hour access by TouchTone(TM) telephone to obtain information
               on account balances, Fund performance or dividends. You can also
               make exchanges, purchases and redemptions in existing accounts,
               request literature about any Janus fund, or order duplicate
               statements. Janus XpressLine is accessed by calling
               1-888-979-7737. Calls are limited to five minutes.
 
               JANUS WEB SITE
 
               Janus maintains a Web site located at janus.com. You can
               purchase, exchange and redeem shares and access information such
               as your account balance and the Fund's NAV through the Web site.
               In order to engage in transactions on our Web site, you must
               authorize us to transmit account information online and accept
               online instructions (see janus.com and follow the procedures
               accordingly). You may also need to have bank account information,
               wire instructions or other options established on your account.
               The Fund and its agents will not be responsible for any losses
               resulting from unauthorized transactions on our Web site when
               procedures designed for engaging in such transactions are
               followed. If you have questions, please call 1-800-975-9932 to
               speak to a Janus representative.
 
               ACCOUNT MINIMUMS
 
               Due to the proportionately higher costs of maintaining small
               accounts, Janus reserves the right to deduct a $10 minimum
               balance fee (or the value of the account if less than $10) from
               accounts with values below the minimums described on page 17 or
               to close such accounts. This policy will apply to accounts
               participating in the Automatic Monthly Investment Program only if
               your account balance does not reach the required minimum initial
               investment or falls below such minimum and you have discontinued
               monthly investments. This policy does not apply to
 
                                                        Shareholder's manual  29
<PAGE> 
 
               accounts that fall below the minimums solely as a result of
               market value fluctuations. It is expected that accounts will be
               valued in September and the $10 fee will be assessed on the
               second Friday of September of each year. You will receive notice
               before we charge the $10 fee or close your account so that you
               may increase your account balance to the required minimum.
 
               TRANSACTIONS THROUGH PROCESSING ORGANIZATIONS
 
               You may purchase or sell Fund shares through a broker-dealer,
               bank or other financial institution, or an organization that
               provides recordkeeping and consulting services to 401(k) plans or
               other employee benefit plans (a "Processing Organization").
               Processing Organizations may charge you a fee for this service
               and may require different minimum initial and subsequent
               investments than the Fund. Processing Organizations may also
               impose other charges or restrictions different from those
               applicable to shareholders who invest in the Fund directly. A
               Processing Organization, rather than its customer, may be the
               shareholder of record of your shares. The Fund is not responsible
               for the failure of any Processing Organization to carry out its
               obligations to its customers. Certain Processing Organizations
               may receive compensation from Janus Capital or its affiliates and
               certain Processing Organizations may receive compensation from
               the Fund for shareholder recordkeeping and similar services.
 
               TAXPAYER IDENTIFICATION NUMBER
 
               On the application or other appropriate form, you will be asked
               to certify that your Social Security or taxpayer identification
               number is correct and that you are not subject to backup
               withholding for failing to report income to the IRS. If you are
               subject to the 31% backup withholding or you did not certify your
               taxpayer identification number, the IRS requires the Fund to
               withhold 31% of any dividends paid and redemption or exchange
               proceeds. In addition to the 31% backup withholding, you may be
               subject to a $50 fee to reimburse the Fund for any penalty that
               the IRS may impose.
 
 30 Janus Worldwide Fund
<PAGE> 
 
               SHARE CERTIFICATES
 
               Most shareholders choose not to hold their shares in certificate
               form because account transactions such as exchanges and
               redemptions cannot be completed until the certificate has been
               returned to the Fund. The Fund will issue share certificates upon
               written request only and reserves the right to charge a fee for
               this service. Share certificates will not be issued until the
               shares have been held for at least 15 days and will not be issued
               for accounts that do not meet the minimum investment
               requirements. Share certificates cannot be issued for retirement
               accounts. In addition, if the certificate is lost, there may be a
               replacement charge.
 
               INVOLUNTARY REDEMPTIONS
 
               The Fund reserves the right to close an account if the
               shareholder is deemed to engage in activities which are illegal
               or otherwise believed to be detrimental to the Fund.
 
               TELEPHONE TRANSACTIONS
 
               You may initiate many transactions by telephone. The Fund and its
               agents will not be responsible for any losses resulting from
               unauthorized transactions when procedures designed to verify the
               identity of the caller are followed.
 
               It may be difficult to reach an Investor Service Representative
               by telephone during periods of unusual market activity. If you
               are unable to reach a representative by telephone, please
               consider sending written instructions, stopping by a Service
               Center, calling the Janus XpressLine or visiting our Web site.
 
               TEMPORARY SUSPENSION OF SERVICES
 
               The Fund or its agents may, in case of emergency, temporarily
               suspend telephone transactions and other shareholder services.
 
                                                        Shareholder's manual  31
<PAGE> 
 
               ADDRESS CHANGES
 
               To change the address on your account, call 1-800-525-3713 or
               send a written request signed by all account owners. Include the
               name of the Fund, the account number(s), the name(s) on the
               account and both the old and new addresses. Certain options may
               be suspended for 10 days following an address change unless a
               signature guarantee is provided.
 
               REGISTRATION CHANGES
 
               To change the name on an account, the shares are generally
               transferred to a new account. In some cases, legal documentation
               may be required. For more information call 1-800-525-3713.
 
               STATEMENTS AND REPORTS
 
               Investors participating in an automatic investment program will
               receive quarterly confirmations of all transactions. Dividend
               information will be distributed annually. In addition, the Fund
               will send you an immediate transaction confirmation statement
               after every non-systematic transaction.
 
               The Fund produces financial reports, which include a list of the
               Fund's portfolio holdings, semiannually and updates its
               prospectus annually. To reduce expenses, the Fund may choose to
               mail only one report or prospectus to your household, even if
               more than one person in the household has a Fund account. Please
               call 1-800-525-3713 if you would like to receive additional
               reports or prospectuses. The Fund reserves the right to charge a
               fee for additional statement and report requests.
 
 32 Janus Worldwide Fund
<PAGE> 
                                                          Management of the fund
 
INVESTMENT ADVISER
 
               Janus Capital Corporation, 100 Fillmore Street, Denver, Colorado
               80206-4928, is the investment adviser to the Fund and is
               responsible for the day-to-day management of its investment
               portfolio and other business affairs.
 
               Janus Capital began serving as investment adviser to Janus Fund
               in 1970 and currently serves as investment adviser to all of the
               Janus funds, acts as sub-adviser for a number of private-label
               mutual funds and provides separate account advisory services for
               institutional accounts.
 
               Janus Capital furnishes continuous advice and recommendations
               concerning the Fund's investments. Janus Capital also furnishes
               certain administrative, compliance and accounting services for
               the Fund, and may be reimbursed by the Fund 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 Fund and pays the salaries, fees and expenses of
               all Fund officers and those Trustees who are affiliated with
               Janus Capital.
 
               The Fund pays Janus Capital a management fee which is calculated
               daily and paid monthly. The advisory agreement with the Fund
               spells out the management fee and other expenses that the Fund
               must pay.
 
               The Fund incurs 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 Annual Fund Operating Expenses
               table on page 5 lists the actual management fee and total
               operating expenses of the Fund for the most recent fiscal year.
 
                                                     Management of the funds  33
<PAGE> 
 
PORTFOLIO MANAGER
 
HELEN YOUNG HAYES
- --------------------------------------------------------------------------------
                   is Executive Vice President and portfolio manager of the
                   Fund. She is also Executive Vice President and co-manager
                   of Janus Overseas Fund. Ms. Hayes joined Janus Capital in
                   1987 and has managed or co-managed the Fund and Janus
                   Overseas Fund since their inceptions. She holds a Bachelor
                   of Arts in Economics from Yale University and received the
                   Chartered Financial Analyst designation.
 
ASSISTANT PORTFOLIO MANAGER
 
LAURENCE J. CHANG
- --------------------------------------------------------------------------------
                   is assistant portfolio manager of the Fund. He is also
                   Executive Vice President and co-manager of Janus Overseas
                   Fund. Mr. Chang joined Janus Capital in 1993 as a research
                   analyst. 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. Mr. Chang received the Chartered Financial
                   Analyst designation.
 
 34 Janus Worldwide Fund
<PAGE> 
                                                               Other information
 
               SIZE OF THE FUND
 
               Although there is no present intention to do so, the Fund may
               discontinue sales of its shares if management and the Trustees
               believe that continued sales may adversely affect the Fund's
               ability to achieve its investment objective. If sales of the Fund
               are discontinued, it is expected that existing shareholders of
               the Fund would be permitted to continue to purchase shares and to
               reinvest any dividends or capital gains distributions, absent
               highly unusual circumstances.
 
               YEAR 2000
 
               Preparing for Year 2000 is a high priority for Janus Capital,
               which has established a dedicated group to address this issue.
               Janus Capital has devoted considerable internal resources and has
               engaged one of the foremost experts in the field to help achieve
               Year 2000 readiness. Janus Capital does not anticipate that Year
               2000-related issues will have a material impact on its ability to
               continue to provide the Fund with service at current levels;
               however, Janus Capital cannot make any assurances that the steps
               it has taken to ensure Year 2000 readiness will be successful. In
               addition, there can be no assurance that Year 2000 issues will
               not affect the companies in which the Fund invests or worldwide
               markets and economies.
 
                                                           Other information  35
<PAGE> 
Distributions and taxes
 
DISTRIBUTIONS
 
               To avoid taxation of the Fund, the Internal Revenue Code requires
               the Fund to distribute net income and any net gains realized on
               its investments annually. The Fund's income from dividends and
               interest and any net realized short-term gains are paid to
               shareholders as ordinary income dividends. Net realized long-term
               gains are paid to shareholders as capital gains distributions.
               Dividends and capital gains distributions are normally declared
               and paid in December.
 
               HOW DISTRIBUTIONS AFFECT A FUND'S NAV
 
               Distributions are paid to shareholders as of the record date of
               the distribution of the Fund, regardless of how long the shares
               have been held. Dividends and capital gains awaiting distribution
               are included in the Fund's daily NAV. The share price of the Fund
               drops by the amount of the distribution, net of any subsequent
               market fluctuations. As an example, assume that on December 31,
               the Fund declared a dividend in the amount of $0.25 per share. If
               the Fund's share price was $10.00 on December 30, the Fund's
               share price on December 31 would be $9.75, barring market
               fluctuations. Shareholders should be aware that distributions
               from a taxable mutual fund are not value-enhancing and may create
               income tax obligations.
 
               "BUYING A DIVIDEND"
 
               If you purchase shares of the Fund just before the distribution,
               you will pay the full price for the shares and receive a portion
               of the purchase price back as a taxable distribution. This is
               referred to as "buying a dividend." In the above example, if you
               bought shares on December 30, you would have paid $10.00 per
               share. On December 31, the Fund would pay you $0.25 per share as
               a dividend and your shares would now be worth $9.75 per share.
               Unless your account is set up as a tax-deferred account,
               dividends paid to you would be included in your gross income for
               tax purposes, even though you may not have participated in the
 
 36 Janus Worldwide Fund
<PAGE> 
 
               increase in NAV of the Fund, whether or not you reinvested the
               dividends.
 
DISTRIBUTION OPTIONS
 
               When you open an account, you must specify on your application
               how you want to receive your distributions. You may change your
               distribution option at any time by writing the Fund at one of the
               addresses listed on page 16 or calling 1-800-525-3713. The Fund
               offers the following options:
 
               1. REINVESTMENT OPTION. You may reinvest your income dividends
                  and capital gains distributions in additional shares. This
                  option is assigned automatically if no other choice is made.
 
               2. CASH OPTION. You may receive your income dividends and capital
                  gains distributions in cash.
 
               3. REINVEST AND CASH OPTION. You may receive either your income
                  dividends or capital gains distributions in cash and reinvest
                  the other in additional shares.
 
               4. REDIRECT OPTION. You may direct your dividends or capital
                  gains to purchase shares of another Janus fund.
 
               The Fund reserves the right to reinvest into your account
               undeliverable and uncashed dividend and distribution checks that
               remain outstanding for six months in shares of the Fund at the
               NAV next computed after the check is cancelled. Subsequent
               distributions may also be reinvested.
 
TAXES
 
               As with any investment, you should consider the tax consequences
               of investing in the Fund. Any time you sell or exchange shares of
               a Fund in a taxable account, it is considered a taxable event.
               Depending on the purchase price and the sale price, you may have
               a gain or loss on the transaction. Any tax liabilities generated
               by your transactions are your responsibility.
 
               The following discussion does not apply to tax-deferred accounts,
               nor is it a complete analysis of the federal tax implications of
               investing in the Fund. You may wish to consult your own tax
 
                                                     Distributions and taxes  37
<PAGE> 
 
               adviser. Additionally, state or local taxes may apply to your
               investment, depending upon the laws of your state of residence.
 
               TAXES ON DISTRIBUTIONS
               Dividends and distributions by the Fund are subject to federal
               income tax, regardless of whether the distribution is made in
               cash or reinvested in additional shares of the Fund.
               Distributions may be taxable at different rates depending on the
               length of time the Fund holds a security. In certain states, a
               portion of the dividends and distributions (depending on the
               source of the Fund's income) may be exempt from state and local
               taxes. Information regarding the tax status of income dividends
               and capital gains distributions will be mailed to shareholders on
               or before January 31st of each year.
 
               TAXATION OF THE FUND
               Dividends, interest and some capital gains received by the Fund
               on foreign securities may be subject to tax withholding or other
               foreign taxes. The Fund 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 Fund.
 
               The Fund does not expect to pay federal income or excise taxes
               because it intends to meet certain requirements of the Internal
               Revenue Code. It is important that the Fund meets these
               requirements so that any earnings on your investment will not be
               taxed twice.
 
 38 Janus Worldwide Fund
<PAGE> 
                                                            Financial highlights
 
               The financial highlights table is intended to help you understand
               the Fund's financial performance for the past 5 years through
               October 31st of each fiscal year shown. Items 1 through 9 reflect
               financial results for a single Fund share. The total returns in
               the table represent the rate that an investor would have earned
               (or lost) on an investment in the Fund (assuming reinvestment of
               all dividends and distributions). This information has been
               audited by PricewaterhouseCoopers LLP, whose report, along with
               the Fund's financial statements, are included in the Annual
               Report, which is available upon request and incorporated by
               reference into the SAI.
 
<TABLE>
<CAPTION>
                                 JANUS WORLDWIDE FUND
- ---------------------------------------------------------------------------------------
                                                   Periods ending October 31st
                                            1998      1997      1996     1995     1994
<S>                                        <C>       <C>       <C>      <C>      <C>
  1. NET ASSET VALUE, BEGINNING OF PERIOD   $40.05    $34.60   $27.65   $27.00   $24.16
     INCOME FROM INVESTMENT OPERATIONS:
  2. Net investment income                    1.26    (0.08)     0.49     0.81     0.15
  3. Net gains or (losses) on securities
     (both realized and unrealized)           3.01      7.73     7.79     1.39     3.34
  4. Total from investment operations         4.27      7.65     8.28     2.20     3.49
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment
     income)                                (1.35)    (0.15)   (0.26)   (0.54)   (0.27)
  6. Dividends (in excess of net
     investment income)                         --        --       --       --       --
  7. Distributions (from capital gains)     (1.45)    (2.05)   (1.07)   (1.01)   (0.38)
  8. Total distributions                    (2.80)    (2.20)   (1.33)   (1.55)   (0.65)
  9. NET ASSET VALUE, END OF PERIOD         $41.52    $40.05   $34.60   $27.65   $27.00
 10. Total return                           11.40%    23.34%   31.00%    8.89%   14.76%
 11. Net assets, end of period (in
     millions)                             $13,932   $10,358   $4,467   $1,804   $1,587
 12. Average net assets for the period
     (in millions)                         $13,078    $7,784   $2,953   $1,622   $1,244
 13. Ratio of gross expenses to average
     net assets                              0.92%     0.97%    1.02%    1.24%      N/A
 14. Ratio of net expenses to average net
     assets                                  0.90%     0.95%    1.01%    1.23%    1.12%
 15. Ratio of net investment
     income/(loss) to average net assets     0.47%     0.65%    0.73%    0.99%    0.42%
 16. Portfolio turnover rate                   86%       79%      80%     142%     158%
- ---------------------------------------------------------------------------------------
</TABLE>
 
                                                        Financial highlights  39
<PAGE> 
Glossary of investment terms
 
               This glossary provides a more detailed description of some of the
               types of securities and other instruments in which the Fund may
               invest. The Fund may invest in these instruments to the extent
               permitted by its investment objective and policies. The Fund 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
               Fund may purchase commercial paper issued in private placements
               under Section 4(2) of the Securities Act of 1933.
 
               COMMON STOCKS are equity securities representing shares of
               ownership in a company, and usually carry voting rights and earns
               dividends. Unlike preferred stock, dividends on common stock are
               not fixed but are declared at the discretion of the issuer's
               board of directors.
 
               CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a
               fixed dividend or interest payment and are convertible into
               common stock at a specified price or conversion ratio.
 
               DEBT SECURITIES are equity securities representing money borrowed
               that must be repaid at a later date. Such securities have
               specific maturities and usually a specific rate of interest or an
               original purchase discount.
 
 40 Janus Worldwide Fund
<PAGE> 
 
               DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
               corporation that entitle the holder to dividends and capital
               gains on the underlying security. Receipts include those issued
               by domestic banks (American Depositary Receipts), foreign banks
               (Global or European Depositary Receipts) and broker- dealers
               (depositary shares).
 
               FIXED-INCOME SECURITIES are securities that pay a specified rate
               of return. The term generally includes short- and long-term
               government, corporate and municipal obligations that pay a
               specified rate of interest or coupons for a specified period of
               time and preferred stock, which pays fixed dividends. Coupon and
               dividend rates may be fixed for the life of the issue or, in the
               case of adjustable and floating rate securities, for a shorter
               period.
 
               HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated
               below investment grade by the primary rating agencies (e.g., BB
               or lower by Standard & Poor's and Ba or lower by Moody's). Other
               terms commonly used to describe such securities include "lower
               rated bonds," "noninvestment grade bonds" and "junk bonds."
 
               MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
               mortgages or other debt. These securities are generally pass-
               through securities, which means that principal and interest
               payments on the underlying securities (less servicing fees) are
               passed through to shareholders on a pro rata basis. These
               securities involve prepayment risk, which is the risk that the
               underlying mortgages or other debt may be refinanced or paid off
               prior to their maturities during periods of declining interest
               rates. In that case, the portfolio manager may have to reinvest
               the proceeds from the securities at a lower rate. Potential
               market gains on a security subject to prepayment risk may be more
               limited than potential market gains on a comparable security that
               is not subject to prepayment risk.
 
               PASSIVE FOREIGN INVESTMENT COMPANIES ("PFICs") are any foreign
               corporations which generate amounts of passive income or hold
 
                                                Glossary of investment terms  41
<PAGE> 
 
               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 Fund must pay if
               these investments are profitable, the Fund may make various
               elections permitted by the tax laws. These elections could
               require that the Fund recognize taxable income, which in turn
               must be distributed, before the securities are sold and before
               cash is received to pay the distributions.
 
               PREFERRED STOCKS are equity securities that generally pay
               dividends at a specified rate and have preference over common
               stock in the payment of dividends and liquidation. Preferred
               stock generally does not carry voting rights.
 
               REPURCHASE AGREEMENTS involve the purchase of a security by the
               Fund and a simultaneous agreement by the seller (generally a bank
               or dealer) to repurchase the security from the Fund 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 Fund 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 Fund to another party (generally a bank or dealer) in return
               for cash and an agreement by the Fund to buy the security back at
               a specified price and time. This technique will be used primarily
               to provide cash to satisfy unusually heavy redemption requests,
               or for other temporary or emergency purposes.
 
               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
 
 42 Janus Worldwide Fund
<PAGE> 
 
               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 stocks
               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
               Fund does not earn interest on such securities until settlement
               and bears the risk of market value fluctuations in between the
               purchase and settlement dates. New issues of stocks and bonds,
               private placements and U.S. government securities may be sold in
               this manner.
 
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
 
                                                Glossary of investment terms  43
<PAGE> 
 
               traded and are typically negotiated on an individual basis. The
               Fund may enter into forward currency contracts to hedge against
               declines on the value of securities denominated in, or whose
               value is tied to, a currency other than the U.S. dollar or to
               reduce the impact of currency appreciation on purchases of such
               securities. It may also enter into forward contracts to purchase
               or sell securities or other financial indices.
 
               FUTURES CONTRACTS are contracts that obligate the buyer to
               receive and the seller to deliver an instrument or money at a
               specified price on a specified date. The Fund 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 Fund
               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 Fund bears the
               market risk of an investment in the underlying instruments, as
               well as the credit risk of the issuer.
 
               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 Fund may purchase and
               write put and call options on securities, securities indices and
               foreign currencies.
 
 44 Janus Worldwide Fund
<PAGE> 
 
                       This page intentionally left blank
<PAGE> 
 
[JANUS LOGO]
          1-800-525-3713
          P.O. Box 173375
          Denver, Colorado 80217-3375
          janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting Janus at 1-800-525-3713 or visiting our Web
site at janus.com. In the Fund's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Fund's performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell shares of the Fund.
 
The Statement of Additional Information provides detailed
information about the Fund and is incorporated into this Prospectus
by reference. You may review the Fund's Statement of Additional
Information at the Public Reference Room of the SEC or get text only
copies for a fee, by writing to or calling the Public Reference
Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You may obtain
the Statement of Additional Information for free from the SEC's Web
site at http://www.sec.gov.
 
              Investment Company Act File No. 811-1879
 
  3184
<PAGE> 
 
                                                     [JANUS LOGO]
 
                         Janus Twenty Fund
 
                                       PROSPECTUS
                                       FEBRUARY 17, 1999
 
                         THE SECURITIES AND EXCHANGE COMMISSION HAS NOT APPROVED
                         OR DISAPPROVED OF THESE SECURITIES OR PASSED ON THE
                         ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY
                         REPRESENTATION TO THE CONTRARY IS A CRIMINAL OFFENSE.
<PAGE> 
 
    [JANUS LOGO]
<PAGE> 
 
                                                               Table of contents
 
<TABLE>
                <S>                                               <C>
                RISK/RETURN SUMMARY
                   Janus Twenty Fund............................    2
                   Fees and expenses............................    5
                INVESTMENT OBJECTIVE, PRINCIPAL INVESTMENT
                STRATEGIES AND RISKS
                   Investment objective and principal investment
                   strategies...................................    6
                   General portfolio policies...................    7
                   Risks........................................    9
                SHAREHOLDER'S MANUAL
                   Minimum investments..........................   17
                   Types of account ownership...................   17
                   How to open your Janus account...............   19
                   How to purchase shares.......................   20
                   How to exchange shares.......................   22
                   How to redeem shares.........................   24
                   Shareholder services and account policies....   28
                MANAGEMENT OF THE FUND
                   Investment adviser...........................   33
                   Portfolio manager............................   34
                OTHER INFORMATION............... ...............   35
                DISTRIBUTIONS AND TAXES
                   Distributions................................   36
                   Taxes........................................   37
                FINANCIAL HIGHLIGHTS.............. .............   39
                GLOSSARY
                   Glossary of investment terms.................   40
 
</TABLE>
 
                                                            Table of contents  1
<PAGE> 
Risk return summary
 
JANUS TWENTY FUND
 
1. WHAT IS THE INVESTMENT OBJECTIVE OF JANUS TWENTY FUND?
 
               The Fund seeks long-term growth of capital.
 
               The Fund's Trustees may change this objective without a
               shareholder vote and the Fund will notify you of any changes that
               are material. If there is a material change in the Fund's
               objective or policies, you should consider whether the Fund
               remains an appropriate investment for you. There is no guarantee
               that the Fund will meet its objective.
 
2. WHAT ARE THE MAIN INVESTMENT STRATEGIES OF JANUS TWENTY FUND?
 
               The Fund invests primarily in common stocks selected for their
               growth potential. The Fund normally concentrates its investments
               in a core group of 20-30 common stocks.
 
               The portfolio manager applies a "bottom up" approach in choosing
               investments. In other words, he looks for companies with earnings
               growth potential one at a time. If the portfolio manager is
               unable to find investments with earnings growth potential, a
               significant portion of the Fund's assets may be in cash or
               similar investments.
 
3. WHAT ARE THE MAIN RISKS OF INVESTING IN JANUS TWENTY FUND?
 
               The biggest risk of investing in this Fund is that its returns
               may vary and you could lose money. If you are considering
               investing in the Fund, remember that it is designed for long-term
               investors who can accept the risks of investing in a portfolio
               with significant common stock holdings. Common stocks tend to be
               more volatile than other investment choices.
 
               The value of the Fund's portfolio may decrease if the value of an
               individual company in the portfolio decreases. The value of the
               Fund's portfolio could also decrease if the stock market goes
               down. If the value of the Fund's portfolio decreases, the Fund's
               net asset value (NAV) will also decrease which means if you sell
               your shares in the Fund you would get back less money.
 
 2 Janus Twenty Fund
<PAGE> 
 
               The Fund is nondiversified. In other words, it may hold larger
               positions in a smaller number of securities than a diversified
               fund. As a result, a single security's increase or decrease in
               value may have a greater impact on the Fund's NAV and total
               return. Since the Fund normally concentrates in a core portfolio
               of 20-30 stocks, this risk may be increased.
 
               An investment in the Fund is not a bank deposit and is not
               insured or guaranteed by the Federal Deposit Insurance
               Corporation or any other government agency.
 
                                                          Risk return summary  3
<PAGE> 
 
               The following information illustrates how the Fund's performance
               has varied over time. The bar chart depicts the change in the
               Fund's performance from year-to-year during the periods
               indicated. The table compares the Fund's average annual returns
               for the periods indicated to a broad-based securities market
               index.
 
                JANUS TWENTY FUND

A BAR CHART showing Annual Total Returns for Janus Twenty Fund from 1989 through
1998:
 
                Annual returns for periods ended 12/31

<TABLE>
                  <S>     <C>    <C>     <C>    <C>    <C>      <C>     <C>     <C>     <C>
                  23.93%  0.59%  69.21%  1.97%  3.43%  (6.73%)  36.22%  27.85%  29.70%  73.39%
                   1989   1990    1991   1992   1993    1994     1995    1996    1997    1998
</TABLE>

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

                Best Quarter: 4th-1998 26.67%; Worst Quarter: 3rd-1990  (17.71%)

<TABLE>
<CAPTION>
                          Average annual total return for periods ended 12/31/98
                          ------------------------------------------------------
                                                                                Since Inception
                                                 1 year   5 years   10 years       (4/30/85)
                <S>                              <C>      <C>       <C>         <C>                
                Janus Twenty Fund                73.39%   29.58%     25.76%         20.90%
                S&P 500 Index*                   28.74%   24.08%     19.20%         18.51%
                                             --------------------------------------------- 
</TABLE>
 
                * The S&P 500 is the Standard & Poor's Composite Index of 500
                  Stocks, a widely recognized, unmanaged index of common stock
                  prices.
 
               The Fund's past performance does not necessarily indicate how it
               will perform in the future.
 
 4 Janus Twenty Fund
<PAGE> 
 
FEES AND EXPENSES
 
               SHAREHOLDER FEES, such as sales loads, redemption fees or
               exchange fees, are charged directly to an investor's account. All
               Janus funds are no-load investments, so you will not pay any
               shareholder fees when you buy or sell shares of the Fund.
 
               ANNUAL FUND OPERATING EXPENSES are paid out of the Fund's assets
               and include fees for portfolio management, maintenance of
               shareholder accounts, shareholder servicing, accounting and other
               services. You do not pay these fees directly but, as the example
               below shows, these costs are borne indirectly by all
               shareholders.
 
This table describes the fees and expenses that you may pay if you buy and hold
shares of the Fund. It is based upon gross expenses (without the effect of
expense offset arrangements) for the fiscal year ended October 31, 1998.
 
<TABLE>
<CAPTION>
                                                             Janus Twenty Fund
   <S>                                                       <C>              
   Management Fee                                                  0.65%
   Other Expenses                                                  0.26%
   Total Annual Fund Operating Expenses                            0.91%
</TABLE>
 
- --------------------------------------------------------------------------------
 
  EXAMPLE:
  This example is intended to help you compare the cost of investing in the
  Fund with the cost of investing in other mutual funds. The example
  assumes that you invest $10,000 in the Fund for the time periods
  indicated then redeem all of your shares at the end of those periods. The
  example also assumes that your investment has a 5% return each year and
  that the Fund's operating expenses remain the same. Although your actual
  costs may be higher or lower, based on these assumptions your costs would
  be:
 
<TABLE>
<CAPTION>
                                               1 Year     3 Years    5 Years    10 Years
   <S>                                         <C>        <C>        <C>        <C>     
                                               -----------------------------------------
   Janus Twenty Fund                            $ 93       $290       $504       $1,120
</TABLE>
 
                                                          Risk return summary  5
<PAGE> 
 
Investment objective, principal
           investment strategies
           and risks
 
               This section takes a closer look at the investment objective of
               the Fund, its principal investment strategies and certain risks
               of investing in the Fund. Strategies and policies that are noted
               as "fundamental" cannot be changed without a shareholder vote.
 
               Please carefully review the "Risks" section of this Prospectus on
               pages 9-12 for a discussion of risks associated with certain
               investment techniques. We've also included a Glossary with
               descriptions of investment terms used throughout this Prospectus.
 
INVESTMENT OBJECTIVE AND PRINCIPAL INVESTMENT STRATEGIES
 
               Janus Twenty Fund seeks long-term growth of capital. It pursues
               its objective by investing primarily in common stocks selected
               for their growth potential. The Fund normally concentrates its
               investments in a core group of 20-30 common stocks.
 
               The Fund may invest substantially all of its assets in common
               stocks if its portfolio manager believes that common stocks will
               appreciate in value. The portfolio manager generally takes a
               "bottom up" approach to selecting companies. In other words, he
               seeks to identify individual companies with earnings growth
               potential that may not be recognized by the market at large. He
               makes this assessment by looking at companies one at a time,
               regardless of size, country of organization, place of principal
               business activity, or other similar selection criteria.
               Realization of income is not a significant consideration when
               choosing investments for the Fund. Income realized on the Fund's
               investments will be incidental to its objective.
 
               Foreign securities are generally selected on a stock-by-stock
               basis without regard to any defined allocation among countries or
               geographic regions. However, certain factors such as expected
               levels of inflation, government policies influencing business
               conditions, the outlook for currency relationships, and prospects
               for economic growth among countries, regions or geographic areas
               may warrant greater consideration in selecting foreign
               securities. There are no limitations on the countries in which
               the Fund
 
 6 Janus Twenty Fund
<PAGE> 
 
               may invest and the Fund may at times have significant foreign
               exposure.
 
GENERAL PORTFOLIO POLICIES
 
               In investing its portfolio assets, the Fund 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. So, for example, if
               the Fund exceeds a limit as a result of market fluctuations or
               the sale of securities, it will not be required to dispose of any
               securities.
 
               CASH POSITION
               When the Fund's portfolio manager believes that market conditions
               are unfavorable for profitable investing, or when he is otherwise
               unable to locate attractive investment opportunities, the Fund's
               cash or similar investments may increase. In other words, the
               Fund does not always stay fully invested in stocks. Cash or
               similar investments generally are a residual - they represent the
               assets that remain after the portfolio manager has committed
               available assets to desirable investment opportunities. However,
               the portfolio manager may also temporarily increase the Fund's
               cash position to protect its assets or maintain liquidity. When
               the Fund's investments in cash or similar investments increase,
               it may not participate in market advances or declines to the same
               extent that it would if the Fund remained more fully invested in
               stocks.
 
               OTHER TYPES OF INVESTMENTS
               The Fund invests primarily in domestic and foreign equity
               securities, which may include preferred stocks, common stocks,
               warrants and securities convertible into common or preferred
               stocks, but it may also invest to a lesser degree in other types
               of securities. These securities (which are described in the
               Glossary) may include:
 
               - debt securities
 
               - indexed/structured securities
 
              Investment objective, principal investment strategies and risks  7
<PAGE> 
 
               - high-yield/high-risk securities (less than 35% of the Fund's
                 assets)
 
               - options, futures, forwards and other types of derivatives for
                 hedging purposes or for non-hedging purposes such as seeking to
                 enhance return
 
               - securities purchased on a when-issued, delayed delivery or
                 forward commitment basis
 
               ILLIQUID INVESTMENTS
               The Fund may invest up to 15% of its net assets in illiquid
               investments. An illiquid investment is a security or other
               position that cannot be disposed of quickly in the normal course
               of business. For example, some securities are not registered
               under the U.S. securities laws and cannot be sold to the U.S.
               public because of SEC regulations (these are known as "restricted
               securities"). Under procedures adopted by the Fund's Trustees,
               certain restricted securities may be deemed liquid, and will not
               be counted toward this 15% limit.
 
               FOREIGN SECURITIES
               The Fund may invest without limit in foreign equity and debt
               securities. The Fund may invest directly in foreign securities
               denominated in a foreign currency and not publicly traded in the
               United States. Other ways of investing in foreign securities
               include depositary receipts or shares, and passive foreign
               investment companies.
 
               SPECIAL SITUATIONS
               The Fund may invest in special situations. A special situation
               arises when, in the opinion of the Fund'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. The
 
 8 Janus Twenty Fund
<PAGE> 
 
               Fund's performance could suffer if the anticipated development in
               a "special situation" investment does not occur or does not
               attract the expected attention.
 
               PORTFOLIO TURNOVER
               The Fund generally intends to purchase securities for long-term
               investment although, to a limited extent, the Fund may purchase
               securities in anticipation of relatively short-term price gains.
               Short-term transactions may also result from liquidity needs,
               securities having reached a price or yield objective, changes in
               interest rates or the credit standing of an issuer, or by reason
               of economic or other developments not foreseen at the time of the
               investment decision. The Fund may also sell one security and
               simultaneously purchase the same or a comparable security to take
               advantage of short-term differentials in bond yields or
               securities prices. Changes are made in the Fund's portfolio
               whenever its portfolio manager believes such changes are
               desirable. Portfolio turnover rates are generally not a factor in
               making buy and sell decisions.
 
               Increased portfolio turnover may result in higher costs for
               brokerage commissions, dealer mark-ups and other transaction
               costs and may also result in taxable capital gains. Higher costs
               associated with increased portfolio turnover may offset gains in
               the Fund's performance.
 
RISKS
 
               Because the Fund may invest substantially all of its assets in
               common stocks, the main risk is the risk that the value of the
               stocks it holds might decrease in response to the activities of
               an individual company or in response to general market and/or
               economic conditions. If this occurs, the Fund's share price may
               also decrease. The Fund's performance may also be affected by
               risks specific to certain types of investments, such as foreign
               securities, derivative investments, non-investment grade debt
               securities or companies with relatively small market
               capitalizations.
 
              Investment objective, principal investment strategies and risks  9
<PAGE> 
 
The following questions and answers are designed to help you better understand
some of the risks of investing in the Fund.
 
1. THE FUND MAY INVEST IN SMALLER OR NEWER COMPANIES. DOES THIS CREATE ANY
   SPECIAL RISKS?
 
               Many attractive investment opportunities may be smaller, start-up
               companies offering emerging products or services. Smaller or
               newer companies may suffer more significant losses as well as
               realize more substantial growth than larger or more established
               issuers because they may lack depth of management, be unable to
               generate funds necessary for growth or potential development, or
               be developing or marketing new products or services for which
               markets are not yet established and may never become established.
               In addition, such companies may be insignificant factors in their
               industries and may become subject to intense competition from
               larger or more established companies. Securities of smaller or
               newer companies may have more limited trading markets than the
               markets for securities of larger or more established issuers, and
               may be subject to wide price fluctuations. Investments in such
               companies tend to be more volatile and somewhat more speculative.
 
2. HOW DOES THE NONDIVERSIFIED STATUS OF THE FUND AFFECT ITS RISK?
 
               Diversification is a way to reduce risk by investing in a broad
               range of stocks or other securities. A "nondiversified" fund has
               the ability to take larger positions in a smaller number of
               issuers. Because the appreciation or depreciation of a single
               stock may have a greater impact on the NAV of a nondiversified
               fund, its share price can be expected to fluctuate more than a
               comparable diversified fund. This fluctuation, if significant,
               may affect the performance of the Fund.
 
3. I'VE HEARD A LOT ABOUT HOW THE CHANGE TO THE YEAR 2000 COULD AFFECT COMPUTER
   SYSTEMS. DOES THIS CREATE ANY SPECIAL RISKS?
 
               The portfolio manager carefully researches each potential
               investment before making an investment decision and, among other
               things, considers Year 2000 readiness when selecting portfolio
 
 10 Janus Twenty Fund
<PAGE> 
 
               holdings. However, there is no guarantee that the information the
               portfolio manager receives regarding a company's Year 2000
               readiness is completely accurate. If a company has not
               satisfactorily addressed Year 2000 issues, the Fund's performance
               could suffer.
 
4. HOW COULD THE FUND'S INVESTMENTS IN FOREIGN SECURITIES AFFECT ITS
   PERFORMANCE?
 
               The Fund may invest without limit in foreign securities either
               indirectly (e.g., depositary receipts) or directly in foreign
               markets. Investments in foreign securities, including those of
               foreign governments, may involve greater risks than investing in
               domestic securities because the Fund's performance may depend on
               issues other than the performance of a particular company. These
               issues include:
 
               - currency risk
 
               - political and economic risk
 
               - regulatory risk
 
               - market risk
 
               - transaction costs
 
               These risks are described in the SAI.
 
5. ARE THERE SPECIAL RISKS ASSOCIATED WITH INVESTMENTS IN HIGH-YIELD/HIGH-RISK
   SECURITIES?
 
               High-yield/high-risk securities (or "junk" bonds) are securities
               rated below investment grade by the primary rating agencies such
               as Standard & Poor's and Moody's. The value of lower quality
               securities generally is more dependent on credit risk, or the
               ability of the issuer to meet interest and principal payments,
               than investment grade debt securities. Issuers of high-yield
               securities may not be as strong financially as those issuing
               bonds with higher credit ratings and are more vulnerable to real
               or perceived economic changes, political changes or adverse
               developments specific to the issuer.
 
             Investment objective, principal investment strategies and risks  11
<PAGE> 
 
               Please refer to the SAI for a description of bond rating
               categories.
 
6. HOW DOES THE FUND TRY TO REDUCE RISK?
 
               The Fund may use futures, options and other derivative
               instruments to "hedge" or protect its portfolio from adverse
               movements in securities prices and interest rates. The Fund may
               also use a variety of currency hedging techniques, including
               forward currency contracts, to manage exchange rate risk. The
               Fund believes the use of these instruments will benefit the Fund.
               However, the Fund's performance could be worse than if the Fund
               had not used such instruments if the portfolio manager's
               judgement proves incorrect. Risks associated with the use of
               derivative instruments are described in the SAI.
 
 12 Janus Twenty Fund
<PAGE> 
 
                       This page intentionally left blank
 
                                                                              13
<PAGE> 
 
Janus Twenty Fund
<PAGE> 
 
                   Shareholder's
                   Manual
 
                   This section will help you become
                   familiar with the different types
                   of accounts you can establish with
                   Janus. It also explains in detail
                   the wide array of services and
                   features you can establish on your
                   account, as well as account
                   policies and fees that may apply
                   to your account. Account policies
                   (including fees), services and
                   features may be modified or
                   discontinued without shareholder
                   approval or prior notice.
 
                                                    [JANUS LOGO]
<PAGE> 
 
HOW TO GET IN TOUCH WITH JANUS
 
               Janus offers two Investor Service Centers for those individuals
               who would like to conduct their investing in person. Our
               representatives will be happy to assist you at either of the
               following locations Monday-Friday 7:00 a.m. to 6:00 p.m. Mountain
               time and Saturday 9:00 a.m. to 1:00 p.m. Mountain time.
 
               100 Fillmore Street, Suite 100
               Denver, CO 80206
 
               3773 Cherry Creek North Drive, Suite 101
               Denver, CO 80209
 
 QUICK ADDRESS AND TELEPHONE REFERENCE
- --------------------------------------------------------------------------------
 
<TABLE>
  <S>                                           <C>
  MAILING ADDRESS                               JANUS XPRESSLINE(TM)
  Janus                                         1-888-979-7737
  P.O. Box 173375                               For 24-hour access to account
  Denver, CO 80217-3375                         and fund information,
                                                exchanges, purchases and
  FOR OVERNIGHT CARRIER                         redemptions, automated daily
  Janus                                         quotes on fund share prices,
  Suite 101                                     yields and total returns.
  3773 Cherry Creek North Drive
  Denver, CO 80209-3811                         TDD
                                                1-800-525-0056
  INVESTOR SERVICE REPRESENTATIVES              A telecommunications device
  If you have any questions while reading       for our hearing- and
  this Prospectus, please call one of our       speech-impaired shareholders.
  Investor Service Representatives at
  1-800-525-3713 Monday-Friday: 8:00            JANUS LITERATURE LINE
  a.m.-8:00 p.m., and Saturday: 10:00           1-800-525-8983
  a.m.-4:00 p.m., New York time.                To request a prospectus,
                                                shareholder reports or
  JANUS INTERNET ADDRESS                        marketing materials 24 hours a
  janus.com                                     day.
</TABLE>
 
 16 Janus Twenty Fund
<PAGE> 
 
MINIMUM INVESTMENTS*
- ---------------------------------------------
 
<TABLE>
<S>                                 <C>
To open a new regular account        $2,500

To open a new retirement,
education or UGMA/UTMA account       $  500

To open a new regular account with
an Automatic Investment Program      $  500**

To add to any type of an account     $  100+
</TABLE>
 
 * The Fund reserves the right to
   change the amount of these
   minimums from time to time or
   to waive them in whole or in
   part for certain types of
   accounts.
** An Automatic Investment Program
   requires a $100 minimum
   automatic investment per month
   until the account balance
   reaches $2,500.
 + The minimum subsequent
   investment for IRA UGMA/UTMA
   accounts is $50.
 
TYPES OF ACCOUNT OWNERSHIP
 
               If you are investing in the Fund for the first time, you will
               need to establish an account. You can establish the following
               types of accounts by completing a New Account Application. To
               request an application, call 1-800-525-3713 or visit our Web site
               at janus.com to download an application.
 
               INDIVIDUAL OR JOINT OWNERSHIP
               Individual accounts are owned by one person. Joint accounts have
               two or more owners.
 
               A GIFT OR TRANSFER TO MINOR (UGMA OR UTMA)
               An UGMA/ UTMA account is a custodial account managed for the
               benefit of a minor. To open an UGMA or UTMA account, you must
               include the minor's Social Security number on the application.
 
               TRUST
               An established trust can open an account. The names of each
               trustee, the name of the trust and the date of the trust
               agreement must be included on the application.
 
                                                        Shareholder's manual  17
<PAGE> 
 
               BUSINESS ACCOUNTS
               Corporations and partnerships may also open an account. The
               application must be signed by an authorized officer of the
               corporation or a general partner of the partnership.
 
TAX-DEFERRED ACCOUNTS
 
               If you are eligible, you may set up one or more tax-deferred
               accounts. A tax-deferred account allows you to shelter your
               investment income and capital gains from current income taxes. A
               contribution to certain of these plans may also be tax
               deductible. Tax-deferred accounts include retirement plans
               described below and the Education IRA. Distributions from these
               plans are generally subject to income tax and may be subject to
               an additional tax if withdrawn prior to age 59 1/2 or used for a
               nonqualifying purpose. Investors should consult their tax adviser
               or legal counsel before selecting a tax-deferred account.
 
               Investors Fiduciary Trust Company serves as custodian for the
               tax-deferred accounts offered by the Fund. You will be charged an
               annual account maintenance fee of $12 for each taxpayer
               identification number no matter how many tax-deferred accounts
               you have with Janus. You may pay the fee by check or have it
               automatically deducted from your account (usually in December).
               The custodian reserves the right to change the amount of this fee
               or to waive it in whole or in part for certain types of accounts.
 
               The following plans require a special application. For an
               application and more details about our Retirement Plans, call
               1-800-525-3713.
 
               TRADITIONAL AND ROTH INDIVIDUAL RETIREMENT ACCOUNTS
               Both types of IRAs allow most individuals with earned income to
               contribute up to the lesser of $2,000 ($4,000 for most married
               couples) or 100% of compensation annually. Please refer to the
               Janus IRA booklet for more complete information regarding the
               different types of IRAs.
 
 18 Janus Twenty Fund
<PAGE> 
 
               EDUCATION IRA
               This plan allows individuals, subject to certain income
               limitations, to contribute up to $500 annually on behalf of any
               child under the age of 18. Please refer to the Janus IRA booklet
               for more complete information regarding the Education IRA.
 
               SIMPLIFIED EMPLOYEE PENSION PLAN
               This plan allows small business owners (including sole
               proprietors) to make tax-deductible contributions for themselves
               and any eligible employee(s). A SEP requires an IRA (a SEP-IRA)
               to be set up for each SEP participant.
 
               PROFIT SHARING OR MONEY PURCHASE PENSION PLAN
               These plans are open to corporations, partnerships and sole
               proprietors to benefit their employees and themselves.
 
               SECTION 403(b)(7) PLAN
               Employees of educational organizations or other qualifying, tax-
               exempt organizations may be eligible to participate in a Section
               403(b)(7) Plan.
 
HOW TO OPEN YOUR JANUS ACCOUNT
 
               Complete and sign the appropriate application. Please be sure to
               provide your Social Security or taxpayer identification number on
               the application and make your check payable to Janus. The Fund is
               available only to U.S. citizens or residents, and your
               application will be returned to you if you do not meet these
               criteria. Send all items to one of addresses listed in the "Quick
               Address and Telephone Reference" on page 16.
 
                                                        Shareholder's manual  19
<PAGE> 
 
HOW TO PURCHASE SHARES
 
               PAYING FOR SHARES
 
               When you purchase shares, your request will be processed at the
               next NAV calculated after your order is received and accepted.
               Please note the following:
 
               - Cash, credit cards, third party checks and credit card checks
                 will not be accepted.
 
               - All purchases must be made in U.S. dollars.
 
               - Checks must be drawn on a U.S. bank and made payable to Janus.
 
               - If a check does not clear your bank, the Fund reserves the
                 right to cancel the purchase.
 
               - If the Fund is unable to debit your predesignated bank account
                 on the day of purchase, it may make additional attempts or
                 cancel the purchase.
 
               - The Fund reserves the right to reject any specific purchase
                 request.
 
               If your purchase is cancelled you will be responsible for any
               losses or fees imposed by your bank and losses that may be
               incurred as a result of any decline in the value of the cancelled
               purchase. The Fund (or its agents) has the authority to redeem
               shares in your account(s) to cover any such losses due to
               fluctuations in share price. Any profit on such cancellation will
               accrue to the Fund.
 
               ONCE YOU HAVE OPENED YOUR JANUS ACCOUNT, THE MINIMUM AMOUNT FOR
               AN ADDITIONAL INVESTMENT IS $100 ($50 FOR IRAs OR UGMA/UTMA
               ACCOUNTS). You may add to your account at any time through any of
               the following options:
 
               BY MAIL
 
               Complete the remittance slip attached at the bottom of your
               confirmation statement. If you are making a purchase into a
               retirement account, please indicate whether the purchase is a
 
 20 Janus Twenty Fund
<PAGE> 
 
               rollover or a current or prior year contribution. Send your check
               made payable to Janus and remittance slip or written instructions
               to one of the addresses listed previously. You may also request a
               booklet of remittance slips for non-retirement accounts.
 
               BY TELEPHONE
 
               This service allows you to purchase additional shares quickly and
               conveniently through an electronic transfer of money. To purchase
               shares by telephone, call an Investor Service Representative at
               1-800-525-3713 during normal business hours or call the Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. When you make an additional purchase by telephone, Janus
               will automatically debit your predesignated bank account for the
               desired amount. To establish the telephone purchase option on
               your new account, complete the "Telephone Purchase of Shares
               Option" section on the application and attach a "voided" check or
               deposit slip from your bank account. If your account is already
               established, call 1-800-525-3713 to request the appropriate form.
               This option will become effective ten business days after the
               form is received.
 
               BY WIRE
 
               Purchases may also be made by wiring money from your bank account
               to your Janus account. Call 1-800-525-3713 to receive wiring
               instructions.
 
               BY INTERNET
 
               You must pre-establish the "Telephone Purchase of Shares Option"
               to make a purchase on our Web site at janus.com. If you have
               questions, please call 1-800-975-9932 to speak to a Janus
               representative.
 
               AUTOMATIC INVESTMENT PROGRAMS
 
               Janus offers several automatic investment programs to help you
               achieve your financial goals as simply and conveniently as
 
                                                        Shareholder's manual  21
<PAGE> 
 
               possible. You may open a new account with a $500 initial purchase
               and $100 automatic subsequent investments.
 
               AUTOMATIC MONTHLY INVESTMENT PROGRAM
               You select the day each month that your money ($100 minimum) will
               be electronically transferred from your bank account to your Fund
               account. To establish this option, complete the "Automatic
               Monthly Investment Program" section on the application and attach
               a "voided" check from your bank account. If your Fund account is
               already established, call 1-800-525-3713 to request the
               appropriate form.
 
               PAYROLL DEDUCTION
               If your employer can initiate an automatic payroll deduction, you
               may have all or a portion of your paycheck ($100 minimum)
               invested directly into your Fund account. To obtain information
               on establishing this option, call 1-800-525-3713.
 
               SYSTEMATIC EXCHANGE
               With a Systematic Exchange you determine the amount of money
               ($100 minimum) you would like automatically exchanged from one
               Janus account to another on any day of the month. For more
               information on how to establish this option, call 1-800-525-3713.
 
HOW TO EXCHANGE SHARES
 
               On any business day, you may exchange all or a portion of your
               shares into any other available Janus fund.
 
               IN WRITING
 
               To request an exchange in writing, please follow the instructions
               for written requests on page 26.
 
               BY TELEPHONE
 
               All accounts are automatically eligible for the telephone
               exchange option. To exchange shares by telephone, call an
               Investor Service Representative at 1-800-525-3713 during normal
               business hours
 
 22 Janus Twenty Fund
<PAGE> 
 
               or call the Janus XpressLine, 1-888-979-7737, for access to this
               option 24 hours a day.
 
               BY SYSTEMATIC EXCHANGE
 
               As noted above, you may establish a Systematic Exchange for as
               little as $100 per month on established accounts. You may
               establish a new account with a $500 initial purchase and
               subsequent $100 systematic exchanges. If the balance in the
               account you are exchanging from falls below the systematic
               exchange amount, all remaining shares will be exchanged and the
               program will be discontinued.
 
               BY INTERNET
 
               Exchanges may also be made on our Web site at janus.com.
 
               EXCHANGE POLICIES
 
               - Except for Systematic Exchanges, new accounts established by
                 exchange must be opened with $2,500 or the total account value
                 if the value of the account you are exchanging from is less
                 than $2,500.
 
               - Exchanges between existing accounts must meet the $100
                 subsequent investment requirement.
 
               - You may make four exchanges out of the Fund during a calendar
                 year (exclusive of Systematic Exchanges). Exchanges in excess
                 of this limit may be subject to an exchange fee or may result
                 in termination of the exchange privilege.
 
               - The Fund reserves the right to reject any exchange request and
                 to modify or terminate the exchange privilege at any time. For
                 example, the Fund may reject exchanges from accounts engaged in
                 or known to engage in trading in excess of the limit above
                 (including market timing transactions).
 
               - Exchanges between accounts will be accepted only if the
                 registrations are identical.
 
                                                        Shareholder's manual  23
<PAGE> 
 
               - If the shares you are exchanging are held in certificate form,
                 you must return the certificate to the Fund prior to making any
                 exchanges.
 
               - Be sure to read the prospectus for the fund into which you are
                 exchanging.
 
               - An exchange represents the sale of shares from one fund and the
                 purchase of shares of another fund, which may produce a taxable
                 gain or loss in a non-tax deferred account.
 
HOW TO REDEEM SHARES
 
               On any business day, you may redeem all or a portion of your
               shares. If the shares are held in certificate form, the
               certificate must be returned with or before your redemption
               request. Your transaction will be processed at the next NAV
               calculated after your order is received and accepted. The
               redemption may be suspended for 10 days following an address
               change unless a signature guarantee is provided.
 
               IN WRITING
 
               To request a redemption in writing, please follow the
               instructions for written requests noted on page 26.
 
               BY TELEPHONE
 
               Most accounts have the telephone redemption option, unless this
               option was specifically declined on the application or in
               writing. This option enables you to request redemptions daily
               from your account by calling 1-800-525-3713 by the close of the
               regular trading session of the New York Stock Exchange ("NYSE")
               normally 4:00 p.m. New York time. You may also use Janus
               XpressLine, 1-888-979-7737, for access to this option 24 hours a
               day. (There is a daily limit of $100,000 per account for
               redemptions payable by check.)
 
 24 Janus Twenty Fund
<PAGE> 
 
               BY INTERNET
 
               Redemptions may also be made on our Web site at janus.com.
 
               SYSTEMATIC REDEMPTION OPTION
 
               The Systematic Redemption Option allows you to redeem a specific
               dollar amount from your Fund account on a regular basis. For more
               information or to request the appropriate form, please call
               1-800-525-3713.
 
PAYMENT OF REDEMPTION PROCEEDS
 
               BY CHECK
               Redemption proceeds will be sent to the shareholder(s) of record
               at the address of record within seven days after receipt of a
               valid redemption request.
 
               BY ELECTRONIC TRANSFER
               If you have established the electronic redemption option, your
               redemption proceeds can be electronically transferred to your
               predesignated bank account on the next bank business day after
               receipt of your redemption request (wire transfer) or the second
               bank business day after receipt of your redemption request (ACH
               transfer). Wire transfers will be charged an $8 fee per wire and
               your bank may charge an additional fee to receive the wire. ACH
               transfers are made free of charge. Wire redemptions are not
               available for retirement accounts.
 
               If you would like to establish the electronic redemption option
               on an existing account, please call 1-800-525-3713 to request the
               appropriate form.
 
               IF THE SHARES BEING REDEEMED WERE PURCHASED BY CHECK, TELEPHONE,
               ON OUR WEB SITE, OR THROUGH THE AUTOMATIC MONTHLY INVESTMENT
               PROGRAM, THE FUND MAY DELAY THE PAYMENT OF YOUR REDEMPTION
               PROCEEDS FOR UP TO 15 DAYS FROM THE DAY OF PURCHASE TO ALLOW THE
               PURCHASE TO CLEAR. Unless you provide alternate instructions,
               your proceeds will be invested in Janus
 
                                                        Shareholder's manual  25
<PAGE> 
 
               Money Market Fund - Investor Shares during the 15 day hold
               period.
 
WRITTEN INSTRUCTIONS
 
               To redeem all or part of your shares in writing, your request
               should be sent to one of the addresses listed on page 16 and must
               include the following information:
 
               - the name of the Fund
 
               - the account number
 
               - the amount of money or number of shares being redeemed or
                 exchanged
 
               - the name(s) on the account registration
 
               - the signature(s) of all registered account owners
 
               - your daytime telephone number
 
               SIGNATURE REQUIREMENTS BASED ON ACCOUNT TYPE
 
               INDIVIDUAL, JOINT TENANTS, TENANTS IN COMMON
               Written instructions must be signed by each shareholder, exactly
               as the names appear in the account registration.
 
               UGMA OR UTMA
               Written instructions must be signed by the custodian in his/her
               capacity as it appears in the account registration.
 
               SOLE PROPRIETOR, GENERAL PARTNER
               Written instructions must be signed by an authorized individual
               in his/her capacity as it appears on the account registration.
 
               CORPORATION, ASSOCIATION
               Written instructions must be signed by the person(s) authorized
               to act on the account. In addition, a certified copy of the
               corporate resolution authorizing the signer to act must accompany
               the request.
 
 26 Janus Twenty Fund
<PAGE> 
 
               TRUST
               Written instructions must be signed by the trustee(s). If the
               name(s) of the current trustee(s) does not appear in the account
               registration, a certificate of incumbency dated within 60 days
               must also be submitted.
 
               IRA
               Written instructions must be signed by the account owner. If you
               do not want federal income tax withheld from your redemption, you
               must state that you elect not to have such withholding apply. In
               addition, your instructions must state whether the distribution
               is normal (after age 59 1/2) or premature (before age 59 1/2)
               and, if premature, whether any exceptions such as death or
               disability apply with regard to the 10% additional tax on early
               distributions.
 
SIGNATURE GUARANTEE
 
               In addition to the signature requirements, A SIGNATURE GUARANTEE
               IS ALSO REQUIRED if any of the following is applicable:
 
               - You request a redemption by check that exceeds $100,000.
 
               - You would like the check made payable to anyone other than the
                 shareholder(s) of record.
 
               - You would like the check mailed to an address which has been
                 changed within 10 days of the redemption request.
 
               - You would like the check mailed to an address other than the
                 address of record.
 
               THE FUND RESERVES THE RIGHT TO REQUIRE A SIGNATURE GUARANTEE
               UNDER OTHER CIRCUMSTANCES OR TO REJECT OR DELAY A REDEMPTION ON
               CERTAIN LEGAL GROUNDS. FOR MORE INFORMATION PERTAINING TO
               SIGNATURE GUARANTEES, PLEASE CALL 1-800-525-3713.
 
               HOW TO OBTAIN A SIGNATURE GUARANTEE
 
               A signature guarantee assures that a signature is genuine. The
               signature guarantee protects shareholders from unauthorized
               account transfers. The following financial institutions may
               guaran-
 
                                                        Shareholder's manual  27
<PAGE> 
 
               tee signatures: banks, savings and loan associations, trust
               companies, credit unions, broker-dealers, and member firms of a
               national securities exchange. Call your financial institution to
               see if they have the ability to guarantee a signature. A
               signature guarantee cannot be provided by a notary public.
 
               If you live outside the United States, a foreign bank properly
               authorized to do business in your country of residence or a U.S.
               consulate may be able to authenticate your signature.
 
PRICING OF FUND SHARES
 
               All purchases, redemptions and exchanges will be processed at the
               NAV next calculated after your request is received and accepted
               by the Fund (or the Fund's agent or authorized designee). The
               Fund's NAV is calculated at the close of the regular trading
               session of the NYSE (normally 4:00 p.m. New York time) each day
               that the NYSE is open. The NAV of Fund shares is not determined
               on days the NYSE is closed (generally, New Year's Day, Martin
               Luther King Day, Presidents' Day, Good Friday, Memorial Day,
               Independence Day, Labor Day, Thanksgiving and Christmas). In
               order to receive a day's price, your order must be received by
               the close of the regular trading session of the NYSE. Securities
               are valued at market value or, if a market quotation is not
               readily available, at their fair value determined in good faith
               under procedures established by and under the supervision of the
               Trustees. Short-term instruments maturing within 60 days are
               valued at amortized cost, which approximates market value. See
               the SAI for more detailed information.
 
SHAREHOLDER SERVICES AND ACCOUNT POLICIES
 
               JANUS XPRESSLINE(TM)
 
               Janus XpressLine, our electronic telephone service, offers you
               24-hour access by TouchTone(TM) telephone to obtain information
               on account balances, Fund performance or dividends. You can also
               make exchanges, purchases and redemptions in existing accounts,
               request literature about any Janus fund, or order duplicate
 
 28 Janus Twenty Fund
<PAGE> 
 
               statements. Janus XpressLine is accessed by calling
               1-888-979-7737. Calls are limited to five minutes.
 
               JANUS WEB SITE
 
               Janus maintains a Web site located at janus.com. You can
               purchase, exchange and redeem shares and access information such
               as your account balance and the Fund's NAV through the Web site.
               In order to engage in transactions on our Web site, you must
               authorize us to transmit account information online and accept
               online instructions (see janus.com and follow the procedures
               accordingly). You may also need to have bank account information,
               wire instructions or other options established on your account.
               The Fund and its agents will not be responsible for any losses
               resulting from unauthorized transactions on our Web site when
               procedures designed for engaging in such transactions are
               followed. If you have questions, please call 1-800-975-9932 to
               speak to a Janus representative.
 
               ACCOUNT MINIMUMS
 
               Due to the proportionately higher costs of maintaining small
               accounts, Janus reserves the right to deduct a $10 minimum
               balance fee (or the value of the account if less than $10) from
               accounts with values below the minimums described on page 17 or
               to close such accounts. This policy will apply to accounts
               participating in the Automatic Monthly Investment Program only if
               your account balance does not reach the required minimum initial
               investment or falls below such minimum and you have discontinued
               monthly investments. This policy does not apply to accounts that
               fall below the minimums solely as a result of market value
               fluctuations. It is expected that, for purposes of this policy,
               accounts will be valued in September, and the $10 fee will be
               assessed on the second Friday of September of each year. You will
               receive notice before we charge the $10 fee or close your account
               so that you may increase your account balance to the required
               minimum.
 
                                                        Shareholder's manual  29
<PAGE> 
 
               TRANSACTIONS THROUGH PROCESSING ORGANIZATIONS
 
               You may purchase or sell Fund shares through a broker-dealer,
               bank or other financial institution, or an organization that
               provides recordkeeping and consulting services to 401(k) plans or
               other employee benefit plans (a "Processing Organization").
               Processing Organizations may charge you a fee for this service
               and may require different minimum initial and subsequent
               investments than the Fund. Processing Organizations may also
               impose other charges or restrictions different from those
               applicable to shareholders who invest in the Fund directly. A
               Processing Organization, rather than its customer, may be the
               shareholder of record of your shares. The Fund is not responsible
               for the failure of any Processing Organization to carry out its
               obligations to its customers. Certain Processing Organizations
               may receive compensation from Janus Capital or its affiliates and
               certain Processing Organizations may receive compensation from
               the Fund for shareholder recordkeeping and similar services.
 
               TAXPAYER IDENTIFICATION NUMBER
 
               On your application or other appropriate form, you will be asked
               to certify that your Social Security or taxpayer identification
               number is correct and that you are not subject to backup
               withholding for failing to report income to the IRS. If you are
               subject to the 31% backup withholding or you did not certify your
               taxpayer identification number, the IRS requires the Fund to
               withhold 31% of any dividends paid and redemption or exchange
               proceeds. In addition to the 31% backup withholding, you may be
               subject to a $50 fee to reimburse the Fund for any penalty that
               the IRS may impose.
 
               SHARE CERTIFICATES
 
               Most shareholders choose not to hold their shares in certificate
               form because account transactions such as exchanges and
               redemptions cannot be completed until the certificate has been
               returned to the Fund. The Fund will issue share certificates upon
               written request only and reserves the right to charge a fee for
               this service.
 
 30 Janus Twenty Fund
<PAGE> 
 
               Share certificates will not be issued until the shares have been
               held for at least 15 days and will not be issued for accounts
               that do not meet the minimum investment requirements. Share
               certificates cannot be issued for retirement accounts. In
               addition, if the certificate is lost, there may be a replacement
               charge.
 
               INVOLUNTARY REDEMPTIONS
 
               The Fund reserves the right to close an account if the
               shareholder is deemed to engage in activities which are illegal
               or otherwise believed to be detrimental to the Fund.
 
               TELEPHONE TRANSACTIONS
 
               You may initiate many transactions by telephone. The Fund and its
               agents will not be responsible for any losses resulting from
               unauthorized transactions when procedures designed to verify the
               identity of the caller are followed.
 
               It may be difficult to reach an Investor Service Representative
               by telephone during periods of unusual market activity. If you
               are unable to reach a representative by telephone, please
               consider sending written instructions, stopping by a Service
               Center, calling the Janus XpressLine or visiting our Web site.
 
               TEMPORARY SUSPENSION OF SERVICES
 
               The Fund or its agents may, in case of emergency, temporarily
               suspend telephone transactions or other shareholder services.
 
               ADDRESS CHANGES
 
               To change the address on your account, call 1-800-525-3713 or
               send a written request signed by all account owners. Include the
               name of the Fund, the account number(s), the name(s) on the
               account and both the old and new addresses. Certain options may
               be suspended for 10 days following an address change unless a
               signature guarantee is provided.
 
                                                        Shareholder's manual  31
<PAGE> 
 
               REGISTRATION CHANGES
 
               To change the name on an account, the shares are generally
               transferred to a new account. In some cases, legal documentation
               may be required. For more information call 1-800-525-3713.
 
               STATEMENTS AND REPORTS
 
               Investors participating in an automatic investment program will
               receive quarterly confirmations of all transactions. Dividend
               information will be distributed annually. In addition, the Fund
               will send you an immediate transaction confirmation statement
               after every non-systematic transaction.
 
               The Fund produces financial reports, which include a list of the
               Fund's portfolio holdings, semiannually and updates its
               prospectus annually. To reduce expenses, the Fund may choose to
               mail only one report or prospectus to your household, even if
               more than one person in the household has a Fund account. Please
               call 1-800-525-3713 if you would like to receive additional
               reports or prospectuses. The Fund reserves the right to charge a
               fee for additional statement and report requests.
 
 32 Janus Twenty Fund
<PAGE> 
                                                          Management of the fund
 
INVESTMENT ADVISER
 
               Janus Capital Corporation, 100 Fillmore Street, Denver, Colorado
               80206-4928, is the investment adviser to the Fund and is
               responsible for the day-to-day management of its investment
               portfolio and other business affairs.
 
               Janus Capital began serving as investment adviser to Janus Fund
               in 1970 and currently serves as investment adviser to all of the
               Janus funds, acts as sub-adviser for a number of private-label
               mutual funds and provides separate account advisory services for
               institutional accounts.
 
               Janus Capital furnishes continuous advice and recommendations
               concerning the Fund's investments. Janus Capital also furnishes
               certain administrative, compliance and accounting services for
               the Fund, and may be reimbursed by the Fund 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 Fund and pays the salaries, fees and expenses of
               all Fund officers and those Trustees who are affiliated with
               Janus Capital.
 
               The Fund pays Janus Capital a management fee which is calculated
               daily and paid monthly. The advisory agreement with the Fund
               spells out the management fee and other expenses that the Fund
               must pay.
 
               The Fund incurs 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 Annual Fund Operating Expenses
               table on page 5 lists the actual management fee and total
               operating expenses of the Fund for the most recent fiscal year.
 
                                                      Management of the fund  33
<PAGE> 
 
PORTFOLIO MANAGER
 
SCOTT W. SCHOELZEL
- --------------------------------------------------------------------------------
                   is Executive Vice President and portfolio manager of the
                   Fund, which he has managed since August 1997. He
                   previously managed Janus Olympus Fund from its inception
                   to August 1997. Mr. Schoelzel joined Janus Capital in
                   January 1994. He holds a Bachelor of Arts in Business from
                   Colorado College.
 
 34 Janus Twenty Fund
<PAGE> 
                                                               Other information
 
               SIZE OF THE FUND
 
               Although there is no present intention to do so, the Fund may
               discontinue sales of its shares if management and the Trustees
               believe that continued sales may adversely affect the Fund's
               ability to achieve its investment objective. If sales of the Fund
               are discontinued, it is expected that existing shareholders of
               the Fund would be permitted to continue to purchase shares and to
               reinvest any dividends or capital gains distributions, absent
               highly unusual circumstances.
 
               YEAR 2000
 
               Preparing for Year 2000 is a high priority for Janus Capital,
               which has established a dedicated group to address this issue.
               Janus Capital has devoted considerable internal resources and has
               engaged one of the foremost experts in the field to help achieve
               Year 2000 readiness. Janus Capital does not anticipate that Year
               2000-related issues will have a material impact on its ability to
               continue to provide the Fund with service at current levels;
               however, Janus Capital cannot make any assurances that the steps
               it has taken to ensure Year 2000 readiness will be successful. In
               addition, there can be no assurance that Year 2000 issues will
               not affect the companies in which the Fund invests or worldwide
               markets and economies.
 
                                                           Other information  35
<PAGE> 
 
Distributions and taxes
 
DISTRIBUTIONS
 
               To avoid taxation of the Fund, the Internal Revenue Code requires
               the Fund to distribute net income and any net capital gains
               realized on its investments annually. The Fund's income from
               dividends and interest and any net realized short-term gains are
               paid to shareholders as ordinary income dividends. Net realized
               long-term gains are paid to shareholders as capital gains
               distributions. Dividends and capital gains distributions are
               normally declared and paid in December.
 
               HOW DISTRIBUTIONS AFFECT THE FUND'S NAV
 
               Distributions are paid to shareholders as of the record date of
               the distribution of the Fund, regardless of how long the shares
               have been held. Dividends and capital gains awaiting distribution
               are included in the Fund's daily NAV. The share price of the Fund
               drops by the amount of the distribution, net of any subsequent
               market fluctuations. As an example, assume that on December 31,
               the Fund declared a dividend in the amount of $0.25 per share. If
               the Fund's share price was $10.00 on December 30, the Fund's
               share price on December 31 would be $9.75, barring market
               fluctuations. Shareholders should be aware that distributions
               from a taxable mutual fund are not value-enhancing and may create
               income tax obligations.
 
               "BUYING A DIVIDEND"
 
               If you purchase shares of the Fund just before the distribution,
               you will pay the full price for the shares and receive a portion
               of the purchase price back as a taxable distribution. This is
               referred to as "buying a dividend." In the above example, if you
               bought shares on December 30, you would have paid $10.00 per
               share. On December 31, the Fund would pay you $0.25 per share as
               a dividend and your shares would now be worth $9.75 per share.
               Unless your account is set up as a tax-deferred account,
               dividends paid to you would be included in your gross income for
               tax purposes, even though you may not have participated in the
 
 36 Janus Twenty Fund
<PAGE> 
 
               increase in NAV of the Fund, whether or not you reinvested the
               dividends.
 
DISTRIBUTION OPTIONS
 
               When you open an account, you must specify on your application
               how you want to receive your distributions. You may change your
               distribution option at any time by writing the Fund at one of the
               addresses on page 16 or calling 1-800-525-3713. The Fund offers
               the following options:
 
               1. REINVESTMENT OPTION. You may reinvest your income dividends
                  and capital gains distributions in additional shares. This
                  option is assigned automatically if no other choice is made.
 
               2. CASH OPTION. You may receive your income dividends and capital
                  gains distributions in cash.
 
               3. REINVEST AND CASH OPTION. You may receive either your income
                  dividends or capital gains distributions in cash and reinvest
                  the other in additional shares.
 
               4. REDIRECT OPTION. You may direct your dividends or capital
                  gains to purchase shares of another Janus fund.
 
               The Fund reserves the right to reinvest into your account
               undeliverable and uncashed dividend and distribution checks that
               remain outstanding for six months in shares of the Fund at the
               NAV next computed after the check is cancelled. Subsequent
               distributions may also be reinvested.
 
TAXES
 
               As with any investment, you should consider the tax consequences
               of investing in the Fund. Any time you sell or exchange shares of
               a Fund in a taxable account, it is considered a taxable event.
               Depending on the purchase price and the sale price, you may have
               a gain or loss on the transaction. Any tax liabilities generated
               by your transactions are your responsibility.
 
               The following discussion does not apply to tax-deferred accounts,
               nor is it a complete analysis of the federal tax implications of
               investing in the Fund. You may wish to consult your own tax
 
                                                     Distributions and taxes  37
<PAGE> 
 
               adviser. Additionally, state or local taxes may apply to your
               investment, depending upon the laws of your state of residence.
 
               TAXES ON DISTRIBUTIONS
 
               Dividends and distributions by the Fund are subject to federal
               income tax, regardless of whether the distribution is made in
               cash or reinvested in additional shares of the Fund.
               Distributions may be taxable at different rates depending on the
               length of time the Fund holds a security. In certain states, a
               portion of the dividends and distributions (depending on the
               source of the Fund's income) may be exempt from state and local
               taxes. Information regarding the tax status of income dividends
               and capital gains distributions will be mailed to shareholders on
               or before January 31st of each year. Account tax information will
               also be sent to the IRS.
 
               TAXATION OF THE FUND
 
               Dividends, interest, and some capital gains received by the Fund
               on foreign securities may be subject to tax withholding or other
               foreign taxes. The Fund 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 Fund.
 
               The Fund does not expect to pay federal income or excise taxes
               because it intends to meet certain requirements of the Internal
               Revenue Code. It is important that the Fund meet these
               requirements so that any earnings on your investment will not be
               taxed twice.
 
 38 Janus Twenty Fund
<PAGE> 
 
                                                            Financial highlights
 
               The financial highlights table is intended to help you understand
               the Fund's financial performance for the past 5 years through
               October 31st of each fiscal year shown. Items 1 through 9 reflect
               financial results for a single Fund share. The total returns in
               the table represent the rate that an investor would have earned
               (or lost) on an investment in the Fund (assuming reinvestment of
               all dividends and distributions). This information has been
               audited by PricewaterhouseCoopers LLP, whose report, along with
               the Fund's financial statements, are included in the Annual
               Report, which is available upon request and incorporated by
               reference into the SAI.
 
<TABLE>
<CAPTION>
                            JANUS TWENTY FUND
- -------------------------------------------------------------------------
                                               Periods ending October 31st
                                        1998      1997     1996     1995     1994
<S>                                    <C>       <C>      <C>      <C>      <C>
  1. NET ASSET VALUE, BEGINNING OF
     PERIOD                             $35.16   $31.90   $30.12   $24.24    $25.85
     INCOME FROM INVESTMENT
     OPERATIONS:
  2. Net investment income                0.12   (0.09)     0.37     0.01      0.16
  3. Net gains or (losses) on
     securities (both realized and
     unrealized)                         12.26     8.85     6.68     5.94    (1.07)
  4. Total from investment operations    12.38     8.76     7.05     5.95    (0.91)
     LESS DISTRIBUTIONS:
  5. Dividends (from net investment
     income)                            (0.10)   (0.18)       --   (0.07)    (0.25)
  6. Dividends (in excess of net
     investment income)                     --       --       --       --        --
  7. Distributions (from capital
     gains)                             (4.46)   (5.32)   (5.27)       --    (0.45)
  8. Total distributions                (4.56)   (5.50)   (5.27)   (0.07)    (0.70)
  9. NET ASSET VALUE, END OF PERIOD     $42.98   $35.16   $31.90   $30.12    $24.24
 10. Total return                       40.58%   31.65%   27.59%   24.67%    (3.52%)
 11. Net assets, end of period (in
     millions)                         $11,255   $5,871   $3,937   $2,996    $2,743
 12. Average net assets for the
     period (in millions)               $8,025   $4,990   $3,386   $2,716    $3,051
 13. Ratio of gross expenses to
     average net assets                  0.91%    0.93%    0.93%    1.00%       N/A
 14. Ratio of net expenses to average
     net assets                          0.90%    0.91%    0.92%    0.99%     1.02%
 15. Ratio of net investment
     income/(loss) to average net
     assets                              0.39%    0.33%    0.67%    0.62%     0.57%
 16. Portfolio turnover rate               54%     123%     137%     147%      102%
- -----------------------------------------------------------------------------------
</TABLE>
 
                                                        Financial highlights  39
<PAGE> 
 
Glossary of investment terms
 
               This glossary provides a more detailed description of some of the
               types of securities and other instruments in which the Fund may
               invest. The Fund may invest in these instruments to the extent
               permitted by its investment objective and policies. The Fund 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
               Fund may purchase commercial paper issued in private placements
               under Section 4(2) of the Securities Act of 1933.
 
               COMMON STOCKS are equity securities representing shares of
               ownership in a company, and usually carry voting rights and earns
               dividends. Unlike preferred stock, dividends on common stocks are
               not fixed but are declared at the discretion of the issuer's
               board of directors.
 
               CONVERTIBLE SECURITIES are preferred stocks or bonds that pay a
               fixed dividend or interest payment and are convertible into
               common stock at a specified price or conversion ratio.
 
               DEBT SECURITIES are equity securities representing money borrowed
               that must be repaid at a later date. Such securities have
               specific maturities and usually a specific rate of interest or an
               original purchase discount.
 
               DEPOSITARY RECEIPTS are receipts for shares of a foreign-based
               corporation that entitle the holder to dividends and capital
               gains on the underlying security. Receipts include those issued
               by domestic banks (American Depositary Receipts), foreign banks
 
 40 Janus Twenty Fund
<PAGE> 
 
               (Global or European Depositary Receipts) and broker-dealers
               (depositary shares).
 
               FIXED-INCOME SECURITIES are securities that pay a specified rate
               of return. The term generally includes short- and long-term
               government, corporate and municipal obligations that pay a
               specified rate of interest or coupons for a specified period of
               time, and preferred stock, which pays fixed dividends. Coupon and
               dividend rates may be fixed for the life of the issue or, in the
               case of adjustable and floating rate securities, for a shorter
               period.
 
               HIGH-YIELD/HIGH-RISK SECURITIES are securities that are rated
               below investment grade by the primary rating agencies (e.g., BB
               or lower by Standard & Poor's and Ba or lower by Moody's). Other
               terms commonly used to describe such securities include "lower
               rated bonds," "noninvestment grade bonds" and "junk bonds."
 
               MORTGAGE- AND ASSET-BACKED SECURITIES are shares in a pool of
               mortgages or other debt. These securities are generally pass-
               through securities, which means that principal and interest
               payments on the underlying securities (less servicing fees) are
               passed through to shareholders on a pro rata basis. These
               securities involve prepayment risk, which is the risk that the
               underlying mortgages or other debt may be refinanced or paid off
               prior to their maturities during periods of declining interest
               rates. In that case, the portfolio managers 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 Fund
               must pay if these investments are profitable, the Fund may make
               various elections permitted by the tax laws. These elections
               could require that the Fund recognize taxable income, which in
               turn
 
                                                Glossary of investment terms  41
<PAGE> 
 
               must be distributed, before the securities are sold and before
               cash is received to pay the distributions.
 
               PREFERRED STOCKS are equity securities that generally pay
               dividends at a specified rate and have preference over common
               stock in the payment of dividends and liquidation. Preferred
               stock generally does not carry voting rights.
 
               REPURCHASE AGREEMENTS involve the purchase of a security by the
               Fund and a simultaneous agreement by the seller (generally a bank
               or dealer) to repurchase the security from the Fund 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 Fund 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 Fund to another party (generally a bank or dealer) in return
               for cash and an agreement by the Fund to buy the security back at
               a specified price and time. This technique will be used primarily
               to provide cash to satisfy unusually heavy redemption requests,
               or for other temporary or emergency purposes.
 
               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.
 
 42 Janus Twenty Fund
<PAGE> 
 
               WARRANTS are securities, typically issued with preferred stocks
               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
               Fund does not earn interest on such securities until settlement
               and bears the risk of market value fluctuations in between the
               purchase and settlement dates. New issues of stocks and bonds,
               private placements and U.S. government securities may be sold in
               this manner.
 
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
               Fund may enter into forward currency contracts to hedge against
               declines in the value of securities denominated in, or whose
               value is tied to, a currency other than the U.S. dollar or to
               reduce the impact of currency appreciation on purchases of such
               securities. It may also enter into forward contracts to purchase
               or sell securities or other financial indices.
 
               FUTURES CONTRACTS are contracts that obligate the buyer to
               receive and the seller to deliver an instrument or money at a
               specified price on a specified date. The Fund 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 Fund
               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.
 
                                                Glossary of investment terms  43
<PAGE> 
 
               INDEXED/STRUCTURED SECURITIES are typically short- to
               intermediate-term debt securities whose value at maturity or
               interest rate is linked to currencies, interest rates, equity
               securities, indices, commodity prices or other financial
               indicators. Such securities may be positively or negatively
               indexed (i.e., their value may increase or decrease if the
               reference index or instrument appreciates). Indexed/structured
               securities may have return characteristics similar to direct
               investments in the underlying instrument and may be more volatile
               than the underlying instrument. The Fund bears the market risk of
               an investment in the underlying instrument, as well as the credit
               risk of the issuer.
 
               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 Fund may purchase and
               write put and call options on securities, securities indices and
               foreign currencies.
 
 44 Janus Twenty Fund
<PAGE> 
 
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<PAGE> 
 
[JANUS LOGO]
          1-800-525-3713
          P.O. Box 173375
          Denver, Colorado 80217-3375
          janus.com
 
You can request other information, including a Statement of
Additional Information, Annual Report or Semiannual Report, free of
charge, by contacting Janus at 1-800-525-3713 or visiting our Web
site at janus.com. In the Fund's Annual Report, you will find a
discussion of the market conditions and investment strategies that
significantly affected the Fund's performance during their last
fiscal year. Other information is also available from financial
intermediaries that sell shares of the Fund.
 
The Statement of Additional Information provides detailed
information about the Fund and is incorporated into this Prospectus
by reference. You may review the Fund's Statement of Additional
Information at the Public Reference Room of the SEC or get text only
copies for a fee, by writing to or calling the Public Reference
Room, Washington, D.C. 20549-6009 (1-800-SEC-0330). You may obtain
the Statement of Additional Information for free from the SEC's Web
site at http://www.sec.gov.
 
              Investment Company Act File No. 811-1879
 
  3181



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