MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
497, 1998-12-02
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<PAGE>
              PROSPECTUS
              NOVEMBER 24, 1998
 
              Morgan Stanley Dean Witter Convertible Securities Trust (the
"Fund") is an open-end diversified management investment company whose
investment objective is to seek a high level of total return on its assets
through a combination of current income and capital appreciation. It seeks to
achieve its investment objective by investing principally in "convertible
securities," that is, bonds, notes, debentures, preferred stocks and other
securities which are convertible into common stock. INVESTORS SHOULD CAREFULLY
CONSIDER THE RELATIVE RISKS OF INVESTING IN HIGH YIELD SECURITIES, WHICH ARE
COMMONLY KNOWN AS JUNK BONDS. BONDS OF THIS TYPE ARE CONSIDERED TO BE
SPECULATIVE WITH REGARD TO THE PAYMENT OF INTEREST AND RETURN OF PRINCIPAL.
INVESTORS SHOULD ALSO BE COGNIZANT OF THE FACT THAT SUCH SECURITIES ARE NOT
GENERALLY MEANT FOR SHORT-TERM INVESTING AND SHOULD ASSESS THE RISKS ASSOCIATED
WITH AN INVESTMENT IN THE FUND. (See "Investment Objective and Policies.")
 
               The Fund offers four classes of shares (each, a "Class"), each
with a different combination of sales charges, ongoing fees and other features.
The different distribution arrangements permit an investor to choose the method
of purchasing shares that the investor believes is most beneficial given the
amount of the purchase, the length of time the investor expects to hold the
shares and other relevant circumstances. (See "Purchase of Fund
Shares--Alternative Purchase Arrangements.")
 
               This Prospectus sets forth concisely the information you should
know before investing in the Fund. It should be read and retained for future
reference. Additional information about the Fund is contained in the Statement
of Additional Information, dated November 24, 1998, which has been filed with
the Securities and Exchange Commission, and which is available at no charge upon
request of the Fund at the address or telephone numbers listed on this page. The
Statement of Additional Information is incorporated herein by reference.
 
     MORGAN STANLEY DEAN WITTER
     DISTRIBUTORS INC.
      DISTRIBUTOR
 
      TABLE OF CONTENTS
 
Prospectus Summary/2
Summary of Fund Expenses/4
Financial Highlights/6
The Fund and its Management/9
Investment Objective and Policies/9
  Risk Considerations/10
Investment Restrictions/17
Purchase of Fund Shares/18
Shareholder Services/30
Redemptions and Repurchases/33
Dividends, Distributions and Taxes/34
Performance Information/35
Additional Information/36
Appendix--Ratings of Investments/38
 
SHARES OF THE FUND ARE NOT DEPOSITS OR OBLIGATIONS OF, OR GUARANTEED OR ENDORSED
BY, ANY BANK, AND THE SHARES ARE NOT FEDERALLY INSURED BY THE FEDERAL DEPOSIT
INSURANCE CORPORATION, THE FEDERAL RESERVE BOARD, OR ANY OTHER AGENCY.
 
THESE SECURITIES HAVE NOT BEEN APPROVED OR DISAPPROVED BY THE SECURITIES AND
EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION NOR HAS THE SECURITIES
AND EXCHANGE COMMISSION OR ANY STATE SECURITIES COMMISSION PASSED UPON THE
ACCURACY OR ADEQUACY OF THIS PROSPECTUS. ANY REPRESENTATION TO THE CONTRARY IS A
CRIMINAL OFFENSE.
 
    Morgan Stanley Dean Witter
    Convertible Securities Trust
    Two World Trade Center
    New York, New York 10048
    (212) 392-2550 or
    (800) 869-NEWS (toll-free)
<PAGE>
PROSPECTUS SUMMARY
- --------------------------------------------------------------------------------
 
<TABLE>
<S>                 <C>
The                 The Fund is organized as a Trust, commonly known as a Massachusetts business trust, and is an open-end
Fund                diversified management investment company investing principally in corporate securities that can be converted
                    into common stock.
- ------------------------------------------------------------------------------------------------------------------------------------
Shares Offered      Shares of beneficial interest with $0.01 par value (see page 36). The Fund offers four Classes of shares, each
                    with a different combination of sales charges, ongoing fees and other features (see pages 18-29).
- ------------------------------------------------------------------------------------------------------------------------------------
Minimum             The minimum initial investment for each Class is $1,000 ($100 if the account is opened through EasyInvest-SM-).
Purchase            Class D shares are only available to persons investing $5 million ($25 million for certain qualified plans) or
                    more and to certain other limited categories of investors. For the purpose of meeting the minimum $5 million
                    investment for Class D shares, and subject to the $1,000 minimum initial investment for each Class of the Fund,
                    an investor's existing holdings of Class A and Class D shares and shares of funds for which Morgan Stanley Dean
                    Witter Advisors Inc. serves as investment manager ("Morgan Stanley Dean Witter Funds") that are sold with a
                    front-end sales charge, and concurrent investments in Class D shares of the Fund and other Morgan Stanley Dean
                    Witter Funds that are multiple class funds, will be aggregated. The minimum subsequent investment is $100 (see
                    page 18).
- ------------------------------------------------------------------------------------------------------------------------------------
Investment          The investment objective of the Fund is to seek a high level of total return on its assets through a combination
Objective           of current income and capital appreciation. It seeks to achieve this objective by investing principally in
                    "convertible securities," that is bonds, notes, debentures, preferred stocks and other securities which are
                    convertible into common stock (see page 9).
- ------------------------------------------------------------------------------------------------------------------------------------
Investment          Morgan Stanley Dean Witter Advisors Inc., the Investment Manager of the Fund, and its wholly-owned subsidiary,
Manager             Morgan Stanley Dean Witter Services Company Inc., serve in various investment management, advisory, management
                    and administrative capacities to 100 investment companies and other portfolios with assets of approximately
                    $117.3 billion at October 31, 1998 (see page 9).
- ------------------------------------------------------------------------------------------------------------------------------------
Management          The Investment Manager receives a monthly fee at the annual rate of 0.60 of 1% of the Fund's net assets not
Fee                 exceeding $750 million, scaled down at various asset levels to 0.425 of 1% of the Fund's daily net assets
                    exceeding $3 billion, determined as of the close of each business day (see page 9).
- ------------------------------------------------------------------------------------------------------------------------------------
Distributor and     Morgan Stanley Dean Witter Distributors Inc. is the Distributor of the Fund's shares. The Fund has adopted a
Distribution Fee    distribution plan pursuant to Rule 12b-1 under the Investment Company Act (the "12b-1 Plan") with respect to the
                    distribution fees paid by the Class A, Class B and Class C shares of the Fund to the Distributor. The entire
                    12b-1 fee payable by Class A and a portion of the 12b-1 fee payable by each of Class B and Class C equal to
                    0.25% of the average daily net assets of the Class are currently each characterized as a service fee within the
                    meaning of the National Association of Securities Dealers, Inc. guidelines. The remaining portion of the 12b-1
                    fee, if any, is characterized as an asset-based sales charge (see pages 18 and 27).
- ------------------------------------------------------------------------------------------------------------------------------------
Alternative         Four classes of shares are offered:
Purchase
Arrangements        - Class A shares are offered with a front-end sales charge, starting at 5.25% and reduced for larger purchases.
                    Investments of $1 million or more (and investments by certain other limited categories of investors) are not
                    subject to any sales charge at the time of purchase but a contingent deferred sales charge ("CDSC") of 1.0% may
                    be imposed on redemptions within one year of purchase. The Fund is authorized to reimburse the Distributor for
                    specific expenses incurred in promoting the distribution of the Fund's Class A shares and servicing shareholder
                    accounts pursuant to the Fund's 12b-1 Plan. Reimbursement may in no event exceed an amount equal to payments at
                    an annual rate of 0.25% of average daily net assets of the Class (see pages 18, 21 and 27).
</TABLE>
 
- --------------------------------------------------------------------------------
 
                                       2
<PAGE>
- --------------------------------------------------------------------------------
<TABLE>
<S>                 <C>
                    - Class B shares are offered without a front-end sales charge, but will in most cases be subject to a CDSC
                    (scaled down from 5.0% to 1.0%) if redeemed within six years after purchase. The CDSC will be imposed on any
                    redemption of shares if after such redemption the aggregate current value of a Class B account with the Fund
                    falls below the aggregate amount of the investor's purchase payments made during the six years preceding the
                    redemption. A different CDSC schedule applies to investments by certain qualified plans. Class B shares are also
                    subject to a 12b-1 fee assessed at the annual rate of 1.0% of the lesser of: (a) the average daily net sales of
                    the Fund's Class B shares or (b) the average daily net assets of Class B. All shares of the Fund held prior to
                    July 28, 1997 have been designated Class B shares. Shares held before May 1, 1997 will convert to Class A shares
                    in May, 2007. In all other instances, Class B shares convert to Class A shares approximately ten years after the
                    date of the original purchase (see pages 18, 24 and 27).
                    - Class C shares are offered without a front-end sales charge, but will in most cases be subject to a CDSC of
                    1.0% if redeemed within one year after purchase. The Fund is authorized to reimburse the Distributor for
                    specific expenses incurred in promoting the distribution of the Fund's Class C shares and servicing shareholder
                    accounts pursuant to the Fund's 12b-1 Plan. Reimbursement may in no event exceed an amount equal to payments at
                    an annual rate of 1.0% of average daily net assets of the Class (see pages 18, 26 and 27).
                    - Class D shares are offered only to investors meeting an initial investment minimum of $5 million ($25 million
                    for certain qualified plans) and to certain other limited categories of investors. Class D shares are offered
                    without a front-end sales charge or CDSC and are not subject to any 12b-1 fee (see pages 18 and 27).
- ------------------------------------------------------------------------------------------------------------------------------------
Dividends           Income dividends are paid quarterly; capital gains, if any, are paid at least once per year. The Fund may,
and                 however, determine to retain all or part of any net long-term capital gains in any year for reinvestment.
Capital Gains       Dividends and capital gains distributions paid on shares of a Class are automatically reinvested in additional
Distributions       shares of the same Class at net asset value unless the shareholder elects to receive cash. Shares acquired by
                    dividend and distribution reinvestment will not be subject to any sales charge or CDSC (see pages 30 and 34).
- ------------------------------------------------------------------------------------------------------------------------------------
Redemption          Shares are redeemable by the shareholder at net asset value less any applicable CDSC on Class A, Class B or
                    Class C shares. An account may be involuntarily redeemed if the total value of the account is less than $100 or,
                    if the account was opened through EasyInvest-SM-, if after twelve months the shareholder has invested less than
                    $1,000 in the account (see page 33).
- ------------------------------------------------------------------------------------------------------------------------------------
Risks               The net asset value of the Fund's shares will fluctuate with changes in the market value of its portfolio
                    securities. Emphasis on convertible securities will result in price fluctuations of the Fund's portfolio
                    securities with varying interest rates and with changes in the prices of the common stocks associated with their
                    conversion rights. In addition, the investor is directed to the discussions of corporate fixed-income securities
                    (certain of which may be lower rated securities commonly known as "junk bonds" or securities which are unrated
                    by recognized rating agencies), "enhanced" and "synthetic" convertible securities, when-issued and delayed
                    delivery securities and forward commitments, when, as and if issued securities, options, futures contracts,
                    foreign securities, repurchase agreements, and options on futures (see pages 10-16).
</TABLE>
 
- --------------------------------------------------------------------------------
 
  THE ABOVE IS QUALIFIED IN ITS ENTIRETY BY THE DETAILED INFORMATION APPEARING
         ELSEWHERE IN THE PROSPECTUS AND IN THE STATEMENT OF ADDITIONAL
                                  INFORMATION.
 
                                       3
<PAGE>
SUMMARY OF FUND EXPENSES
- --------------------------------------------------------------------------------
 
    The following table illustrates all expenses and fees that a shareholder of
the Fund will incur. The expenses and fees set forth in the table are based on
the expenses and fees for the fiscal year ended September 30, 1998.
 
<TABLE>
<CAPTION>
                                                                                   CLASS A    CLASS B    CLASS C    CLASS D
                                                                                  ---------   -------   ---------   -------
<S>                                                                               <C>         <C>       <C>         <C>
SHAREHOLDER TRANSACTION EXPENSES
- --------------------------------------------------------------------------------
Maximum Sales Charge Imposed on Purchases (as a percentage of offering price)...   5.25%(1)    None      None        None
Sales Charge Imposed on Dividend Reinvestments..................................   None        None      None        None
Maximum Contingent Deferred Sales Charge (as a percentage of original purchase
  price or redemption proceeds).................................................   None(2)     5.00%(3)  1.00%(4)    None
Redemption Fees.................................................................   None        None      None        None
Exchange Fee....................................................................   None        None      None        None
ANNUAL FUND OPERATING EXPENSES (AS A PERCENTAGE OF AVERAGE NET ASSETS)
- --------------------------------------------------------------------------------
Management Fees.................................................................   0.60%       0.60%     0.60%       0.60%
12b-1 Fees (5) (6)..............................................................   0.24%       1.00%     1.00%       None
Other Expenses..................................................................   0.21%       0.21%     0.21%       0.21%
Total Fund Operating Expenses...................................................   1.05%       1.81%     1.81%       0.81%
</TABLE>
 
- ------------
(1) REDUCED FOR PURCHASES OF $25,000 AND OVER (SEE "PURCHASE OF FUND
    SHARES--INITIAL SALES CHARGE ALTERNATIVE--CLASS A SHARES").
 
(2) INVESTMENTS THAT ARE NOT SUBJECT TO ANY SALES CHARGE AT THE TIME OF PURCHASE
    ARE SUBJECT TO A CDSC OF 1.00% THAT WILL BE IMPOSED ON REDEMPTIONS MADE
    WITHIN ONE YEAR AFTER PURCHASE, EXCEPT FOR CERTAIN SPECIFIC CIRCUMSTANCES
    (SEE "PURCHASE OF FUND SHARES--INITIAL SALES CHARGE ALTERNATIVE--CLASS A
    SHARES").
 
(3) THE CDSC IS SCALED DOWN TO 1.00% DURING THE SIXTH YEAR, REACHING ZERO
    THEREAFTER.
 
(4) ONLY APPLICABLE TO REDEMPTIONS MADE WITHIN ONE YEAR AFTER PURCHASE (SEE
    "PURCHASE OF FUND SHARES-- LEVEL LOAD ALTERNATIVE--CLASS C SHARES").
 
(5) THE 12b-1 FEE IS ACCRUED DAILY AND PAYABLE MONTHLY. THE ENTIRE 12b-1 FEE
    PAYABLE BY CLASS A AND A PORTION OF THE 12b-1 FEE PAYABLE BY EACH OF CLASS B
    AND CLASS C EQUAL TO 0.25% OF THE AVERAGE DAILY NET ASSETS OF THE CLASS ARE
    CURRENTLY EACH CHARACTERIZED AS A SERVICE FEE WITHIN THE MEANING OF NATIONAL
    ASSOCIATION OF SECURITIES DEALERS, INC. ("NASD") GUIDELINES AND ARE PAYMENTS
    MADE FOR PERSONAL SERVICE AND/OR MAINTENANCE OF SHAREHOLDER ACCOUNTS. THE
    REMAINDER OF THE 12b-1 FEE, IF ANY, IS AN ASSET-BASED SALES CHARGE, AND IS A
    DISTRIBUTION FEE PAID TO THE DISTRIBUTOR TO COMPENSATE IT FOR THE SERVICES
    PROVIDED AND THE EXPENSES BORNE BY THE DISTRIBUTOR AND OTHERS IN THE
    DISTRIBUTION OF THE FUND'S SHARES (SEE "PURCHASE OF FUND SHARES--PLAN OF
    DISTRIBUTION").
 
(6) UPON CONVERSION OF CLASS B SHARES TO CLASS A SHARES, SUCH SHARES WILL BE
    SUBJECT TO THE LOWER 12b-1 FEE APPLICABLE TO CLASS A SHARES. NO SALES CHARGE
    IS IMPOSED AT THE TIME OF CONVERSION OF CLASS B SHARES TO CLASS A SHARES.
    CLASS C SHARES DO NOT HAVE A CONVERSION FEATURE AND, THEREFORE, ARE SUBJECT
    TO AN ONGOING 1.00% DISTRIBUTION FEE (SEE "PURCHASE OF FUND
    SHARES--ALTERNATIVE PURCHASE ARRANGEMENTS").
 
                                       4
<PAGE>
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
EXAMPLES                                            1 Year   3 Years   5 Years   10 Years
- --------------------------------------------------  ------   -------   -------   --------
<S>                                                 <C>      <C>       <C>       <C>
You would pay the following expenses on a $1,000
 investment assuming (1) a 5% annual return and
 (2) redemption at the end of each time period:
    Class A.......................................   $63       $84       $107      $174
    Class B.......................................   $68       $87       $118      $213
    Class C.......................................   $28       $57       $98       $213
    Class D.......................................   $ 8       $26       $45       $100
 
You would pay the following expenses on the same
 $1,000 investment assuming no redemption at the
 end of the period:
    Class A.......................................   $63       $84       $107      $174
    Class B.......................................   $18       $57       $98       $213
    Class C.......................................   $18       $57       $98       $213
    Class D.......................................   $ 8       $26       $45       $100
</TABLE>
 
    THE ABOVE EXAMPLES SHOULD NOT BE CONSIDERED A REPRESENTATION OF PAST OR
FUTURE EXPENSES OR PERFORMANCE. ACTUAL EXPENSES OF EACH CLASS MAY BE GREATER OR
LESS THAN THOSE SHOWN.
 
    The purpose of this table is to assist the investor in understanding the
various costs and expenses that an investor in the Fund will bear directly or
indirectly. For a more complete description of these costs and expenses, see
"The Fund and its Management," "Purchase of Fund Shares--Plan of Distribution"
and "Redemptions and Repurchases."
 
    Long-term shareholders of Class B and Class C may pay more in sales charges,
including distribution fees, than the economic equivalent of the maximum
front-end sales charges permitted by the NASD.
 
                                       5
<PAGE>
FINANCIAL HIGHLIGHTS
- --------------------------------------------------------------------------------
 
    The following ratios and per share data for a share of beneficial interest
outstanding throughout each period have been audited by PricewaterhouseCoopers
LLP, independent accountants. The financial highlights should be read in
conjunction with the financial statements, the notes thereto and the unqualified
report of independent accountants, which are contained in the Statement of
Additional Information. Further information about the performance of the Fund is
contained in the Fund's Annual Report to Shareholders, which may be obtained
without charge upon request to the Fund.
 
<TABLE>
<CAPTION>
                                                           FOR THE YEAR ENDED SEPTEMBER 30,
                   ----------------------------------------------------------------------------------------------------------------
CLASS B SHARES      1998++      1997*++      1996       1995        1994       1993       1992        1991       1990        1989
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
<S>                <C>         <C>         <C>        <C>         <C>        <C>        <C>         <C>        <C>         <C>
PER SHARE
 OPERATING
 PERFORMANCE:
Net asset value,
 beginning of
 period..........  $   15.07   $   12.72   $  11.67   $   10.75   $  10.62   $   8.92       $8.67      $7.65       $9.68      $8.63
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
Net investment
 income..........       0.65        0.60       0.55        0.60       0.42       0.37        0.34       0.37        0.46       0.48
Net realized and
 unrealized gain
 (loss)..........      (2.70)       2.31       1.12        0.82       0.11       1.67        0.15       1.05       (2.06)      1.20
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
Total from
 investment
 operations......      (2.05)       2.91       1.67        1.42       0.53       2.04        0.49       1.42       (1.60)      1.68
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
Less dividends
 from net
 investment
 income..........      (0.57)      (0.56)     (0.62)      (0.50)     (0.40)     (0.34)      (0.24)     (0.40)      (0.43)     (0.63)
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
Net asset value,
 end of period...     $12.45      $15.07     $12.72      $11.67     $10.75     $10.62       $8.92      $8.67       $7.65      $9.68
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
                   ---------   ---------   --------   ---------   --------   --------   ---------   --------   ---------   --------
 
TOTAL INVESTMENT
 RETURN+.........   (14.01)%      23.38%     14.70%      13.68%      5.02%     23.22%       5.69%     18.93%    (16.93)%     20.20%
 
RATIOS TO AVERAGE
 NET ASSETS:
Expenses.........      1.81%(1)     1.84%     1.89%       1.96%      1.93%      1.93%       1.92%      1.92%       1.88%      1.76%
Net investment
 income..........      4.40%(1)     4.45%     4.78%       5.24%      3.68%      3.44%       3.43%      4.34%       4.96%      4.93%
 
SUPPLEMENTAL
 DATA:
Net assets, end
 of period, in
 millions........       $263        $317       $234        $185       $190       $208        $218       $297        $413       $822
Portfolio
 turnover rate...        95%        182%       171%        138%       184%       221%        145%       133%         92%       167%
</TABLE>
 
- -------------
 * PRIOR TO JULY 28, 1997, THE FUND ISSUED ONE CLASS OF SHARES. ALL SHARES OF
   THE FUND HELD PRIOR TO THAT DATE HAVE BEEN DESIGNATED CLASS B SHARES.
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 + DOES NOT REFLECT THE DEDUCTION OF SALES CHARGE. CALCULATED BASED ON THE NET
   ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE PERIOD.
(1) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
    EXPENSES.
 
                                       6
<PAGE>
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                                  FOR THE PERIOD
                                                                FOR THE YEAR      JULY 28, 1997*
                                                                   ENDED             THROUGH
                                                               SEPTEMBER 30,      SEPTEMBER 30,
CLASS A SHARES                                                     1998++             1997++
                                                              ----------------   ----------------
<S>                                                           <C>                <C>
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period........................      $ 15.07            $ 14.31
                                                                  -------            -------
Net investment income.......................................         0.75               0.13
Net realized and unrealized gain (loss).....................        (2.68)              0.78
                                                                  -------            -------
Total from investment operations............................        (1.93)              0.91
                                                                  -------            -------
Less dividends from net investment income...................        (0.69)             (0.15)
                                                                  -------            -------
Net asset value, end of period..............................      $ 12.45            $ 15.07
                                                                  -------            -------
                                                                  -------            -------
TOTAL INVESTMENT RETURN+....................................       (13.38)%             6.40%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses....................................................         1.05%(3)           1.15%(2)
Net investment income.......................................         5.16%(3)           5.03%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands.....................         $963                $50
Portfolio turnover rate.....................................           95%               182%
 
CLASS C SHARES
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period........................      $ 15.06            $ 14.31
                                                                  -------            -------
Net investment income.......................................         0.64               0.12
Net realized and unrealized gain (loss).....................        (2.68)              0.77
                                                                  -------            -------
Total from investment operations............................        (2.04)              0.89
                                                                  -------            -------
Less dividends from net investment income...................        (0.59)             (0.14)
                                                                  -------            -------
Net asset value, end of period..............................      $ 12.43            $ 15.06
                                                                  -------            -------
                                                                  -------            -------
TOTAL INVESTMENT RETURN+....................................       (14.07)%             6.26%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses....................................................         1.81%(3)           1.92%(2)
Net investment income.......................................         4.40%(3)           4.52%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands.....................       $2,390               $620
Portfolio turnover rate.....................................           95%               182%
</TABLE>
 
- -------------
 * THE DATE SHARES WERE FIRST ISSUED.
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 + DOES NOT REFLECT THE DEDUCTION OF SALES CHARGE. CALCULATED BASED ON THE NET
   ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE PERIOD.
(1) NOT ANNUALIZED.
(2) ANNUALIZED.
(3) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
    EXPENSES.
 
                                       7
<PAGE>
- --------------------------------------------------------------------------------
 
<TABLE>
<CAPTION>
                                                                                            FOR THE PERIOD
                                                                          FOR THE YEAR      JULY 28, 1997*
                                                                             ENDED             THROUGH
                                                                         SEPTEMBER 30,      SEPTEMBER 30,
CLASS D SHARES                                                               1998++             1997++
                                                                        ----------------   ----------------
<S>                                                                     <C>                <C>
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................      $ 15.08            $ 14.31
                                                                            -------            -------
Net investment income.................................................         0.79               0.13
Net realized and unrealized gain (loss)...............................        (2.71)              0.80
                                                                            -------            -------
Total from investment operations......................................        (1.92)              0.93
                                                                            -------            -------
Less dividends from net investment income.............................        (0.72)             (0.16)
                                                                            -------            -------
Net asset value, end of period........................................      $ 12.44            $ 15.08
                                                                            -------            -------
                                                                            -------            -------
 
TOTAL INVESTMENT RETURN+..............................................       (13.19)%             6.42%(1)
 
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         0.81%(3)           0.89%(2)
Net investment income.................................................         5.40%(3)           4.94%(2)
 
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................       $1,696                $21
Portfolio turnover rate...............................................           95%               182%
</TABLE>
 
- -------------
 * THE DATE SHARES WERE FIRST ISSUED.
 ++ THE PER SHARE AMOUNTS WERE COMPUTED USING AN AVERAGE NUMBER OF SHARES
    OUTSTANDING DURING THE PERIOD.
 + CALCULATED BASED ON THE NET ASSET VALUE AS OF THE LAST BUSINESS DAY OF THE
   PERIOD.
(1) NOT ANNUALIZED.
(2) ANNUALIZED.
(3) REFLECTS OVERALL FUND RATIOS FOR INVESTMENT INCOME AND NON-CLASS SPECIFIC
    EXPENSES.
 
                                       8
<PAGE>
THE FUND AND ITS MANAGEMENT
- --------------------------------------------------------------------------------
 
    Morgan Stanley Dean Witter Convertible Securities Trust (formerly named Dean
Witter Convertible Securities Trust) (the "Fund") is an open-end diversified
management investment company. The Fund is a trust of the type commonly known as
a "Massachusetts business trust" and was organized under the laws of the
Commonwealth of Massachusetts on May 21, 1985.
 
    Morgan Stanley Dean Witter Advisors Inc. ("MSDW Advisors" or the "Investment
Manager"), whose address is Two World Trade Center, New York, New York 10048, is
the Fund's Investment Manager. The Investment Manager is a wholly-owned
subsidiary of Morgan Stanley Dean Witter & Co., a preeminent global financial
services firm that maintains leading market positions in each of its three
primary businesses--securities, asset management and credit services. The
Investment Manager, which was incorporated in July, 1992 under the name Dean
Witter InterCapital Inc., changed its name to Morgan Stanley Dean Witter
Advisors Inc. on June 22, 1998.
 
    MSDW Advisors and its wholly-owned subsidiary, Morgan Stanley Dean Witter
Services Company Inc. ("MSDW Services"), serve in various investment management,
advisory, management and administrative capacities to 100 investment companies,
28 of which are listed on the New York Stock Exchange, with combined total net
assets of approximately $112.9 billion as of October 31, 1998. The Investment
Manager also manages and advises managers of portfolios of pension plans, other
institutions and individuals which aggregated approximately $4.4 billion at such
date.
 
    The Fund has retained the Investment Manager to provide administrative
services, manage its business affairs and manage the investment of the Fund's
assets, including the placing of orders for the purchase and sale of portfolio
securities. MSDW Advisors has retained MSDW Services to perform the
aforementioned administrative services for the Fund.
 
    The Fund's Trustees review the various services provided by or under the
direction of the Investment Manager to ensure that the Fund's general investment
policies and programs are being properly carried out and that administrative
services are being provided to the Fund in a satisfactory manner.
 
    As full compensation for the services and facilities furnished to the Fund
and for expenses of the Fund assumed by the Investment Manager, the Fund pays
the Investment Manager monthly compensation calculated daily by applying the
following annual rates to the Fund's net assets determined as of the close of
each business day: 0.60% of the portion of the daily net assets not exceeding
$750 million, scaled down at various asset levels to 0.425% of the portion of
the daily net assets exceeding $3 billion. For the fiscal year ended September
30, 1998, the Fund accrued total compensation to the Investment Manager
amounting to 0.60% of the Fund's average daily net assets and the total expenses
of each Class amounted to 1.05%, 1.81%, 1.81% and 0.81% of the average daily net
assets of Class A, Class B, Class C and Class D, respectively.
 
INVESTMENT OBJECTIVE AND POLICIES
- --------------------------------------------------------------------------------
 
    The investment objective of the Fund is to seek a high level of total return
on its assets through a combination of current income and capital appreciation.
There is no assurance that this objective will be achieved. It is a fundamental
policy of the Fund and cannot be changed without shareholder approval. The
following policies may be changed by the Trustees of the Fund without
shareholder approval.
 
   (1) The Fund will normally invest at least 65% of its total assets (taken at
current value) in
 
                                       9
<PAGE>
"convertible securities," i.e., securities (bonds, debentures, corporate notes,
preferred stocks and other securities) which are convertible into common stock.
Securities received upon conversion may be retained in the Fund's portfolio to
permit orderly disposition or to establish long-term holding periods for federal
income tax purposes. The Fund is not required to sell these securities for the
purpose of assuring that 65% of its assets are invested in convertible
securities.
 
   (2) The Fund may invest up to 35% of its total assets (taken at current value
and subject to any restrictions appearing elsewhere in this Prospectus) in any
combination and quantity of the following securities: (a) common stock; (b)
nonconvertible preferred stock; (c) nonconvertible corporate debt securities;
(d) options on debt and equity securities; (e) financial futures contracts and
related options thereon; and (f) money market instruments.
 
   (3) Notwithstanding paragraphs (1) and (2) above, when market conditions
dictate a "defensive" investment strategy, the Fund may invest without limit in
money market instruments, including commercial paper, certificates of deposit,
bankers' acceptances and other obligations of domestic banks or domestic
branches of foreign banks, or foreign branches of domestic banks, in each case
having total assets of at least $500 million, and obligations issued or
guaranteed by the United States Government, or foreign governments or their
respective instrumentalities or agencies.
 
    The Fund may invest in fixed-income securities rated Baa or lower by Moody's
Investors Service, Inc. ("Moody's"), or BBB or lower by Standard & Poor's
Corporation ("S&P"). Fixed-income securities rated Baa by Moody's or BBB by S&P
have speculative characteristics greater than those of more highly rated bonds,
while fixed-income securities rated Ba or BB or lower by Moody's and S&P,
respectively, are considered to be speculative investments. Furthermore, the
Fund does not have any minimum quality rating standard for its investments. As
such, the Fund may invest in securities rated as low as Caa, Ca or C by Moody's
or CCC, CC, C or C1 by S&P. Fixed-income securities rated Caa or Ca by Moody's
may already be in default on payment of interest or principal, while bonds rated
C by Moody's, their lowest bond rating, can be regarded as having extremely poor
prospects of ever attaining any real investment standing. Bonds rated C1 by S&P,
their lowest bond rating, are no longer making interest payments.
 
    Non-rated securities are also considered for investment by the Fund when the
Investment Manager believes that the financial condition of the issuers of such
securities, or the protection afforded by the terms of the securities
themselves, makes them appropriate investments for the Fund.
 
    A general description of Moody's and S&P's ratings is set forth in the
Appendix at the end of this Prospectus.
 
RISK CONSIDERATIONS
 
    CONVERTIBLE SECURITIES.  The Fund will seek to meet its investment objective
by investing primarily in convertible securities in accordance with the
above-stated policies. Investments in these securities can provide a high level
of total return by virtue of their affording current income through interest and
dividend payments and because of the opportunity they provide for capital
appreciation by virtue of their convertibility into common stock. The Fund may
invest in investment grade convertible securities which are rated within the
four highest categories by recognized rating agencies; i.e., S & P and Moody's,
as well as in such securities which are lower rated or which are not rated by
such agencies. See the Statement of Additional Information for a discussion of
S&P and Moody's ratings.
 
    Convertible securities rank senior to common stocks in a corporation's
capital structure and, therefore, entail less risk than the corporation's common
stock. The value of a convertible security is a function of its "investment
value" (its value as if it did not have a conversion privilege), and its
"conversion value" (the security's worth if it were to be exchanged for the
underlying security, at market value, pursuant to its conversion privilege).
 
                                       10
<PAGE>
    To the extent that a convertible security's investment value is greater than
its conversion value, its price will be primarily a reflection of such
investment value and its price will be likely to increase when interest rates
fall and decrease when interest rates rise, as with a fixed-income security (the
credit standing of the issuer and other factors may also have an effect on the
convertible security's value). If the conversion value exceeds the investment
value, the price of the convertible security will rise above its investment
value and, in addition, will sell at some premium over its conversion value.
(This premium represents the price investors are willing to pay for the
privilege of purchasing a fixed-income security with a possibility of capital
appreciation due to the conversion privilege.) At such times the price of the
convertible security will tend to fluctuate directly with the price of the
underlying equity security. Convertible securities may be purchased by the Fund
at varying price levels above their investment values and/or their conversion
values in keeping with the Fund's objective.
 
    The Fund may invest up to 25% of its total assets in "enhanced" convertible
securities. Enhanced convertible securities offer holders the opportunity to
obtain higher current income than would be available from a traditional equity
security issued by the same company, in return for reduced participation or a
cap on appreciation in the underlying common stock of the issuer which the
holder can realize. In addition, in many cases, enhanced convertible securities
are convertible into the underlying common stock of the issuer automatically at
maturity, unlike traditional convertible securities which are convertible only
at the option of the security holder. Enhanced convertible securities may be
more volatile than traditional convertible securities due to the mandatory
conversion feature.
 
    The Fund also may invest up to 10% in "synthetic" convertible securities.
Unlike traditional convertible securities whose conversion values are based on
the common stock of the issuer of the convertible security, "synthetic"
convertible securities are preferred stocks or debt obligations of an issuer
which are combined with an equity component whose conversion value is based on
the value of the common stock of a different issuer or a particular benchmark
(which may include a foreign issuer or basket of foreign stocks, or a company
whose stock is not yet publicly traded). In many cases, "synthetic" convertible
securities are not convertible prior to maturity, at which time the value of the
security is paid in cash by the issuer.
 
    "Synthetic" convertible securities may be less liquid than traditional
convertible securities and their price changes may be more volatile. Reduced
liquidity may have an adverse impact on the Fund's ability to sell particular
synthetic securities promptly at favorable prices and may also make it more
difficult for the Fund to obtain market quotations based on actual trades, for
purposes of valuing the Fund's portfolio securities.
 
    The Fund may invest without limitation in "exchangeable" convertible bonds
and convertible preferred stock which are issued by one company, but convertible
into the common stock of a different publicly traded company. These securities
generally have liquidity trading and risk characteristics similar to traditional
convertible securities noted above.
 
    ZERO COUPON SECURITIES.  A portion of the securities purchased by the Fund
may be zero coupon securities. Such securities are purchased at a discount from
their face amount, giving the purchaser the right to receive their full value at
maturity. The interest earned on such securities is, implicitly, automatically
compounded and paid out at maturity. While such compounding at a constant rate
eliminates the risk of receiving lower yields upon reinvestment of interest if
prevailing interest rates decline, the owner of a zero coupon security will be
unable to participate in higher yields upon reinvestment of interest received on
interest-paying securities if prevailing interest rates rise.
 
    A zero coupon security pays no interest to its holder during its life.
Therefore, to the extent the Fund invests in zero coupon securities, it will not
receive current cash available for distribution to
 
                                       11
<PAGE>
shareholders. In addition, zero coupon securities are subject to substantially
greater price fluctuations during periods of changing prevailing interest rates
than are comparable securities which pay interest on a current basis. Current
federal tax law requires that a holder (such as the Fund) of a zero coupon
security accrue a portion of the discount at which the security was purchased as
income each year even though the Fund receives no interest payments in cash on
the security during the year.
 
    CORPORATE FIXED-INCOME SECURITIES.  In order to generate the current income
needed to achieve its investment objective, the Fund may invest in investment
grade nonconvertible fixed-income securities as well as in such securities which
are in the lower rating categories of S & P and Moody's or which are not rated
by such agencies. Such investments may be deemed speculative in nature.
 
    The ratings of fixed-income securities by Moody's and S & P are a generally
accepted barometer of credit risk. The Investment Manager will primarily rely
upon such ratings in assessing the creditworthiness of the issuers of the
securities it purchases. Nevertheless, the Investment Manager takes into account
in its security selection process the fact that credit ratings evaluate the
safety of a security's continuing payments of principal and interest, rather
than the risk of decline in its market value. Moreover, as credit rating
agencies may fail to make timely changes in their credit ratings to reflect
changing circumstances and events, the Investment Manager will continuously
monitor the issuers of the lower-rated securities held in the Fund's portfolio
to determine whether these issuers have sufficient cash flow and profits to meet
required principal and interest payments.
 
    All fixed-income securities are subject to two types of risks: the credit
risk and the interest rate risk. The credit risk relates to the ability of the
issuer to meet interest or principal payments or both as they come due. The
interest rate risk refers to the fluctuations in net asset value of any
portfolio of fixed-income securities resulting from the inverse relationship
between price and yield of fixed-income securities; that is, when the general
level of interest rates rises, the prices of outstanding fixed-income securities
decline, and when interest rates fall, prices rise.
 
    FOREIGN SECURITIES.  The Fund may invest in securities of foreign companies.
However, the Fund will not invest more than 10% of the value of its total
assets, at the time of purchase, in foreign securities (other than securities of
Canadian issuers registered under the Securities Exchange Act of 1934 or
American Depository Receipts, on which there is no such limit). Foreign
securities investments may be affected by changes in currency rates or exchange
control regulations, changes in governmental administration or economic or
monetary policy (in the United States and abroad) or changed circumstances in
dealings between nations. Costs will be incurred in connection with conversions
between various currencies held by the Fund. Investments in foreign securities
will also occasion risks relating to political and economic developments abroad,
including the possibility of expropriations or confiscatory taxation,
limitations on the use or transfer of Fund assets and any effects of foreign
social, economic or political instability. When purchasing foreign securities,
the Fund may enter into foreign currency exchange transactions or forward
foreign exchange contracts to facilitate settlement. The Fund may utilize
forward foreign exchange contracts in these instances as an attempt to limit the
effect of changes in the relationship between the U.S. dollar and the foreign
currency during the period between the trade date and settlement date for the
transaction.
 
    Many European countries are about to adopt a single European currency, the
euro (the "Euro Conversion"). The consequences of the Euro Conversion for
foreign exchange rates, interest rates and the value of European securities
eligible for purchase by the Fund are presently unclear. Such consequences may
adversely affect the value and/or increase the volatility of securities held by
the Fund.
 
    WHEN-ISSUED AND DELAYED DELIVERY SECURITIES AND FORWARD COMMITMENTS.  From
time to time, in
 
                                       12
<PAGE>
the ordinary course of business, the Fund may purchase securities on a
when-issued or delayed delivery basis or may purchase or sell securities on a
forward commitment basis. When such transactions are negotiated, the price is
fixed at the time of the commitment, but delivery and payment can take place a
month or more after the date of the commitment. While the Fund will only
purchase securities on a when-issued, delayed delivery or forward commitment
basis with the intention of acquiring the securities, the Fund may sell the
securities before the settlement date, if it is deemed advisable. The securities
so purchased or sold are subject to market fluctuation and no interest accrues
to the purchaser during this period.
 
    WHEN, AS AND IF ISSUED SECURITIES.  The Fund may purchase securities on a
"when, as and if issued" basis under which the issuance of the security depends
upon the occurrence of a subsequent event, such as approval of a merger,
corporate reorganization or debt restructuring. If the anticipated event does
not occur and the securities are not issued, the Fund will have lost an
investment opportunity. There is no overall limit on the percentage of the
Fund's assets which may be committed to the purchase of securities on a "when,
as and if issued" basis. An increase in the percentage of the Fund's assets
committed to the purchase of securities on a "when, as and if issued" basis may
increase the volatility of its net asset value.
 
    PRIVATE PLACEMENTS.  The Fund may invest up to 5% of its total assets in
securities which are subject to restrictions on resale because they have not
been registered under the Securities Act of 1933, as amended (the "Securities
Act"), or which are otherwise not readily marketable. (Securities eligible for
resale pursuant to Rule 144A under the Securities Act, and determined to be
liquid pursuant to the procedures discussed in the following paragraph, are not
subject to the foregoing restriction.) These securities are generally referred
to as private placements or restricted securities. Limitations on the resale of
such securities may have an adverse effect on their marketability, and may
prevent the Fund from disposing of them promptly at reasonable prices. The Fund
may have to bear the expense of registering such securities for resale and the
risk of substantial delays in effecting such registration.
 
    The Securities and Exchange Commission has adopted Rule 144A under the
Securities Act, which permits the Fund to sell restricted securities to
qualified institutional buyers without limitation. The Investment Manager,
pursuant to procedures adopted by the Trustees of the Fund, will make a
determination as to the liquidity of each restricted security purchased by the
Fund. If a restricted security is determined to be "liquid," such security will
not be included within the category "illiquid securities," which under current
policy may not exceed 15% of the Fund's net assets. Investing in Rule 144A
securities could have the effect of increasing the level of Fund illiquidity to
the extent the Fund, at a particular time, may be unable to find qualified
institutional buyers interested in purchasing such securities.
 
    INVESTMENT IN REAL ESTATE INVESTMENT TRUSTS. The Fund may invest in real
estate investment trusts, which pool investors' funds for investments primarily
in commercial real estate properties. Investment in real estate investment
trusts may be the most practical available means for the Fund to invest in the
real estate industry (the Fund is prohibited from investing in real estate
directly). As a shareholder in a real estate investment trust, the Fund would
bear its ratable share of the real estate investment trust's expenses, including
its advisory and administration fees. At the same time the Fund would continue
to pay its own investment management fees and other expenses, as a result of
which the Fund and its shareholders in effect will be absorbing duplicate levels
of fees with respect to investments in real estate investment trusts.
 
    RIGHTS AND WARRANTS.  The Fund may acquire rights and/or warrants which are
attached to other securities in its portfolio, or which are issued as a
distribution by the issuer of a security held in its portfolio. Rights and/or
warrants are, in effect, options to purchase equity securities at a specific
 
                                       13
<PAGE>
price, generally valid for a specific period of time, and have no voting rights,
pay no dividends and have no rights with respect to the corporation issuing
them.
 
    REPURCHASE AGREEMENTS.  The Fund may enter into repurchase agreements, which
may be viewed as a type of secured lending by the Fund, and which typically
involve the acquisition by the Fund of government securities or other securities
from a selling financial institution such as a bank, savings and loan
association or broker-dealer. The agreement provides that the Fund will sell
back to the institution, and that the institution will repurchase, the
underlying security at a specified price and at a fixed time in the future,
usually not more than seven days from the date of purchase. While repurchase
agreements involve certain risks not associated with direct investments in debt
securities, including the risks of default or bankruptcy of the selling
financial institution, the Fund follows procedures to minimize such risks. These
procedures include effecting repurchase transactions only with large,
well-capitalized and well-established financial institutions and maintaining
adequate collateralization.
 
    LOWER-RATED SECURITIES.  Because of the special nature of the Fund's
investments in lower rated securities (certain lower rated securities in which
the Fund may invest are commonly known as junk bonds), the Investment Manager
must take account of certain special considerations in assessing the risks
associated with such investments. For example, as the lower rated securities
market is relatively new, its growth had paralleled a long economic expansion
and, until recently, it had not faced adverse economic and market conditions.
Therefore, an economic downturn or increase in interest rates is likely to have
a negative effect on this market and on the value of the lower rated securities
held by the Fund, as well as on the ability of the securities' issuers to repay
principal and interest on their borrowings.
 
    The prices of lower rated securities have been found to be less sensitive to
changes in prevailing interest rates than higher rated investments, but are
likely to be more sensitive to adverse economic changes or individual corporate
developments. During an economic downturn or substantial period of rising
interest rates, highly leveraged issuers may experience financial stress which
would adversely effect their ability to service their principal and interest
payment obligations, to meet their projected business goals or to obtain
additional financing. If the issuer of a fixed-income security owned by the Fund
defaults, the Fund may incur additional expenses to seek recovery. In addition,
periods of economic uncertainty and change can be expected to result in an
increased volatility of market prices of lower rated securities and a
concomitant volatility in the net asset value of a share of the Fund. Moreover,
the market prices of certain of the Fund's portfolio securities which are
structured as zero coupon and payment-in-kind securities are affected to a
greater extent by interest rate changes and thereby tend to be more volatile
than securities which pay interest periodically and in cash (see "Dividends,
Distributions and Taxes" for a discussion of the tax ramifications of
investments in such securities).
 
    The secondary market for lower rated securities may be less liquid than the
markets for higher quality securities and, as such, may have an adverse affect
on the market prices of certain securities. The limited liquidity of the market
may also adversely affect the ability of the Fund's Trustees to arrive at a fair
value for certain lower rated securities at certain times and should make it
difficult for the Fund to sell certain securities. In addition, new laws and
proposed new laws may have an adverse effect upon the value of lower rated
securities and a concomitant negative impact upon the net asset value of a share
of the Fund.
 
    During the fiscal year ended September 30, 1998, the monthly dollar weighted
average ratings of the debt obligations held by the Fund, expressed as a
percentage of the Fund's total investments were as follows:
 
                                       14
<PAGE>
 
<TABLE>
<CAPTION>
                              PERCENTAGE OF
RATINGS                     TOTAL INVESTMENTS
- -------------------------  --------------------
<S>                        <C>
AAA/Aaa..................            5.8%
AA/Aa....................            0.2%
A/A......................            4.3%
BBB/Baa..................            3.8%
BB/Ba....................            7.3%
B/B......................           24.2%
CCC/Caa..................            0.6%
CC/Ca....................            0.1%
C/C......................            0.0%
Unrated..................           15.7%
</TABLE>
 
    OPTIONS AND FUTURES TRANSACTIONS.  The Fund is permitted to enter into call
and put options on U.S. Treasury notes, bonds and bills and equity securities
which are listed on Exchanges and are written in over-the-counter transactions
("OTC options"). Listed options are issued by the Options Clearing Corporation.
OTC options are purchased from or sold (written) to dealers or financial
institutions which have entered into direct agreements with the Fund. The Fund
is permitted to write covered call options on portfolio securities, in an amount
not exceeding 20% of the value of its total assets, in order to aid it in
achieving its investment objective.
 
    The Fund may purchase listed and OTC call and put options in amounts
equalling up to 5% of its total assets. The Fund may purchase call options only
in order to close out a covered call position. The Fund may purchase put options
on securities which it holds (or has the right to acquire) in its portfolio only
to protect itself against a decline in the value of the security. The Fund may
also purchase put options to close out written put positions. There are no other
limits on the Fund's ability to purchase call and put options.
 
    The Fund may purchase and sell financial futures contracts ("futures
contracts") that are traded on U.S. commodity exchanges on such underlying
securities as U.S. Treasury bonds, notes, and bills. The Fund may invest in
financial futures contracts only as a hedge against anticipated interest rate
changes.
 
    The Fund may also purchase and write call and put options on futures
contracts which are traded on an Exchange and enter into closing transactions
with respect to such options to terminate an existing position. The Fund will
purchase and write options on futures contracts for identical purposes to those
set forth above for the purchase of a futures contract and the sale of a futures
contract or to close out a long or short position in futures contracts.
 
    The Fund may not enter into futures contracts or purchase related options
thereon if, immediately thereafter, the amount committed to initial margin plus
the amount paid for premiums for unexpired options on futures contracts exceeds
5% of the value of the Fund's total assets, after taking into account unrealized
gains and unrealized losses on such contracts it has entered into, provided,
however, that in the case of an option that is in-the-money (the exercise price
of the call (put) option is less (more) than the market price of the underlying
security) at the time of purchase, the in-the-money amount may be excluded in
calculating the 5%. Moreover, the Fund may only buy and write options which are
listed on national securities exchanges and may not purchase options if, as a
result, the aggregate cost of all outstanding options exceeds 10% of the Fund's
total assets. In addition, the Fund may not purchase or sell futures contracts
or related options thereon if, immediately thereafter, more than one-third of
its net assets would be hedged.
 
    RISKS OF OPTIONS AND FUTURES TRANSACTIONS. The Fund may close out its
position as writer of an option, or as a buyer or seller of a futures contract
only if a liquid secondary market exists for options or futures contracts of
that series. There is no assurance that such a market will exist particularly in
the case of OTC options, as such options generally will only be closed out by
entering into a closing purchase transaction with the purchasing dealer. Also,
exchanges may limit the amount by which the price of many futures contracts may
move on any day. If the price moves equal the daily limit on successive days,
then it may prove impossible to liquidate a
 
                                       15
<PAGE>
futures position until the daily limit moves have ceased.
 
    While the futures contracts and options transactions to be engaged in by the
Fund for the purpose of hedging the Fund's portfolio securities are not
speculative in nature, there are risks inherent in the use of such instruments.
One such risk is that the Investment Manager could be incorrect in its
expectations as to the direction or extent of various interest rate or price
movements or the time span within which the movements take place. For example,
if the Fund sold futures contracts for the sale of securities in anticipation of
an increase in interest rates, and then interest rates went down instead,
causing bond prices to rise, the Fund would lose money on the sale. Another risk
which may arise in employing futures contracts to protect against the price
volatility of portfolio securities is that the prices of securities and indices
subject to futures contracts (and thereby the futures contract prices) may
correlate imperfectly with the behavior of the cash prices of the Fund's
portfolio securities. See the Statement of Additional Information for further
discussion of such risks.
 
    YEAR 2000.  The investment management services provided to the Fund by the
Investment Manager and the services provided to shareholders by the Distributor
and the Transfer Agent depend on the smooth functioning of their computer
systems. Many computer software systems in use today cannot recognize the year
2000, but revert to 1900 or some other date, due to the manner in which dates
were encoded and calculated. That failure could have a negative impact on the
handling of securities trades, pricing and account services. The Investment
Manager, the Distributor and the Transfer Agent have been actively working on
necessary changes to their own computer systems to prepare for the year 2000 and
expect that their systems will be adapted before that date, but there can be no
assurance that they will be successful, or that interaction with other
non-complying computer systems will not impair their services at that time.
 
    In addition, it is possible that the markets for securities in which the
Fund invests may be detrimentally affected by computer failures throughout the
financial services industry beginning January 1, 2000. Improperly functioning
trading systems may result in settlement problems and liquidity issues. In
addition, corporate and governmental data processing errors may result in
production problems for individual companies and overall economic uncertainties.
Earnings of individual issuers will be affected by remediation costs, which may
be substantial and may be reported inconsistently in U.S. and foreign financial
statements. Accordingly, the Fund's investments may be adversely affected.
 
PORTFOLIO MANAGEMENT
 
   
    The Fund's portfolio is actively managed by its Investment Manager with a
view to achieving the Fund's investment objective. In determining which
securities to purchase for the Fund or hold in the Fund's portfolio, the
Investment Manager will rely on information from various sources, including
rating agencies, research, analysis and appraisals of brokers and dealers,
including Dean Witter Reynolds Inc., Morgan Stanley & Co. Incorporated and other
broker-dealers that are affiliates of the Investment Manager, and the Investment
Manager's own analysis of factors it deems relevant. The Fund's portfolio is
managed within MSDW Advisors' Growth and Income Group, which manages 20 funds
and fund portfolios with approximately $32.9 billion in assets at October 31,
1998. Peter M. Avelar, Senior Vice President of MSDW Advisors and a member of
MSDW Advisors' Growth and Income Group, has been the primary portfolio manager
of the Fund since January 1998 and has been assisted by Ellen Gold since
November 1998. Mr. Avelar has been a portfolio manager with MSDW Advisors for
over five years. Ms. Gold has been a Vice President of MSDW Advisors since May
1998 and prior thereto was an Assistant Vice President and Junior Portfolio
Manager with MSDW Advisors.
    
 
    Orders for transactions in portfolio securities are placed for the Fund with
a number of brokers and dealers, including Dean Witter Reynolds Inc., Morgan
Stanley & Co. Incorporated and other brokers and dealers that are affiliates of
the Investment Manager. The Fund may incur brokerage commissions on transactions
conducted through such affiliates. Pursuant to an order of the Securities and
 
                                       16
<PAGE>
Exchange Commission, the Fund may effect principal transactions in certain money
market instruments with Dean Witter Reynolds Inc.
 
    The portfolio trading engaged in by the Fund may result in its portfolio
turnover rate exceeding 100%. The Fund is expected to incur higher than normal
brokerage commission costs due to its portfolio turnover rate. Short-term gains
and losses taxable at ordinary income rates may result from such portfolio
transactions. See "Dividends, Distributions and Taxes" for a full discussion of
the tax implications of the Fund's trading policy. A more extensive discussion
of the Fund's portfolio brokerage policies is set forth in the Statement of
Additional Information.
 
    Except as specifically noted, all investment objectives, policies and
practices discussed above are not fundamental policies of the Fund and, as such,
may be changed without shareholder approval.
 
INVESTMENT RESTRICTIONS
- --------------------------------------------------------------------------------
 
    The investment restrictions listed below are among the restrictions that
have been adopted by the Fund as fundamental policies. Under the Investment
Company Act of 1940, as amended (the "Act"), a fundamental policy may not be
changed without the vote of a majority of the outstanding voting securities of
the Fund, as defined in the Act.
 
    The Fund may not:
 
   1. Invest more than 5% of the value of its total assets in the securities of
any one issuer (other than obligations issued or guaranteed by the United States
Government, its agencies or instrumentalities).
 
   2. Purchase more than 10% of all outstanding voting securities or any class
of securities of any one issuer. For purposes of compliance with this
restriction, the Fund will not invest in the convertible securities of any one
issuer if, upon conversion of such securities, the Fund would hold more than 10%
of the outstanding voting securities of that issuer.
 
   3. Invest more than 25% of the value of its total assets in securities of
issuers in any one industry. This restriction does not apply to obligations
issued or guaranteed by the United States Government or its agencies or
instrumentalities.
 
   4. Invest more than 5% of the value of its total assets in securities of
issuers having a record, together with predecessors, of less than three years of
continuous operation. This restriction shall not apply to any obligation of the
United States Government, its agencies or instrumentalities.
 
   5. Borrow money, except that the Fund may borrow from a bank for temporary or
emergency purposes in amounts not exceeding 5% (taken at the lower of cost or
current value) of the value of its total assets (not including the amount
borrowed).
 
   6. Invest more than 5% of the value of its total assets in warrants,
including not more than 2% of such assets in warrants not listed on either the
New York or American Stock Exchange. However, the acquisition of warrants
attached to other securities is not subject to this restriction.
 
    If a percentage restriction is adhered to at the time of investment, a later
increase or decrease in percentage resulting from a change in values of
portfolio securities or amount of total or net assets will not be considered a
violation of any of the foregoing restrictions.
 
    Notwithstanding any other investment policy or restriction, the Fund may
seek to achieve its investment objective by investing all or substantially all
of its assets in another investment company having substantially the same
investment objective and policies as the Fund.
 
                                       17
<PAGE>
PURCHASE OF FUND SHARES
- --------------------------------------------------------------------------------
 
GENERAL
 
    The Fund offers each class of its shares for sale to the public on a
continuous basis. Pursuant to a Distribution Agreement between the Fund and
Morgan Stanley Dean Witter Distributors Inc. ("MSDW Distributors" or the
"Distributor"), an affiliate of the Investment Manager, shares of the Fund are
distributed by the Distributor and offered by Dean Witter Reynolds Inc. ("DWR"),
a selected dealer and subsidiary of Morgan Stanley Dean Witter & Co., and other
dealers who have entered into selected dealer agreements with the Distributor
("Selected Broker-Dealers"). It is anticipated that DWR will undergo a change of
corporate name which is expected to incorporate the brand name of "Morgan
Stanley Dean Witter," pending approval of various regulatory authorities. The
principal executive office of the Distributor is located at Two World Trade
Center, New York, New York 10048.
 
    The Fund offers four classes of shares (each, a "Class"). Class A shares are
sold to investors with an initial sales charge that declines to zero for larger
purchases; however, Class A shares sold without an initial sales charge are
subject to a contingent deferred sales charge ("CDSC") of 1.0% if redeemed
within one year of purchase, except for certain specific circumstances. Class B
shares are sold without an initial sales charge but are subject to a CDSC
(scaled down from 5.0% to 1.0%) payable upon most redemptions within six years
after purchase. (Class B shares purchased by certain qualified plans are subject
to a CDSC scaled down from 2.0% to 1.0% if redeemed within three years after
purchase.) Class C shares are sold without an initial sales charge but are
subject to a CDSC of 1.0% on most redemptions made within one year after
purchase. Class D shares are sold without an initial sales charge or CDSC and
are available only to investors meeting an initial investment minimum of $5
million ($25 million for certain qualified plans), and to certain other limited
categories of investors. At the discretion of the Board of Trustees of the Fund,
Class A shares may be sold to categories of investors in addition to those set
forth in this prospectus at net asset value without a front-end sales charge,
and Class D shares may be sold to certain other categories of investors, in each
case as may be described in the then current prospectus of the Fund. See
"Alternative Purchase Arrangements-- Selecting a Particular Class" for a
discussion of factors to consider in selecting which Class of shares to
purchase.
 
    The minimum initial purchase is $1,000 for each Class of shares, although
Class D shares are only available to persons investing $5 million ($25 million
for certain qualified plans) or more and to certain other limited categories of
investors. For the purpose of meeting the minimum $5 million (or $25 million)
initial investment for Class D shares, and subject to the $1,000 minimum initial
investment for each Class of the Fund, an investor's existing holdings of Class
A and Class D shares of the Fund and other Morgan Stanley Dean Witter Funds that
are multiple class funds ("Morgan Stanley Dean Witter Multi-Class Funds") and
shares of Morgan Stanley Dean Witter Funds sold with a front-end sales charge
("FSC Funds") and concurrent investments in Class D shares of the Fund and other
Morgan Stanley Dean Witter Multi-Class Funds will be aggregated. Subsequent
purchases of $100 or more may be made by sending a check, payable to Morgan
Stanley Dean Witter Convertible Securities Trust, directly to Morgan Stanley
Dean Witter Trust FSB (the "Transfer Agent" or "MSDW Trust") at P.O. Box 1040,
Jersey City, NJ 07303 or by contacting a Morgan Stanley Dean Witter Financial
Advisor or other Selected Broker-Dealer representative. When purchasing shares
of the Fund, investors must specify whether the purchase is for Class A, Class
B, Class C or Class D shares. If no Class is specified, the Transfer Agent will
not process the transaction
 
                                       18
<PAGE>
until the proper Class is identified. The minimum initial purchase in the case
of investments through EasyInvest-SM-, an automatic purchase plan (see
"Shareholder Services"), is $100, provided that the schedule of automatic
investments will result in investments totalling $1,000 within the first twelve
months. The minimum initial purchase in the case of an "Education IRA" is $500,
if the Distributor has reason to believe that additional investments will
increase the investment in the account to $1,000 within three years. In the case
of investments pursuant to (i) Systematic Payroll Deduction Plans (including
Individual Retirement Plans), (ii) the MSDW Advisors mutual fund asset
allocation program and (iii) fee-based programs approved by the Distributor,
pursuant to which participants pay an asset based fee for services in the nature
of investment advisory, administrative and/or brokerage services, the Fund, in
its discretion, may accept investments without regard to any minimum amounts
which would otherwise be required, provided, in the case of Systematic Payroll
Deduction Plans, that the Distributor has reason to believe that additional
investments will increase the investment in all accounts under such Plans to at
least $1,000. Certificates for shares purchased will not be issued unless
requested by the shareholder in writing to the Transfer Agent.
 
    Shares of the Fund are sold through the Distributor on a normal three
business day settlement basis; that is, payment is due on the third business day
(settlement date) after the order is placed with the Distributor. Since DWR and
other Selected Broker-Dealers forward investors' funds on settlement date, they
will benefit from the temporary use of the funds if payment is made prior
thereto. As noted above, orders placed directly with the Transfer Agent must be
accompanied by payment. Investors will be entitled to receive income dividends
and capital gains distributions if their order is received by the close of
business on the day prior to the record date for such dividends and
distributions. Sales personnel of a Selected Broker-Dealer are compensated for
selling shares of the Fund by the Distributor or any of its affiliates and/or
the Selected Broker-Dealer. In addition, some sales personnel of the Selected
Broker-Dealer will receive various types of non-cash compensation as special
sales incentives, including trips, educational and/or business seminars and
merchandise. The Fund and the Distributor reserve the right to reject any
purchase orders.
 
ALTERNATIVE PURCHASE ARRANGEMENTS
 
    The Fund offers several Classes of shares to investors designed to provide
them with the flexibility of selecting an investment best suited to their needs.
The general public is offered three Classes of shares: Class A shares, Class B
shares and Class C shares, which differ principally in terms of sales charges
and rate of expenses to which they are subject. A fourth Class of shares, Class
D shares, is offered only to limited categories of investors (see "No Load
Alternative--Class D Shares" below).
 
    Each Class A, Class B, Class C or Class D share of the Fund represents an
identical interest in the investment portfolio of the Fund except that Class A,
Class B and Class C shares bear the expenses of the ongoing shareholder service
fees, Class B and Class C shares bear the expenses of the ongoing distribution
fees and Class A, Class B and Class C shares which are redeemed subject to a
CDSC bear the expense of the additional incremental distribution costs resulting
from the CDSC applicable to shares of those Classes. The ongoing distribution
fees that are imposed on Class A, Class B and Class C shares will be imposed
directly against those Classes and not against all assets of the Fund and,
accordingly, such charges against one Class will not affect the net asset value
of any other Class or have any impact on investors choosing another sales charge
option. See "Plan of Distribution" and "Redemptions and Repurchases."
 
    Set forth below is a summary of the differences between the Classes and the
factors an
 
                                       19
<PAGE>
investor should consider when selecting a particular Class. This summary is
qualified in its entirety by detailed discussion of each Class that follows this
summary.
 
    CLASS A SHARES.  Class A shares are sold at net asset value plus an initial
sales charge of up to 5.25%. The initial sales charge is reduced for certain
purchases. Investments of $1 million or more (and investments by certain other
limited categories of investors) are not subject to any sales charges at the
time of purchase but are subject to a CDSC of 1.0% on redemptions made within
one year after purchase, except for certain specific circumstances. Class A
shares are also subject to a 12b-1 fee of up to 0.25% of the average daily net
assets of the Class. See "Initial Sales Charge Alternative--Class A Shares."
 
    CLASS B SHARES.  Class B shares are offered at net asset value with no
initial sales charge but are subject to a CDSC (scaled down from 5.0% to 1.0%)
if redeemed within six years of purchase. (Class B shares purchased by certain
qualified plans are subject to a CDSC scaled down from 2.0% to 1.0% if redeemed
within three years after purchase.) This CDSC may be waived for certain
redemptions. Class B shares are also subject to an annual 12b-1 fee of 1.0% of
the lesser of: (a) the average daily aggregate gross sales of the Fund's Class B
shares since the inception of the Fund (not including reinvestments of dividends
or capital gains distributions), less the average daily aggregate net asset
value of the Fund's Class B shares redeemed since the Fund's inception upon
which a CDSC has been imposed or waived, or (b) the average daily net assets of
Class B. The Class B shares' distribution fee will cause that Class to have
higher expenses and pay lower dividends than Class A or Class D shares.
 
    After approximately ten (10) years, Class B shares will convert
automatically to Class A shares of the Fund, based on the relative net asset
values of the shares of the two Classes on the conversion date. In addition, a
certain portion of Class B shares that have been acquired through the
reinvestment of dividends and distributions will be converted at that time. See
"Contingent Deferred Sales Charge Alternative--Class B Shares."
 
    CLASS C SHARES.  Class C shares are sold at net asset value with no initial
sales charge but are subject to a CDSC of 1.0% on redemptions made within one
year after purchase. This CDSC may be waived for certain redemptions. They are
subject to an annual 12b-1 fee of up to 1.0% of the average daily net assets of
the Class C shares. The Class C shares' distribution fee may cause that Class to
have higher expenses and pay lower dividends than Class A or Class D shares. See
"Level Load Alternative--Class C Shares."
 
    CLASS D SHARES.  Class D shares are available only to limited categories of
investors (see "No Load Alternative--Class D Shares" below). Class D shares are
sold at net asset value with no initial sales charge or CDSC. They are not
subject to any 12b-1 fees. See "No Load Alternative-- Class D Shares."
 
    SELECTING A PARTICULAR CLASS.  In deciding which Class of Fund shares to
purchase, investors should consider the following factors, as well as any other
relevant facts and circumstances:
 
    The decision as to which Class of shares is more beneficial to an investor
depends on the amount and intended length of his or her investment. Investors
who prefer an initial sales charge alternative may elect to purchase Class A
shares. Investors qualifying for significantly reduced or, in the case of
purchases of $1 million or more, no initial sales charges may find Class A
shares particularly attractive because similar sales charge reductions are not
available with respect to Class B or Class C shares. Moreover, Class A shares
are subject to lower ongoing expenses than are Class B or Class C shares over
the term of the investment. As an alternative, Class B and Class C shares are
sold without any initial sales charge so the entire purchase price is
immediately
 
                                       20
<PAGE>
invested in the Fund. Any investment return on these additional investment
amounts may partially or wholly offset the higher annual expenses of these
Classes. Because the Fund's future return cannot be predicted, however, there
can be no assurance that this would be the case.
 
    Finally, investors should consider the effect of the CDSC period and any
conversion rights of the Classes in the context of their own investment time
frame. For example, although Class C shares are subject to a significantly lower
CDSC upon redemptions, they do not, unlike Class B shares, convert into Class A
shares after approximately ten years, and, therefore, are subject to an ongoing
12b-1 fee of 1.0% (rather than the 0.25% fee applicable to Class A shares) for
an indefinite period of time. Thus, Class B shares may be more attractive than
Class C shares to investors with longer term investment outlooks. Other
investors, however, may elect to purchase Class C shares if, for example, they
determine that they do not wish to be subject to a front-end sales charge and
they are uncertain as to the length of time they intend to hold their shares.
 
    For the purpose of meeting the $5 million (or $25 million) minimum
investment amount for Class D shares, holdings of Class A and Class D shares in
all Morgan Stanley Dean Witter Multi-Class Funds, shares of FSC Funds and shares
of Morgan Stanley Dean Witter Funds for which such shares have been exchanged
will be included together with the current investment amount.
 
    Sales personnel may receive different compensation for selling each Class of
shares. Investors should understand that the purpose of a CDSC is the same as
that of the initial sales charge in that the sales charges applicable to each
Class provide for the financing of the distribution of shares of that Class.
 
    Set forth below is a chart comparing the sales charge, 12b-1 fees and
conversion options applicable to each Class of shares:
 
<TABLE>
<CAPTION>
- -----------------------------------------------------------
                                              CONVERSION
  CLASS       SALES CHARGE     12b-1 FEE       FEATURE
<C>        <S>                 <C>         <C>
- -----------------------------------------------------------
    A      Maximum 5.25%         0.25%            No
           initial sales
           charge reduced for
           purchases of
           $25,000 and over;
           shares sold
           without an initial
           sales charge
           generally subject
           to a 1.0% CDSC
           during first year.
- -----------------------------------------------------------
    B      Maximum 5.0% CDSC      1.0%     B shares convert
           during the first                to A shares
           year decreasing to              automatically
           0 after six years               after
                                           approximately
                                           ten years
- -----------------------------------------------------------
    C      1.0% CDSC during       1.0%            No
           first year
- -----------------------------------------------------------
    D             None            None            No
</TABLE>
 
    See "Purchase of Fund Shares" and "The Fund and its Management" for a
complete description of the sales charges and service and distribution fees for
each Class of shares and "Determination of Net Asset Value," "Dividends,
Distributions and Taxes" and "Shareholder Services--Exchange Privilege" for
other differences between the Classes of shares.
 
INITIAL SALES CHARGE ALTERNATIVE--
CLASS A SHARES
 
    Class A shares are sold at net asset value plus an initial sales charge. In
some cases, reduced sales charges may be available, as described below.
Investments of $1 million or more (and investments by certain other limited
categories of investors) are not subject to any sales charges at the time of
purchase but are subject to a CDSC of 1.0% on redemptions made within one year
after purchase (calculated from the last day of the month in which the shares
were purchased), except for certain specific circumstances. The CDSC will be
assessed on an amount equal to the lesser of the current market
 
                                       21
<PAGE>
value or the cost of the shares being redeemed. The CDSC will not be imposed (i)
in the circumstances set forth below in the section "Contingent Deferred Sales
Charge Alternative--Class B Shares--CDSC Waivers," except that the references to
six years in the first paragraph of that section shall mean one year in the case
of Class A shares, and (ii) in the circumstances identified in the section
"Additional Net Asset Value Purchase Options" below. Class A shares are also
subject to an annual 12b-1 fee of up to 0.25% of the average daily net assets of
the Class.
 
    The offering price of Class A shares will be the net asset value per share
next determined following receipt of an order (see "Determination of Net Asset
Value" below), plus a sales charge (expressed as a percentage of the offering
price) on a single transaction as shown in the following table:
 
<TABLE>
<CAPTION>
                                          SALES CHARGE
                           ------------------------------------------
                              PERCENTAGE OF          APPROXIMATE
        AMOUNT OF            PUBLIC OFFERING    PERCENTAGE OF AMOUNT
   SINGLE TRANSACTION             PRICE               INVESTED
- -------------------------  -------------------  ---------------------
<S>                        <C>                  <C>
Less than $25,000........           5.25%                 5.44%
$25,000 but less
     than $50,000........           4.75%                 4.99%
$50,000 but less
     than $100,000.......           4.00%                 4.17%
$100,000 but less
     than $250,000.......           3.00%                 3.09%
$250,000 but less
     than $1 million.....           2.00%                 2.04%
$1 million and over......              0                     0
</TABLE>
 
    Upon notice to all Selected Broker-Dealers, the Distributor may reallow up
to the full applicable sales charge as shown in the above schedule during
periods specified in such notice. During periods when 90% or more of the sales
charge is reallowed, such Selected Broker-Dealers may be deemed to be
underwriters as that term is defined in the Securities Act of 1933.
 
    The above schedule of sales charges is applicable to purchases in a single
transaction by, among others: (a) an individual; (b) an individual, his or her
spouse and their children under the age of 21 purchasing shares for his, her or
their own accounts; (c) a trustee or other fiduciary purchasing shares for a
single trust estate or a single fiduciary account; (d) a pension, profit-sharing
or other employee benefit plan qualified or non-qualified under Section 401 of
the Internal Revenue Code; (e) tax-exempt organizations enumerated in Section
501(c)(3) or (13) of the Internal Revenue Code; (f) employee benefit plans
qualified under Section 401 of the Internal Revenue Code of a single employer or
of employers who are "affiliated persons" of each other within the meaning of
Section 2(a)(3)(c) of the Act; and for investments in Individual Retirement
Accounts of employees of a single employer through Systematic Payroll Deduction
plans; or (g) any other organized group of persons, whether incorporated or not,
provided the organization has been in existence for at least six months and has
some purpose other than the purchase of redeemable securities of a registered
investment company at a discount.
 
    COMBINED PURCHASE PRIVILEGE.  Investors may have the benefit of reduced
sales charges in accordance with the above schedule by combining purchases of
Class A shares of the Fund in single transactions with the purchase of Class A
shares of other Morgan Stanley Dean Witter Multi-Class Funds and shares of FSC
Funds. The sales charge payable on the purchase of the Class A shares of the
Fund, the Class A shares of the other Morgan Stanley Dean Witter Multi-Class
Funds and the shares of the FSC Funds will be at their respective rates
applicable to the total amount of the combined concurrent purchases of such
shares.
 
    RIGHT OF ACCUMULATION.  The above persons and entities may benefit from a
reduction of the sales charges in accordance with the above schedule if the
cumulative net asset value of Class A shares purchased in a single transaction,
 
                                       22
<PAGE>
together with shares of the Fund and other Morgan Stanley Dean Witter Funds
previously purchased at a price including a front-end sales charge (including
shares of the Fund and other Morgan Stanley Dean Witter Funds acquired in
exchange for those shares, and including in each case shares acquired through
reinvestment of dividends and distributions), which are held at the time of such
transaction, amounts to $25,000 or more. If such investor has a cumulative net
asset value of shares of FSC Funds and Class A and Class D shares that, together
with the current investment amount, is equal to at least $5 million ($25 million
for certain qualified plans), such investor is eligible to purchase Class D
shares subject to the $1,000 minimum initial investment requirement of that
Class of the Fund. See "No Load Alternative--Class D Shares" below.
 
    The Distributor must be notified by DWR or a Selected Broker-Dealer or the
shareholder at the time a purchase order is placed that the purchase qualifies
for the reduced charge under the Right of Accumulation. Similar notification
must be made in writing by the dealer or shareholder when such an order is
placed by mail. The reduced sales charge will not be granted if: (a) such
notification is not furnished at the time of the order; or (b) a review of the
records of the Selected Broker-Dealer or the Transfer Agent fails to confirm the
investor's represented holdings.
 
    LETTER OF INTENT.  The foregoing schedule of reduced sales charges will also
be available to investors who enter into a written Letter of Intent providing
for the purchase, within a thirteen-month period, of Class A shares of the Fund
from DWR or other Selected Broker-Dealers. The cost of Class A shares of the
Fund or shares of other Morgan Stanley Dean Witter Funds which were previously
purchased at a price including a front-end sales charge during the 90-day period
prior to the date of receipt by the Distributor of the Letter of Intent, or of
Class A shares of the Fund or shares of other Morgan Stanley Dean Witter Funds
acquired in exchange for shares of such funds purchased during such period at a
price including a front-end sales charge, which are still owned by the
shareholder, may also be included in determining the applicable reduction.
 
    ADDITIONAL NET ASSET VALUE PURCHASE OPTIONS. In addition to investments of
$1 million or more, Class A shares also may be purchased at net asset value by
the following:
 
    (1) trusts for which MSDW Trust (which is an affiliate of the Investment
Manager) provides discretionary trustee services;
 
    (2) persons participating in a fee-based program approved by the
Distributor, pursuant to which such persons pay an asset based fee for services
in the nature of investment advisory, administrative and/or brokerage services
(such investments are subject to all of the terms and conditions of such
programs, which may include termination fees, mandatory redemption upon
termination and such other circumstances as specified in the programs'
agreements, and restrictions on transferability of Fund shares);
 
    (3) employer-sponsored 401(k) and other plans qualified under Section 401(a)
of the Internal Revenue Code ("Qualified Retirement Plans") with at least 200
eligible employees and for which MSDW Trust serves as Trustee or DWR's
Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement;
 
    (4) Qualified Retirement Plans for which MSDW Trust serves as Trustee or
DWR's Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement whose Class B shares have converted to Class A
shares, regardless of the plan's asset size or number of eligible employees;
 
    (5) investors who are clients of a Morgan Stanley Dean Witter Financial
Advisor who joined
 
                                       23
<PAGE>
Morgan Stanley Dean Witter from another investment firm within six months prior
to the date of purchase of Fund shares by such investors, if the shares are
being purchased with the proceeds from a redemption of shares of an open-end
proprietary mutual fund of the Financial Advisor's previous firm which imposed
either a front-end or deferred sales charge, provided such purchase was made
within sixty days after the redemption and the proceeds of the redemption had
been maintained in the interim in cash or a money market fund; and
 
    (6) other categories of investors, at the discretion of the Board, as
disclosed in the then current prospectus of the Fund.
 
    No CDSC will be imposed on redemptions of shares purchased pursuant to
paragraphs (1), (2) or (5), above.
 
    For further information concerning purchases of the Fund's shares, contact
DWR or another Selected Broker-Dealer or consult the Statement of Additional
Information.
 
CONTINGENT DEFERRED SALES CHARGE
ALTERNATIVE--CLASS B SHARES
 
    Class B shares are sold at net asset value next determined without an
initial sales charge so that the full amount of an investor's purchase payment
may be immediately invested in the Fund. A CDSC, however, will be imposed on
most Class B shares redeemed within six years after purchase. The CDSC will be
imposed on any redemption of shares if after such redemption the aggregate
current value of a Class B account with the Fund falls below the aggregate
amount of the investor's purchase payments for Class B shares made during the
six years (or, in the case of shares held by certain Qualified Retirement Plans,
three years) preceding the redemption. In addition, Class B shares are subject
to an annual 12b-1 fee of 1.0% of the lesser of: (a) the average daily aggregate
gross sales of the Fund's Class B shares since the inception of the Fund (not
including reinvestments of dividends or capital gains distributions), less the
average daily aggregate net asset value of the Fund's Class B shares redeemed
since the Fund's inception upon which a CDSC has been imposed or waived, or (b)
the average daily net assets of Class B.
 
    Except as noted below, Class B shares of the Fund which are held for six
years or more after purchase (calculated from the last day of the month in which
the shares were purchased) will not be subject to any CDSC upon redemption.
Shares redeemed earlier than six years after purchase may, however, be subject
to a CDSC which will be a percentage of the dollar amount of shares redeemed and
will be assessed on an amount equal to the lesser of the current market value or
the cost of the shares being redeemed. The size of this percentage will depend
upon how long the shares have been held, as set forth in the following table:
 
<TABLE>
<CAPTION>
         YEAR SINCE PURCHASE            CDSC AS A PERCENTAGE
             PAYMENT MADE                OF AMOUNT REDEEMED
- --------------------------------------  ---------------------
<S>                                     <C>
First.................................          5.0%
Second................................          4.0%
Third.................................          3.0%
Fourth................................          2.0%
Fifth.................................          2.0%
Sixth.................................          1.0%
Seventh and thereafter................          None
</TABLE>
 
    In the case of Class B shares of the Fund purchased on or after July 28,
1997 by Qualified Retirement Plans for which MSDW Trust serves as Trustee or
DWR's Retirement Plan Services serves as recordkeeper pursuant to written
Recordkeeping Services Agreement, shares held for three years or more after
purchase (calculated as described in the paragraph above) will not be subject to
any CDSC upon redemption. However, shares redeemed earlier than three years
after purchase may be subject to a CDSC (calculated as described in the
paragraph above), the percentage of which will depend on how long the
 
                                       24
<PAGE>
shares have been held, as set forth in the following table:
 
<TABLE>
<CAPTION>
         YEAR SINCE PURCHASE            CDSC AS A PERCENTAGE
             PAYMENT MADE                OF AMOUNT REDEEMED
- --------------------------------------  ---------------------
<S>                                     <C>
First.................................          2.0%
Second................................          2.0%
Third.................................          1.0%
Fourth and thereafter.................          None
</TABLE>
 
    CDSC WAIVERS.  A CDSC will not be imposed on: (i) any amount which
represents an increase in value of shares purchased within the six years (or, in
the case of shares held by certain Qualified Retirement Plans, three years)
preceding the redemption; (ii) the current net asset value of shares purchased
more than six years (or, in the case of shares held by certain Qualified
Retirement Plans, three years) prior to the redemption; and (iii) the current
net asset value of shares purchased through reinvestment of dividends or
distributions and/or shares acquired in exchange for shares of FSC Funds or of
other Morgan Stanley Dean Witter Funds acquired in exchange for such shares.
Moreover, in determining whether a CDSC is applicable it will be assumed that
amounts described in (i), (ii) and (iii) above (in that order) are redeemed
first. In addition, no CDSC will be imposed on redemptions of shares which are
attributable to reinvestment of dividends or distributions from, or the proceeds
of, certain Unit Investment Trusts.
 
    In addition, the CDSC, if otherwise applicable, will be waived in the case
of:
 
    (1) redemptions of shares held at the time a shareholder dies or becomes
disabled, only if the shares are:  (A) registered either in the name of an
individual shareholder (not a trust), or in the names of such shareholder and
his or her spouse as joint tenants with right of survivorship; or  (B) held in a
qualified corporate or self-employed retirement plan, Individual Retirement
Account ("IRA") or Custodial Account under Section 403(b)(7) of the Internal
Revenue Code ("403(b) Custodial Account"), provided in either case that the
redemption is requested within one year of the death or initial determination of
disability;
 
    (2) redemptions in connection with the following retirement plan
distributions:  (A) lump-sum or other distributions from a qualified corporate
or self-employed retirement plan following retirement (or, in the case of a "key
employee" of a "top heavy" plan, following attainment of age 59 1/2);  (B)
distributions from an IRA or 403(b) Custodial Account following attainment of
age 59 1/2; or  (C) a tax-free return of an excess contribution to an IRA;
 
    (3) all redemptions of shares held for the benefit of a participant in a
401(k) plan or other Qualified Retirement Plan which offers investment companies
managed by the Investment Manager or its subsidiary, MSDW Services, as
self-directed investment alternatives and for which MSDW Trust serves as Trustee
or DWR's Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement ("Eligible Plan"), provided that either:  (A)
the plan continues to be an Eligible Plan after the redemption; or  (B) the
redemption is in connection with the complete termination of the plan involving
the distribution of all plan assets to participants; and
 
    (4) certain redemptions pursuant to the Fund's Systematic Withdrawal Plan
(see "Shareholder Services--Systematic Withdrawal Plan").
 
    With reference to (1) above, for the purpose of determining disability, the
Distributor utilizes the definition of disability contained in Section 72(m)(7)
of the Internal Revenue Code, which relates to the inability to engage in
gainful employment. With reference to (2) above, the term "distribution" does
not encompass a direct transfer of IRA, 403(b) Custodial Account or retirement
plan assets to a successor custodian or trustee. All waivers will be granted
only following receipt by the Distributor of confirmation of the shareholder's
entitlement.
 
                                       25
<PAGE>
    CONVERSION TO CLASS A SHARES.  All shares of the Fund held prior to July 28,
1997 have been designated Class B shares. Shares held before May 1, 1997 will
convert to Class A shares in May, 2007. In all other instances Class B shares
will convert automatically to Class A shares, based on the relative net asset
values of the shares of the two Classes on the conversion date, which will be
approximately ten (10) years after the date of the original purchase. The ten
year period is calculated from the last day of the month in which the shares
were purchased or, in the case of Class B shares acquired through an exchange or
a series of exchanges, from the last day of the month in which the original
Class B shares were purchased, provided that shares originally purchased before
May 1, 1997 will convert to Class A shares in May, 2007. The conversion of
shares purchased on or after May 1, 1997 will take place in the month following
the tenth anniversary of the purchase. There will also be converted at that time
such proportion of Class B shares acquired through automatic reinvestment of
dividends and distributions owned by the shareholder as the total number of his
or her Class B shares converting at the time bears to the total number of
outstanding Class B shares purchased and owned by the shareholder. In the case
of Class B shares held by a Qualified Retirement Plan for which MSDW Trust
serves as Trustee or DWR's Retirement Plan Services serves as a recordkeeper
pursuant to a written Recordkeeping Services Agreement, the plan is treated as a
single investor and all Class B shares will convert to Class A shares on the
conversion date of the first shares of a Morgan Stanley Dean Witter Multi-Class
Fund purchased by that plan. In the case of Class B shares previously exchanged
for shares of an "Exchange Fund" (see "Shareholder Services--Exchange
Privilege"), the period of time the shares were held in the Exchange Fund
(calculated from the last day of the month in which the Exchange Fund shares
were acquired) is excluded from the holding period for conversion. If those
shares are subsequently re-exchanged for Class B shares of a Morgan Stanley Dean
Witter Multi-Class Fund, the holding period resumes on the last day of the month
in which Class B shares are reacquired.
 
    If a shareholder has received share certificates for Class B shares, such
certificates must be delivered to the Transfer Agent at least one week prior to
the date for conversion. Class B shares evidenced by share certificates that are
not received by the Transfer Agent at least one week prior to any conversion
date will be converted into Class A shares on the next scheduled conversion date
after such certificates are received.
 
    Effectiveness of the conversion feature is subject to the continuing
availability of a ruling of the Internal Revenue Service or an opinion of
counsel that (i) the conversion of shares does not constitute a taxable event
under the Internal Revenue Code, (ii) Class A shares received on conversion will
have a basis equal to the shareholder's basis in the converted Class B shares
immediately prior to the conversion, and (iii) Class A shares received on
conversion will have a holding period that includes the holding period of the
converted Class B shares. The conversion feature may be suspended if the ruling
or opinion is no longer available. In such event, Class B shares would continue
to be subject to Class B 12b-1 fees.
 
LEVEL LOAD ALTERNATIVE--CLASS C SHARES
 
    Class C shares are sold at net asset value next determined without an
initial sales charge but are subject to a CDSC of 1.0% on most redemptions made
within one year after purchase (calculated from the last day of the month in
which the shares were purchased). The CDSC will be assessed on an amount equal
to the lesser of the current market value or the cost of the shares being
redeemed. The CDSC will not be imposed in the circumstances set forth above in
the section "Contingent Deferred Sales Charge Alternative-- Class B Shares--CDSC
Waivers," except that the references to six years in the first paragraph of that
section shall mean one year in the case of Class C shares. Class C shares are
subject to an
 
                                       26
<PAGE>
annual 12b-1 fee of up to 1.0% of the average daily net assets of the Class.
Unlike Class B shares, Class C shares have no conversion feature and,
accordingly, an investor that purchases Class C shares will be subject to 12b-1
fees applicable to Class C shares for an indefinite period subject to annual
approval by the Fund's Board of Trustees and regulatory limitations.
 
NO LOAD ALTERNATIVE--CLASS D SHARES
 
    Class D shares are offered without any sales charge on purchase or
redemption and without any 12b-1 fee. Class D shares are offered only to
investors meeting an initial investment minimum of $5 million ($25 million for
Qualified Retirement Plans for which MSDW Trust serves as Trustee or DWR's
Retirement Plan Services serves as recordkeeper pursuant to a written
Recordkeeping Services Agreement) and the following categories of investors: (i)
investors participating in the MSDW Advisors mutual fund asset allocation
program pursuant to which such persons pay an asset based fee; (ii) persons
participating in a fee-based program approved by the Distributor, pursuant to
which such persons pay an asset based fee for services in the nature of
investment advisory, administrative and/or brokerage services (subject to all of
the terms and conditions of such programs referred to in (i) and (ii) above,
which may include termination fees, mandatory redemption, and such other
circumstances as specified in the programs' agreements and restrictions on
transferability of Fund shares); (iii) employee benefit plans maintained by
Morgan Stanley Dean Witter & Co. or any of its subsidiaries for the benefit of
certain employees of Morgan Stanley Dean Witter & Co. and its subsidiaries; (iv)
certain Unit Investment Trusts sponsored by DWR; (v) certain other open-end
investment companies whose shares are distributed by the Distributor; (vi)
investors who were shareholders of Dean Witter Retirement Series on September
11, 1998 (with respect to additional purchases for their former Dean Witter
Retirement Series accounts); and (vii) other categories of investors, at the
discretion of the Board, as disclosed in the then current prospectus of the
Fund. Investors who require a $5 million (or $25 million) minimum initial
investment to qualify to purchase Class D shares may satisfy that requirement by
investing that amount in a single transaction in Class D shares of the Fund and
other Morgan Stanley Dean Witter Multi-Class Funds, subject to the $1,000
minimum initial investment required for that Class of the Fund. In addition, for
the purpose of meeting the $5 million (or $25 million) minimum investment
amount, holdings of Class A and Class D shares in all Morgan Stanley Dean Witter
Multi-Class Funds, shares of FSC Funds and shares of Morgan Stanley Dean Witter
Funds for which such shares have been exchanged will be included together with
the current investment amount. If a shareholder redeems Class A shares and
purchases Class D shares, such redemption may be a taxable event.
 
PLAN OF DISTRIBUTION
 
    The Fund has adopted a Plan of Distribution pursuant to Rule 12b-1 under the
Act with respect to the distribution of Class A, Class B and Class C shares of
the Fund. In the case of Class A and Class C shares, the Plan provides that the
Fund will reimburse the Distributor and others for the expenses of certain
activities and services incurred by them specifically on behalf of those shares.
Reimbursements for these expenses will be made in monthly payments by the Fund
to the Distributor, which will in no event exceed amounts equal to payments at
the annual rates of 0.25% and 1.0% of the average daily net assets of Class A
and Class C, respectively. In the case of Class B shares, the Plan provides that
the Fund will pay the Distributor a fee, which is accrued daily and paid
monthly, at the annual rate of 1.0% of the lesser of: (a) the average daily
aggregate gross sales of the Fund's Class B shares since the inception of the
Fund (not including reinvestments of dividends or capital gains distributions),
less the average daily aggregate net asset value of the Fund's Class B shares
redeemed since the Fund's inception upon which a CDSC has been imposed
 
                                       27
<PAGE>
or waived, or (b) the average daily net assets of Class B. The fee is treated by
the Fund as an expense in the year it is accrued. In the case of Class A shares,
the entire amount of the fee currently represents a service fee within the
meaning of the NASD guidelines. In the case of Class B and Class C shares, a
portion of the fee payable pursuant to the Plan, equal to 0.25% of the average
daily net assets of each of these Classes, respectively, is currently
characterized as a service fee. A service fee is a payment made for personal
service and/or the maintenance of shareholder accounts.
 
    Additional amounts paid under the Plan in the case of Class B and Class C
shares are paid to the Distributor for services provided and the expenses borne
by the Distributor and others in the distribution of the shares of those
Classes, including the payment of commissions for sales of the shares of those
Classes and incentive compensation to and expenses of Morgan Stanley Dean Witter
Financial Advisors and others who engage in or support distribution of shares or
who service shareholder accounts, including overhead and telephone expenses;
printing and distribution of prospectuses and reports used in connection with
the offering of the Fund's shares to other than current shareholders; and
preparation, printing and distribution of sales literature and advertising
materials. In addition, the Distributor may utilize fees paid pursuant to the
Plan in the case of Class B shares to compensate DWR and other Selected
Broker-Dealers for their opportunity costs in advancing such amounts, which
compensation would be in the form of a carrying charge on any unreimbursed
expenses.
 
    For the fiscal year ended September 30, 1998, Class B shares of the Fund
accrued payments under the Plan amounting to $3,206,390, which amount is equal
to 1.0% of the average daily net assets of Class B for the fiscal year. These
payments were calculated pursuant to clause (b) of the compensation formula
under the Plan. For the fiscal year ended September 30, 1998, Class A and Class
C shares of the Fund accrued payments under the Plan amounting to $1,753 and
$20,751, respectively, which amounts are equal to 0.24% and 1.0% of the average
daily net assets of Class A and Class C, respectively, for the fiscal year.
 
    In the case of Class B shares, at any given time, the expenses in
distributing Class B shares of the Fund may be in excess of the total of (i) the
payments made by the Fund pursuant to the Plan, and (ii) the proceeds of CDSCs
paid by investors upon the redemption of Class B shares. For example, if $1
million in expenses in distributing Class B shares of the Fund had been incurred
and $750,000 had been received as described in (i) and (ii) above, the excess
expense would amount to $250,000. The Distributor has advised the Fund that such
excess amounts, including the carrying charge described above, totalled
$80,592,295 at September 30, 1998, which was equal to 30.59% of the net assets
of Class B on such date. Of this amount, $1,471,908 represents excess
distribution expenses of TCW/DW Global Convertible Trust, the net assets of
which were combined with those of the Fund on December 22, 1995 as pursuant to
an Agreement and Plan of Reorganization. Because there is no requirement under
the Plan that the Distributor be reimbursed for all distribution expenses or any
requirement that the Plan be continued from year to year, such excess amount
does not constitute a liability of the Fund. Although there is no legal
obligation for the Fund to pay expenses incurred in excess of payments made to
the Distributor under the Plan, and the proceeds of CDSCs paid by investors upon
redemption of shares, if for any reason the Plan is terminated the Trustees will
consider at that time the manner in which to treat such expenses. Any cumulative
expenses incurred, but not yet recovered through distribution fees or CDSCs, may
or may not be recovered through future distribution fees or CDSCs.
 
                                       28
<PAGE>
    In the case of Class A and Class C shares, expenses incurred pursuant to the
Plan in any calendar year in excess of 0.25% or 1.0% of the average daily net
assets of Class A or Class C, respectively, will not be reimbursed by the Fund
through payments in any subsequent year, except that expenses representing a
gross sales commission credited to Morgan Stanley Dean Witter Financial Advisors
and other Selected Broker-Dealer representatives at the time of sale may be
reimbursed in the subsequent calendar year. The Distributor has advised the Fund
that unreimbursed expenses representing a gross sales commission credited to
Morgan Stanley Dean Witter Financial Advisors and other Selected Broker-Dealer
representatives at the time of sale totalled $10,463 in the case of Class C at
December 31, 1997, which amount was equal to 0.77% of the net assets of Class C
on such date, and that there were no such expenses which may be reimbursed in
the subsequent year in the case of Class A on such date. No interest or other
financing charges will be incurred on any Class A or Class C distribution
expenses incurred by the Distributor under the Plan or on any unreimbursed
expenses due to the Distributor pursuant to the Plan.
 
DETERMINATION OF NET ASSET VALUE
 
    The net asset value per share is determined once daily at 4:00 p.m., New
York time, on each day that the New York Stock Exchange is open, or, on days
when the New York Stock Exchange closes prior to 4:00 p.m., at such earlier
time, by taking the net assets of the Fund, dividing by the number of shares
outstanding and adjusting to the nearest cent. The assets belonging to the Class
A, Class B, Class C and Class D shares will be invested together in a single
portfolio. The net asset value of each Class, however, will be determined
separately by subtracting each Class's accrued expenses and liabilities. The net
asset value per share will not be determined on Good Friday and on such other
federal and non-federal holidays as are observed by the New York Stock Exchange.
 
    In the calculation of the Fund's net asset value: (1) an equity portfolio
security listed or traded on the New York or American Stock Exchange or other
domestic or foreign exchange is valued at its latest sale price on that exchange
prior to the time assets are valued; if there were no sales that day, the
security is valued at the latest bid price (in cases where a security is traded
on more than one exchange, the security is valued on the exchange designated as
the primary market pursuant to procedures adopted by the Trustees), and (2) all
other portfolio securities for which over-the-counter market quotations are
readily available are valued at the latest bid price. When market quotations are
not readily available, or when it is determined by the Investment Manager that
sale or bid prices are not reflective of a security's fair value, portfolio
securities are valued at their fair value as determined in good faith under
procedures established by and under the general supervision of the Fund's
Trustees.
 
    Short-term debt securities with remaining maturities of sixty days or less
at the time of purchase are valued at amortized cost, unless the Trustees
determine such does not reflect the securities' market value, in which case
these securities will be valued at their fair value as determined by the
Trustees.
 
    Certain of the Fund's portfolio securities may be valued by an outside
pricing service approved by the Fund's Trustees. The pricing service may utilize
a matrix system incorporating security quality, maturity and coupon as the
evaluation model parameters, and/or research evaluations by its staff, including
review of broker-dealer market price quotations in determining what it believes
is the fair valuation of the portfolio securities valued by such pricing
service.
 
                                       29
<PAGE>
SHAREHOLDER SERVICES
- --------------------------------------------------------------------------------
 
    AUTOMATIC INVESTMENT OF DIVIDENDS AND DISTRIBUTIONS.  All income dividends
and capital gains distributions are automatically paid in full and fractional
shares of the applicable Class of the Fund (or, if specified by the shareholder,
in shares of any other open-end Morgan Stanley Dean Witter Fund), unless the
shareholder requests that they be paid in cash. Shares so acquired are acquired
at net asset value and are not subject to the imposition of a front-end sales
charge or a CDSC (see "Redemptions and Repurchases").
 
    INVESTMENT OF DIVIDENDS AND DISTRIBUTIONS RECEIVED IN CASH.  Any shareholder
who receives a cash payment representing a dividend or capital gains
distribution may invest such dividend or distribution in shares of the
applicable Class at the net asset value per share next determined after receipt
by the Transfer Agent, by returning the check or the proceeds to the Transfer
Agent within thirty days after the payment date. Shares so acquired are acquired
at net asset value and are not subject to the imposition of a front-end sales
charge or a CDSC (see "Redemptions and Repurchases.")
 
    EASYINVEST-SM-.  Shareholders may subscribe to EasyInvest, an automatic
purchase plan which provides for any amount from $100 to $5,000 to be
transferred automatically from a checking or savings account or following
redemption of shares of a Morgan Stanley Dean Witter money market fund, on a
semi-monthly, monthly or quarterly basis, to the Transfer Agent for investment
in shares of the Fund (see "Purchase of Fund Shares" and "Redemptions and
Repurchases-- Involuntary Redemption").
 
    SYSTEMATIC WITHDRAWAL PLAN.  A systematic withdrawal plan (the "Withdrawal
Plan") is available for shareholders whose shares of Morgan Stanley Dean Witter
Funds have an aggregate value of $10,000 or more. Shares of any Fund from which
redemptions will be made pursuant to the Plan must have a value of $1,000 or
more (referred to as a "SWP Fund"). The required share values are determined on
the date the shareholder establishes the Withdrawal Plan. The Withdrawal Plan
provides for monthly, quarterly, semi-annual or annual payments in any amount
not less than $25, or in any whole percentage of the value of the SWP Funds'
shares, on an annualized basis. Any applicable CDSC will be imposed on shares
redeemed under the Withdrawal Plan (see "Purchase of Fund Shares"), except that
the CDSC, if any, will be waived on redemptions under the Withdrawal Plan of up
to 12% annually of the value of each SWP Fund account, based on the share values
next determined after the shareholder establishes the Withdrawal Plan.
Redemptions for which this CDSC waiver policy applies may be in amounts up to 1%
per month, 3% per quarter, 6% semi-annually or 12% annually. Under this CDSC
waiver policy, amounts withdrawn each period will be paid by first redeeming
shares not subject to a CDSC because the shares were purchased by the
reinvestment of dividends or capital gains distributions, the CDSC period has
elapsed or some other waiver of the CDSC applies. If shares subject to a CDSC
must be redeemed, shares held for the longest period of time will be redeemed
first and continuing with shares held the next longest period of time until
shares held the shortest period of time are redeemed. Any shareholder
participating in the Withdrawal Plan will have sufficient shares redeemed from
his or her account so that the proceeds (net of any applicable CDSC) to the
shareholder will be the designated monthly, quarterly, semi-annual or annual
amount.
 
    A shareholder may suspend or terminate participation in the Withdrawal Plan
at any time. A shareholder who has suspended participation may resume payment
under the Withdrawal Plan, without requiring a new determination of the account
value of the 12% CDSC waiver. The Withdrawal Plan may be terminated or revised
at any time by the Fund.
 
                                       30
<PAGE>
    Prior to adding an additional SWP Fund to an existing Withdrawal Plan, the
required $10,000/$1,000 share values must be met, to be calculated on the date
the shareholder adds the additional SWP Fund. However, the addition of a new SWP
Fund will not change the account value for the 12% CDSC waiver for the SWP Funds
already participating in the Withdrawal Plan.
 
    Withdrawal Plan payments should not be considered dividends, yields or
income. If periodic Withdrawal Plan payments continuously exceed net investment
income and net capital gains, the shareholder's original investment will be
correspondingly reduced and ultimately exhausted. Each withdrawal constitutes a
redemption of shares and any gain or loss realized must be recognized for
federal income tax purposes.
 
    Shareholders should contact their Morgan Stanley Dean Witter Financial
Advisor or other Selected Broker-Dealer representive, or the Transfer Agent for
further information about any of the above services.
 
    TAX-SHELTERED RETIREMENT PLANS.  Retirement plans are available through the
Distributor for use by corporations, the self-employed, Individual Retirement
Accounts and Custodial Accounts under Section 403(b)(7) of the Internal Revenue
Code. Adoption of such plans should be on advice of legal counsel or tax
advisor.
 
    For further information regarding plan administration, custodial fees and
other details, investors should contact their Morgan Stanley Dean Witter
Financial Advisor or other Selected Broker-Dealer representive or the Transfer
Agent.
 
EXCHANGE PRIVILEGE
 
    Shares of each Class may be exchanged for shares of the same Class of any
other Dean Witter Multi-Class Fund without the imposition of any exchange fee.
Shares may also be exchanged for shares of the following funds: Morgan Stanley
Dean Witter Short-Term U.S. Treasury Trust, Morgan Stanley Dean Witter Limited
Term Municipal Trust, Morgan Stanley Dean Witter Short-Term Bond Fund and five
Morgan Stanley Dean Witter funds which are money market funds (the "Exchange
Funds"). Class A shares may also be exchanged for shares of Morgan Stanley Dean
Witter Multi-State Municipal Series Trust and Morgan Stanley Dean Witter Hawaii
Municipal Trust, which are Morgan Stanley Dean Witter Funds sold with a
front-end sales charge ("FSC Funds"). Exchanges may be made after the shares of
the Fund acquired by purchase (not by exchange or dividend reinvestment) have
been held for thirty days. There is no waiting period for exchanges of shares
acquired by exchange or dividend reinvestment.
 
    An exchange to another Morgan Stanley Dean Witter Multi-Class Fund, any FSC
Fund or any Exchange Fund that is not a money market fund is on the basis of the
next calculated net asset value per share of each fund after the exchange order
is received. When exchanging into a money market fund from the Fund, shares of
the Fund are redeemed out of the Fund at their next calculated net asset value
and the proceeds of the redemption are used to purchase shares of the money
market fund at their net asset value determined the following business day.
Subsequent exchanges between any of the money market funds and any of the Morgan
Stanley Dean Witter Multi-Class Funds, FSC Funds or any Exchange Fund that is
not a money market fund can be effected on the same basis.
 
    No CDSC is imposed at the time of any exchange of shares, although any
applicable CDSC will be imposed upon ultimate redemption. During the period of
time the shareholder remains in an Exchange Fund (calculated from the last day
of the month in which the Exchange Fund shares were acquired), the holding
period (for the purpose of determining the rate of the CDSC) is frozen. If those
shares are subsequently re-exchanged for shares of a Morgan Stanley Dean Witter
Multi-Class Fund, the holding period previously frozen when the first exchange
was made
 
                                       31
<PAGE>
resumes on the last day of the month in which shares of a Morgan Stanley Dean
Witter Multi-Class Fund are reacquired. Thus, the CDSC is based upon the time
(calculated as described above) the shareholder was invested in shares of a
Morgan Stanley Dean Witter Multi-Class Fund (see "Purchase of Fund Shares"). In
the case of exchanges of Class A shares which are subject to a CDSC, the holding
period also includes the time (calculated as described above) the shareholder
was invested in shares of a FSC Fund. In the case of shares exchanged into an
Exchange Fund on or after April 23, 1990, upon a redemption of shares which
results in a CDSC being imposed, a credit (not to exceed the amount of the CDSC)
will be given in an amount equal to the Exchange Fund 12b-1 distribution fees
incurred on or after that date which are attributable to those shares. (Exchange
Fund 12b-1 distribution fees are described in the prospectuses for those funds.)
Class B shares of the Fund acquired in exchange for Class B shares of another
Morgan Stanley Dean Witter Multi-Class Fund having a different CDSC schedule
than that of this Fund will be subject to the higher CDSC schedule, even if such
shares are subsequently re-exchanged for shares of the fund with the lower CDSC
schedule.
 
    ADDITIONAL INFORMATION REGARDING EXCHANGES. Purchases and exchanges should
be made for investment purposes only. A pattern of frequent exchanges may be
deemed by the Investment Manager to be abusive and contrary to the best
interests of the Fund's other shareholders and, at the Investment Manager's
discretion, may be limited by the Fund's refusal to accept additional purchases
and/or exchanges from the investor. Although the Fund does not have any specific
definition of what constitutes a pattern of frequent exchanges, and will
consider all relevant factors in determining whether a particular situation is
abusive and contrary to the best interests of the Fund and its other
shareholders, investors should be aware that the Fund and each of the other
Morgan Stanley Dean Witter Funds may in their discretion limit or otherwise
restrict the number of times this Exchange Privilege may be exercised by any
investor. Any such restriction will be made by the Fund on a prospective basis
only, upon notice to the shareholder not later than ten days following such
shareholder's most recent exchange. Also the Exchange Privilege may be
terminated or revised at any time by the Fund and/or any of such Morgan Stanley
Dean Witter Funds for which shares of the Fund have been exchanged, upon such
notice as may be required by applicable regulatory agencies. Shareholders
maintaining margin accounts with DWR or another Selected Broker-Dealer are
referred to their Morgan Stanley Dean Witter Financial Advisor or other Selected
Broker-Dealer representative regarding restrictions on exchange of shares of the
Fund pledged in the margin account.
 
    The current prospectus for each fund describes its investment objective(s)
and policies, and shareholders should obtain a copy and examine it carefully
before investing. Exchanges are subject to the minimum investment requirement of
each Class of shares and any other conditions imposed by each fund. In the case
of a shareholder holding a share certificate or certificates, no exchanges may
be made until all applicable share certificates have been received by the
Transfer Agent and deposited in the shareholder's account. An exchange will be
treated for federal income tax purposes the same as a repurchase or redemption
of shares, on which the shareholder may realize a capital gain or loss. However,
the ability to deduct capital losses on an exchange may be limited in situations
where there is an exchange of shares within ninety days after the shares are
purchased. The Exchange Privilege is only available in states where an exchange
may legally be made.
 
    If DWR or other Selected Broker-Dealer is the current dealer of record and
its account numbers are part of the account information, shareholders may
initiate an exchange of shares of the Fund for shares of any of the Morgan
Stanley Dean Witter
 
                                       32
<PAGE>
Funds (for which the Exchange Privilege is available) pursuant to this Exchange
Privilege by contacting their Morgan Stanley Dean Witter Financial Advisor or
other Selected Broker-Dealer representative (no Exchange Privilege Authorization
Form is required). Other shareholders (and those shareholders who are clients of
DWR or another Selected Broker-Dealer but who wish to make exchanges directly by
writing or telephoning the Transfer Agent) must complete and forward to the
Transfer Agent an Exchange Privilege Authorization Form, copies of which may be
obtained from the Transfer Agent, to initiate an exchange. If the Authorization
Form is used, exchanges may be made in writing or by contacting the Transfer
Agent at (800) 869-NEWS (toll-free). The Fund will employ reasonable procedures
to confirm that exchange instructions communicated over the telephone are
genuine. Such procedures may include requiring various forms of personal
identification such as name, mailing address, social security or other tax
identification number and DWR or other Selected Broker-Dealer account number (if
any). Telephone instructions may also be recorded. If such procedures are not
employed, the Fund may be liable for any losses due to unauthorized or
fraudulent instructions.
 
    Telephone exchange instructions will be accepted if received by the Transfer
Agent between 9:00 a.m. and 4:00 p.m., New York time, on any day the New York
Stock Exchange is open. Any shareholder wishing to make an exchange who has
previously filed an Exchange Privilege Authorization Form and who is unable to
reach the Fund by telephone should contact his or her Morgan Stanley Dean Witter
Financial Advisor or other Selected Broker-Dealer representative, if
appropriate, or make a written exchange request. Shareholders are advised that
during periods of drastic economic or market changes, it is possible that the
telephone exchange procedures may be difficult to implement, although this has
not been the case with the Morgan Stanley Dean Witter Funds in the past.
 
    Shareholders should contact their Morgan Stanley Dean Witter Financial
Advisor or other Selected Broker-Dealer representative or the Transfer Agent for
further information about the Exchange Privilege.
 
REDEMPTIONS AND REPURCHASES
- --------------------------------------------------------------------------------
 
    REDEMPTION.  Shares of each Class of the Fund can be redeemed for cash at
any time at the net asset value per share next determined; less the amount of
any applicable CDSC in the case of Class A, Class B or Class C shares (see
"Purchase of Fund Shares"). If shares are held in a shareholder's account
without a share certificate, a written request for redemption sent to the Fund's
Transfer Agent at P.O. Box 983, Jersey City, NJ 07303 is required. If
certificates are held by the shareholder(s), the shares may be redeemed by
surrendering the certificate(s) with a written request for redemption, along
with any additional information required by the Transfer Agent.
 
    REPURCHASE.  DWR and other Selected Broker-Dealers are authorized to
repurchase shares represented by a share certificate which is delivered to any
of their offices. Shares held in a shareholder's account without a share
certificate may also be repurchased by DWR and other Selected Broker-Dealers
upon the telephonic or telegraphic request of the shareholder. The repurchase
price is the net asset value next computed (see "Purchase of Fund Shares") after
such repurchase order is received by DWR or other Selected Broker-Dealer,
reduced by any applicable CDSC.
 
    The CDSC, if any, will be the only fee imposed upon repurchase by the Fund
or the Distributor. The offer by DWR and other Selected Broker-Dealers to
repurchase shares may be suspended without notice by them at any time. In that
event, shareholders may redeem their shares through the Fund's Transfer Agent as
set forth above under "Redemption."
 
                                       33
<PAGE>
    PAYMENT FOR SHARES REDEEMED OR REPURCHASED.  Payment for shares presented
for repurchase or redemption will be made by check within seven days after
receipt by the Transfer Agent of the certificate and/or written request in good
order. Such payment may be postponed or the right of redemption suspended under
unusual circumstances, e.g., when normal trading is not taking place on the New
York Stock Exchange. If the shares to be redeemed have recently been purchased
by check, payment of the redemption proceeds may be delayed for the minimum time
needed to verify that the check used for investment has been honored (not more
than fifteen days from the time of receipt of the check by the Transfer Agent).
Shareholders maintaining margin accounts with DWR or another Selected Broker-
Dealer are referred to their Morgan Stanley Dean Witter Financial Advisor or
other Selected Broker-Dealer representative regarding restrictions on redemption
of shares of the Fund pledged in the margin account.
 
    REINSTATEMENT PRIVILEGE.  A shareholder who has had his or her shares
redeemed or repurchased and has not previously exercised this reinstatement
privilege may, within 35 days after the date of the redemption or repurchase,
reinstate any portion or all of the proceeds of such redemption or repurchase in
shares of the Fund in the same Class from which such shares were redeemed or
repurchased, at the net asset value next determined after a reinstatement
request, together with the proceeds, is received by the Transfer Agent and
receive a pro rata credit for any CDSC paid in connection with such redemption
or repurchase.
 
    INVOLUNTARY REDEMPTION.  The Fund reserves the right, on sixty days' notice,
to redeem, at their net asset value, the shares of any shareholder (other than
shares held in an Individual Retirement Account or Custodial Account under
Section 403(b)(7) of the Internal Revenue Code) whose shares, due to redemptions
by the shareholder, have a value of less than $100 or such lesser amount as may
be fixed by the Fund's Trustees or, in the case of an account opened through
EasyInvest, if after twelve months the Shareholder has invested less than $1,000
in the account. However, before the Fund redeems such shares and sends the
proceeds to the shareholder, it will notify the shareholder that the value of
the shares is less than the applicable amount and allow him or her sixty days to
make an additional investment in an amount which will increase the value of his
or her account to at least the applicable amount or more before the redemption
is processed. No CDSC will be imposed on any involuntary redemption.
 
DIVIDENDS, DISTRIBUTIONS AND TAXES
- --------------------------------------------------------------------------------
 
    DIVIDENDS AND DISTRIBUTIONS.  The Fund declares dividends separately for
each Class of shares and intends to pay quarterly income dividends and to
distribute net short-term and net long-term gains, if any, at least once per
year. The Fund may, however, determine either to distribute or to retain all or
part of any long-term gains in any year for reinvestment.
 
    All dividends and capital gains distributions will be paid in additional
shares of the same Class and automatically credited to the shareholder's account
without issuance of a share certificate unless the shareholder requests in
writing that all dividends and/or distributions be paid in cash. Shares acquired
by dividend and distribution reinvestments will not be subject to any front-end
sales charge or CDSC. Class B shares acquired through dividend and distribution
reinvestments will become eligible for conversion to Class A shares on a pro
rata basis. Distributions paid on Class A and Class D shares will be higher than
for Class B and Class C shares because distribution fees paid by Class B and
Class C shares are higher. (See "Shareholder Services--Automatic Investment of
Dividends and Distributions".)
 
                                       34
<PAGE>
    TAXES.  Because the Fund intends to distribute substantially all of its net
investment income and net capital gains to shareholders and otherwise remain
qualified as a regulated investment company under Subchapter M of the Internal
Revenue Code, it is not expected that the Fund will be required to pay any
federal income tax on such income and capital gains. Shareholders who are
required to pay taxes on their income will normally have to pay federal income
taxes, and any state and local income taxes, on the dividends and distributions
they receive from the Fund. Such dividends and distributions, to the extent they
are derived from net investment income or net short-term capital gains, are
taxable to the shareholder as ordinary dividend income regardless of whether the
shareholder receives such payments in additional shares or in cash. Any
dividends declared in the last quarter of any calendar year which are paid in
the following year prior to February 1 will be deemed, for tax purposes, to have
been received by the shareholder in the prior year.
 
    Gains or losses on the Fund's transactions in listed non-equity options,
futures and options on futures generally are treated as 60% long-term and 40%
short-term. When the Fund engages in options and futures transactions, various
tax regulations applicable to the Fund may have the effect of causing the Fund
to recognize a gain or loss for tax purposes before that gain or loss is
realized, or to defer recognition of a realized loss for tax purposes.
Recognition, for tax purposes, of an unrealized loss may result in a lesser
amount of the Fund's realized gains being available for annual distribution.
 
    With respect to the Fund's investments in zero coupon and payment-in-kind
bonds, the Fund accrues income prior to any actual cash payments by their
issuers. In order to continue to comply with Subchapter M of the Code and remain
able to forego payment of Federal income tax on its income and capital gains,
the Fund must distribute all of its net investment income, including income
accrued from zero coupon and payment-in-kind bonds. As such, the Fund may be
required to dispose of some of its portfolio securities under disadvantageous
circumstances to generate the cash required for distribution.
 
    The Fund may at times make payments from sources other than income or net
capital gains. Payments from such sources would, in effect, represent a return
of a portion of each shareholder's investment. All, or a portion, of such
payments would not be taxable to shareholders.
 
    After the end of the calendar year, shareholders will receive full
information on their dividends and capital gains distributions for tax purposes.
To avoid being subject to a 31% federal backup withholding tax on taxable
dividends, capital gains distributions and the proceeds of redemptions and
repurchases, shareholders' taxpayer identification numbers must be furnished and
certified as to their accuracy.
 
    Distributions of net long-term capital gains, if any, are taxable to
shareholders as long-term capital gains regardless of how long a shareholder has
held the Fund's shares and regardless of whether the distribution is received in
additional shares or in cash. Capital gains distributions are not eligible for
the dividends received deduction.
 
    Shareholders should consult their tax advisors as to the applicability of
the foregoing to their current situation.
 
PERFORMANCE INFORMATION
- --------------------------------------------------------------------------------
 
    From time to time the Fund may quote its "yield" and/or its "total return"
in advertisements and sales literature. These figures are computed separately
for Class A, Class B, Class C and Class D shares. Both the yield and the total
return of the Fund are based on historical earnings and are not intended to
indicate future performance. The yield of each Class of the Fund is computed
 
                                       35
<PAGE>
by dividing the Class's net investment income over a 30-day period by an average
value (using the average number of shares entitled to receive dividends and the
maximum offering price per share at the end of the period), all in accordance
with applicable regulatory requirements. Such amount is compounded for six
months and then annualized for a twelve-month period to derive the Fund's yield
for each Class.
 
    The "average annual total return" of the Fund refers to a figure reflecting
the average annualized percentage increase (or decrease) in the value of an
initial investment in a Class of the Fund of $1,000 over periods of one, five
and ten years, as well as over the life of the Fund. Average annual total return
reflects all income earned by the Fund, any appreciation or depreciation of the
Fund's assets, all expenses incurred by the applicable Class and all sales
charges incurred by shareholders, for the stated periods. It also assumes
reinvestment of all dividends and distributions paid by the Fund.
 
    In addition to the foregoing, the Fund may advertise its total return for
each Class over different periods of time by means of aggregate, average,
year-by-year or other types of total return figures. Such calculations may or
may not reflect the deduction of any sales charge which, if reflected, would
reduce the performance quoted. The Fund may also advertise the growth of
hypothetical investments of $10,000, $50,000 and $100,000 in each Class of
shares of the Fund. The Fund from time to time may also advertise its
performance relative to certain performance rankings and indices compiled by
independent organizations (such as mutual fund performance rankings of Lipper
Analytical Services, Inc.).
 
ADDITIONAL INFORMATION
- --------------------------------------------------------------------------------
 
    VOTING RIGHTS.  All shares of beneficial interest of the Fund are of $0.01
par value and are equal as to earnings, assets and voting privileges except that
each Class will have exclusive voting privileges with respect to matters
relating to distribution expenses borne solely by such Class or any other matter
in which the interests of one Class differ from the interests of any other
Class. In addition, Class B shareholders will have the right to vote on any
proposed material increase in Class A's expenses, if such proposal is submitted
separately to Class A shareholders. Also, as discussed herein, Class A, Class B
and Class C bear the expenses related to the distribution of their respective
shares.
 
    The Fund is not required to hold Annual Meetings of Shareholders and, in
ordinary circumstances, the Fund does not intend to hold such meetings. The
Trustees may call Special Meetings of Shareholders for action by shareholder
vote as may be required by the Act or the Declaration of Trust. Under certain
circumstances the Trustees may be removed by action of the Trustees or by the
Shareholders.
 
    Under Massachusetts law, shareholders of a business trust may, under certain
circumstances, be held personally liable as partners for the obligations of the
Fund. However, the Declaration of Trust contains an express disclaimer of
shareholder liability for acts or obligations of the Fund, requires that Fund
obligations include such disclaimer, and provides for indemnification and
reimbursement of expenses out of the Fund's property for any shareholder held
personally liable for the obligations of the Fund. Thus, the risk of a
shareholder incurring financial loss on account of shareholder liability is
limited to circumstances in which the Fund itself would be unable to meet its
obligations. Given the above limitations on shareholder personal liability and
the nature of the Fund's assets and operations, the possibility of the Fund's
being unable to meet its obligations is remote and, thus, in the opinion of
Massachusetts
 
                                       36
<PAGE>
counsel to the Fund, the risk to Fund shareholders of personal liability is
remote.
 
    CODE OF ETHICS.  Directors, officers and employees of MSDW Advisors, MSDW
Services and MSDW Distributors are subject to a strict Code of Ethics adopted by
those companies. The Code of Ethics is intended to ensure that the interests of
shareholders and other clients are placed ahead of any personal interest, that
no undue personal benefit is obtained from person's employment activities and
that actual and potential conflicts of interest are avoided. To achieve these
goals and comply with regulatory requirements, the Code of Ethics requires,
among other things, that personal securities transactions by employees of the
companies be subject to an advance clearance process to monitor that no Morgan
Stanley Dean Witter Fund is engaged at the same time in a purchase or sale of
the same security. The Code of Ethics bans the purchase of securities in an
initial public offering, and also prohibits engaging in futures and options
transactions and profiting on short-term trading (that is, a purchase within
sixty
days of a sale or a sale within sixty days of a purchase) of a security. In
addition, investment personnel may not purchase or sell a security for their
personal account within thirty days before or after any transaction in any
Morgan Stanley Dean Witter Fund managed by them. Any violations of the Code of
Ethics are subject to sanctions, including reprimand, demotion or suspension or
termination of employment. The Code of Ethics comports with regulatory
requirements and the recommendations in the 1994 report by the Investment
Company Institute Advisory Group on Personal Investing.
 
    MASTER/FEEDER CONVERSION.  The Fund reserves the right to seek to achieve
its investment objective by investing all of its investable assets in a
diversified, open-end management investment company having the same investment
objective and policies and substantially the same investment restrictions as
those applicable to the Fund.
 
    SHAREHOLDER INQUIRIES.  All inquiries regarding the Fund should be directed
to the Fund at the telephone numbers or address set forth on the front cover of
this Prospectus.
 
                                       37
<PAGE>
APPENDIX
- --------------------------------------------------------------------------------
 
RATINGS OF INVESTMENTS
MOODY'S INVESTORS SERVICE INC. ("MOODY'S")
 
                                  BOND RATINGS
 
<TABLE>
<S>        <C>
Aaa        Bonds which are rated Aaa are judged to be of the best quality. They carry the smallest
           degree of investment risk and are generally referred to as "gilt edge." Interest payments
           are protected by a large or by an exceptionally stable margin and principal is secure.
           While the various protective elements are likely to change, such changes as can be
           visualized are most unlikely to impair the fundamentally strong position of such issues.
Aa         Bonds which are rated Aa are judged to be of high quality by all standards. Together with
           the Aaa group they comprise what are generally known as high grade bonds. They are rated
           lower than the best bonds because margins of protection may not be as large as in Aaa
           securities or fluctuation of protective elements may be of greater amplitude or there may
           be other elements present which make the long-term risks appear somewhat larger than in
           Aaa securities.
A          Bonds which are rated A possess many favorable investment attributes and are to be
           considered as upper medium grade obligations. Factors giving security to principal and
           interest are considered adequate, but elements may be present which suggest a
           susceptibility to impairment sometime in the future.
Baa        Bonds which are rated Baa are considered as medium grade obligations; i.e., they are
           neither highly protected nor poorly secured. Interest payments and principal security
           appear adequate for the present but certain protective elements may be lacking or may be
           characteristically unreliable over any great length of time. Such bonds lack outstanding
           investment characteristics and in fact have speculative characteristics as well.
           Bonds rated Aaa, Aa, A and Baa are considered investment grade bonds.
Ba         Bonds which are rated Ba are judged to have speculative elements; their future cannot be
           considered as well assured. Often the protection of interest and principal payments may be
           very moderate, and therefore not well safeguarded during both good and bad times over the
           future. Uncertainty of position characterizes bonds in this class.
B          Bonds which are rated B generally lack characteristics of desirable investments. Assurance
           of interest and principal payments or of maintenance of other terms of the contract over
           any long period of time may be small.
Caa        Bonds which are rated Caa are of poor standing. Such issues may be in default or there may
           be present elements of danger with respect to principal or interest.
Ca         Bonds which are rated Ca present obligations which are speculative in a high degree. Such
           issues are often in default or have other marked shortcomings.
C          Bonds which are rated C are the lowest rated class of bonds, and issues so rated can be
           regarded as having extremely poor prospects of ever attaining any real investment
           standing.
</TABLE>
 
    CONDITIONAL RATING:  Municipal bonds for which the security depends upon the
completion of some act or the fulfillment of some condition are rated
conditionally. These are bonds secured by (a) earnings of projects under
construction, (b) earnings of projects unseasoned in operation experience, (c)
rentals which begin
 
                                       38
<PAGE>
when facilities are completed, or (d) payments to which some other limiting
condition attaches. Parenthetical rating denotes probable credit stature upon
completion of construction or elimination of basis of condition.
 
    RATING REFINEMENTS:  Moody's may apply numerical modifiers, 1, 2 and 3 in
each generic rating classification from Aa through B in its corporate and
municipal bond rating system. The modifier 1 indicates that the security ranks
in the higher end of its generic rating category; the modifier 2 indicates a
mid-range ranking; and a modifier 3 indicates that the issue ranks in the lower
end of its generic rating category.
 
                            COMMERCIAL PAPER RATINGS
 
    Moody's Commercial Paper ratings are opinions of the ability to repay
punctually promissory obligations not having an original maturity in excess of
nine months. Moody's employs the following three designations, all judged to be
investment grade, to indicate the relative repayment capacity of rated issuers:
Prime-1, Prime-2, Prime-3.
 
    Issuers rated Prime-1 have a superior capacity for repayment of short-term
promissory obligations. Issuers rated Prime-2 have a strong capacity for
repayment of short-term promissory obligations; and Issuers rated Prime-3 have
an acceptable capacity for repayment of short-term promissory obligations.
Issuers rated Not Prime do not fall within any of the Prime rating categories.
 
STANDARD & POOR'S CORPORATION ("STANDARD & POOR'S")
 
                                  BOND RATINGS
 
    A Standard & Poor's bond rating is a current assessment of the
creditworthiness of an obligor with respect to a specific obligation. This
assessment may take into consideration obligors such as guarantors, insurers, or
lessees.
 
    The ratings are based on current information furnished by the issuer or
obtained by Standard & Poor's from other sources it considers reliable. The
ratings are based, in varying degrees, on the following considerations: (1)
likelihood of default-capacity and willingness of the obligor as to the timely
payment of interest and repayment of principal in accordance with the terms of
the obligation; (2) nature of and provisions of the obligation; and (3)
protection afforded by, and relative position of, the obligation in the event of
bankruptcy, reorganization or other arrangement under the laws of bankruptcy and
other laws affecting creditors' rights.
 
    Standard & Poor's does not perform an audit in connection with any rating
and may, on occasion, rely on unaudited financial information. The ratings may
be changed, suspended or withdrawn as a result of changes in, or unavailability
of, such information, or for other reasons.
 
<TABLE>
<S>        <C>
AAA        Debt rated AAA has the highest rating assigned by Standard & Poor's. Capacity to pay
           interest and repay principal is extremely strong.
AA         Debt rated AA has a very strong capacity to pay interest and repay principal and differs
           from the highest-rated issues only in small degree.
A          Debt rated A has a strong capacity to pay interest and repay principal although they are
           somewhat more susceptible to the adverse effects of changes in circumstances and economic
           conditions than debt in higher-rated categories.
BBB        Debt rated BBB is regarded as having an adequate capacity to pay interest and repay
           principal. Whereas it normally exhibits adequate protection parameters, adverse economic
           conditions or changing circumstances are more likely to lead to a weakened capacity to
           pay interest and repay principal for debt in this category than for debt in higher-rated
           categories.
</TABLE>
 
                                       39
<PAGE>
<TABLE>
<S>        <C>
           Bonds rated AAA, AA, A and BBB are considered investment grade bonds.
BB         Debt rated BB has less near-term vulnerability to default than other speculative grade
           debt. However, it faces major ongoing uncertainties or exposure to adverse business,
           financial or economic conditions which could lead to inadequate capacity to meet timely
           interest and principal payment.
B          Debt rated B has a greater vulnerability to default but presently has the capacity to
           meet interest payments and principal repayments. Adverse business, financial or economic
           conditions would likely impair capacity or willingness to pay interest and repay
           principal.
CCC        Debt rated CCC has a current identifiable vulnerability to default, and is dependent upon
           favorable business, financial and economic conditions to meet timely payments of interest
           and repayments of principal. In the event of adverse business, financial or economic
           conditions, it is not likely to have the capacity to pay interest and repay principal.
CC         The rating CC is typically applied to debt subordinated to senior debt which is assigned
           an actual or implied CCC rating.
C          The rating C is typically applied to debt subordinated to senior debt which is assigned
           an actual or implied CCC- debt rating.
CI         The rating CI is reserved for income bonds on which no interest is being paid.
NR         Indicates that no rating has been requested, that there is insufficient information on
           which to base a rating or that Standard & Poor's does not rate a particular type of
           obligation as a matter of policy.
           Bonds rated BB, B, CCC, CC and C are regarded as having predominantly speculative
           characteristics with respect to capacity to pay interest and repay principal. BB
           indicates the least degree of speculation and C the highest degree of speculation. While
           such debt will likely have some quality and protective characteristics, these are
           outweighed by large uncertainties or major risk exposures to adverse conditions.
           Plus (+) or minus (-): The ratings from AA to CCC may be modified by the addition of a
           plus or minus sign to show relative standing within the major ratings categories.
           In the case of municipal bonds, the foregoing ratings are sometimes followed by a "p"
           which indicates that the rating is provisional. A provisional rating assumes the
           successful completion of the project being financed by the bonds being rated and
           indicates that payment of debt service requirements is largely or entirely dependent upon
           the successful and timely completion of the project. This rating, however, while
           addressing credit quality subsequent to completion of the project, makes no comment on
           the likelihood or risk of default upon failure of such completion.
</TABLE>
 
                                       40
<PAGE>
                            COMMERCIAL PAPER RATINGS
 
    Standard and Poor's commercial paper rating is a current assessment of the
likelihood of timely payment of debt having an original maturity of no more than
365 days. The commercial paper rating is not a recommendation to purchase or
sell a security. The ratings are based upon current information furnished by the
issuer or obtained by Standard and Poor's from other sources it considers
reliable. The ratings may be changed, suspended, or withdrawn as a result of
changes in or unavailability of such information. Ratings are graded into group
categories, ranging from "A" for the highest quality obligations to "D" for the
lowest. Ratings are applicable to both taxable and tax-exempt commercial paper.
The categories are as follows:
 
    Issues assigned A ratings are regarded as having the greatest capacity for
timely payment. Issues in this category are further refined with the designation
1, 2 and 3 to indicate the relative degree of safety.
 
<TABLE>
<S>        <C>
A-1        indicates that the degree of safety regarding timely payment is very strong.
A-2        indicates capacity for timely payment on issues with this designation is strong. However,
           the relative degree of safety is not as overwhelming as for issues designated "A-1."
A-3        indicates a satisfactory capacity for timely payment. Obligations carrying this
           designation are, however, somewhat more vulnerable to the adverse effects of changes in
           circumstances than obligations carrying the higher designations.
</TABLE>
 
                                       41
<PAGE>
Morgan Stanley Dean Witter
Convertible Securities Trust
Two World Trade Center
New York, New York 10048
 
TRUSTEES
 
Michael Bozic
Charles A. Fiumefreddo
Edwin J. Garn
John R. Haire
Wayne E. Hedien
Dr. Manuel H. Johnson
Michael E. Nugent
Philip J. Purcell
John L. Schroeder
 
OFFICERS
 
Charles A. Fiumefreddo
Chairman and Chief Executive Officer
Barry Fink
Vice President, Secretary and
General Counsel
Peter M. Avelar
Vice President
Ellen Gold
Assistant Vice President
Thomas F. Caloia
Treasurer
 
CUSTODIAN
 
The Bank of New York
90 Washington Street
New York, New York 10286
 
TRANSFER AGENT AND
DIVIDEND DISBURSING AGENT
 
Morgan Stanley Dean Witter Trust FSB
Harborside Financial Center
Plaza Two
Jersey City, New Jersey 07311
 
INDEPENDENT ACCOUNTANTS
 
PricewaterhouseCoopers LLP
1177 Avenue of the Americas
New York, New York 10036
 
INVESTMENT MANAGER
 
Morgan Stanley Dean Witter Advisors Inc.
 
MORGAN STANLEY
DEAN WITTER
CONVERTIBLE
SECURITIES
TRUST
 
                              [GRAPHIC]
                                                 PROSPECTUS -- NOVEMBER 24, 1998
<PAGE>
STATEMENT OF ADDITIONAL INFORMATION
NOVEMBER 24, 1998
 
- --------------------------------------------------------------------------------
 
MORGAN STANLEY DEAN WITTER
CONVERTIBLE
SECURITIES TRUST
 
    Morgan Stanley Dean Witter Convertible Securities Trust (the "Fund") is an
open-end diversified management investment company whose investment objective is
to seek a high level of total return on its assets through a combination of
current income and capital appreciation. It seeks to achieve its investment
objective by investing principally in "convertible securities," that is, bonds,
notes, debentures, preferred stocks and other securities which are convertible
into common stocks. (See "Investment Practices and Policies".)
 
    A Prospectus for the Fund dated November 24, 1998, which provides the basic
information you should know before investing in the Fund, may be obtained
without charge from the Fund at the address or telephone numbers listed below or
from the Fund's Distributor, Morgan Stanley Dean Witter Distributors Inc., or
from Dean Witter Reynolds Inc. at any of its branch offices. This Statement of
Additional Information is not a Prospectus. It contains information in addition
to and more detailed than that set forth in the Prospectus. It is intended to
provide additional information regarding the activities and operations of the
Fund, and should be read in conjunction with the Prospectus.
 
Morgan Stanley Dean Witter
Convertible Securities Trust
Two World Trade Center
New York, New York 10048
(212) 392-2550 or
(800) 869-NEWS (toll-free)
<PAGE>
TABLE OF CONTENTS
- --------------------------------------------------------------------------------
 
   
<TABLE>
<S>                                                                                      <C>
The Fund and its Management............................................................          3
 
Trustees and Officers..................................................................          7
 
Investment Practices and Policies......................................................         13
 
Investment Restrictions................................................................         24
 
Portfolio Transactions and Brokerage...................................................         25
 
The Distributor........................................................................         27
 
Purchase of Fund Shares................................................................         32
 
Shareholder Services...................................................................         34
 
Redemptions and Repurchases............................................................         39
 
Dividends, Distributions and Taxes.....................................................         41
 
Performance Information................................................................         42
 
Description of Shares of the Fund......................................................         44
 
Custodian and Transfer Agent...........................................................         44
 
Independent Accountants................................................................         45
 
Reports to Shareholders................................................................         45
 
Legal Counsel..........................................................................         45
 
Experts................................................................................         45
 
Registration Statement.................................................................         45
 
Report of Independent Accountants......................................................         46
 
Financial Statements -- September 30, 1998.............................................         47
</TABLE>
    
 
                                       2
<PAGE>
THE FUND AND ITS MANAGEMENT
- --------------------------------------------------------------------------------
 
THE FUND
 
    The Fund is a Trust of the type commonly known as a "Massachusetts business
trust" and was organized under the laws of the Commonwealth of Massachusetts on
May 21, 1985 under the name Dean Witter Convertible Securities Trust. On June
22, 1998, the Trustees of the Fund adopted an Amendment to the Declaration of
Trust of the Fund changing the name of the Fund to Morgan Stanley Dean Witter
Convertible Securities Trust.
 
THE INVESTMENT MANAGER
 
    Morgan Stanley Dean Witter Advisors Inc. (the "Investment Manager" or "MSDW
Advisors"), a Delaware corporation, whose address is Two World Trade Center, New
York, New York 10048, is the Fund's Investment Manager. MSDW Advisors is a
wholly-owned subsidiary of Morgan Stanley, Dean Witter & Co. ("MSDW"), a
Delaware corporation. The daily management of the Fund and research relating to
the Fund's portfolio are conducted by or under the direction of officers of the
Fund and of the Investment Manager, subject to review by the Fund's Board of
Trustees. Information as to these Trustees and Officers is contained under the
caption "Trustees and Officers."
 
    MSDW Advisors is the investment manager or investment advisor of the
following investment companies, which are collectively referred to as the
"Morgan Stanley Dean Witter Funds:"
 
<TABLE>
<CAPTION>
OPEN-END FUNDS
<C>        <S>
        1  Active Assets California Tax-Free Trust
        2  Active Assets Government Securities Trust
        3  Active Assets Money Trust
        4  Active Assets Tax-Free Trust
        5  Morgan Stanley Dean Witter American Value Fund
        6  Morgan Stanley Dean Witter Balanced Growth Fund
        7  Morgan Stanley Dean Witter Balanced Income Fund
        8  Morgan Stanley Dean Witter California Tax-Free Daily Income Trust
        9  Morgan Stanley Dean Witter California Tax-Free Income Fund
       10  Morgan Stanley Dean Witter Capital Appreciation Fund
       11  Morgan Stanley Dean Witter Capital Growth Securities
       12  Morgan Stanley Dean Witter Competitive Edge Fund, "BEST IDEAS" Portfolio
       13  Morgan Stanley Dean Witter Convertible Securities Trust
       14  Morgan Stanley Dean Witter Developing Growth Securities Trust
       15  Morgan Stanley Dean Witter Diversified Income Trust
       16  Morgan Stanley Dean Witter Dividend Growth Securities Inc.
       17  Morgan Stanley Dean Witter Equity Fund
       18  Morgan Stanley Dean Witter European Growth Fund Inc.
       19  Morgan Stanley Dean Witter Federal Securities Trust
       20  Morgan Stanley Dean Witter Financial Services Trust
       21  Morgan Stanley Dean Witter Fund of Funds
       22  Morgan Stanley Dean Witter Global Dividend Growth Securities
       23  Morgan Stanley Dean Witter Global Short-Term Income Fund Inc.
       24  Morgan Stanley Dean Witter Global Utilities Fund
       25  Morgan Stanley Dean Witter Growth Fund
       26  Morgan Stanley Dean Witter Hawaii Municipal Trust
       27  Morgan Stanley Dean Witter Health Sciences Trust
       28  Morgan Stanley Dean Witter High Yield Securities Inc.
       29  Morgan Stanley Dean Witter Income Builder Fund
       30  Morgan Stanley Dean Witter Information Fund
       31  Morgan Stanley Dean Witter Intermediate Income Securities
       32  Morgan Stanley Dean Witter International SmallCap Fund
       33  Morgan Stanley Dean Witter Japan Fund
</TABLE>
 
                                       3
<PAGE>
<TABLE>
<C>        <S>
       34  Morgan Stanley Dean Witter Limited Term Municipal Trust
       35  Morgan Stanley Dean Witter Liquid Asset Fund Inc.
       36  Morgan Stanley Dean Witter Market Leader Trust
       37  Morgan Stanley Dean Witter Mid-Cap Dividend Growth Securities
       38  Morgan Stanley Dean Witter Mid-Cap Growth Fund
       39  Morgan Stanley Dean Witter Multi-State Municipal Series Trust
       40  Morgan Stanley Dean Witter Natural Resource Development Securities Inc.
       41  Morgan Stanley Dean Witter New York Municipal Money Market Trust
       42  Morgan Stanley Dean Witter New York Tax-Free Income Fund
       43  Morgan Stanley Dean Witter Pacific Growth Fund Inc.
       44  Morgan Stanley Dean Witter Precious Metals and Minerals Trust
       45  Morgan Stanley Dean Witter Select Dimensions Investment Series
       46  Morgan Stanley Dean Witter Select Municipal Reinvestment Fund
       47  Morgan Stanley Dean Witter Short-Term Bond Fund
       48  Morgan Stanley Dean Witter Short-Term U.S. Treasury Trust
       49  Morgan Stanley Dean Witter Special Value Fund
       50  Morgan Stanley Dean Witter S&P 500 Index Fund
       51  Morgan Stanley Dean Witter S&P 500 Select Fund
       52  Morgan Stanley Dean Witter Strategist Fund
       53  Morgan Stanley Dean Witter Tax-Exempt Securities Trust
       54  Morgan Stanley Dean Witter Tax-Free Daily Income Trust
       55  Morgan Stanley Dean Witter U.S. Government Money Market Trust
       56  Morgan Stanley Dean Witter U.S. Government Securities Trust
       57  Morgan Stanley Dean Witter Utilities Fund
       58  Morgan Stanley Dean Witter Value-Added Market Series
       59  Morgan Stanley Dean Witter Value Fund
       60  Morgan Stanley Dean Witter Variable Investment Series
       61  Morgan Stanley Dean Witter World Wide Income Trust
<CAPTION>
 
CLOSED-END FUNDS
<C>        <S>
        1  InterCapital California Insured Municipal Income Trust
        2  InterCapital California Quality Municipal Securities
        3  Dean Witter Government Income Trust
        4  High Income Advantage Trust
        5  High Income Advantage Trust II
        6  High Income Advantage Trust III
        7  InterCapital Income Securities Inc.
        8  InterCapital Insured California Municipal Securities
        9  InterCapital Insured Municipal Bond Trust
       10  InterCapital Insured Municipal Income Trust
       11  InterCapital Insured Municipal Securities
       12  InterCapital Insured Municipal Trust
       13  Municipal Income Opportunities Trust
       14  Municipal Income Opportunities Trust II
       15  Municipal Income Opportunities Trust III
       16  Municipal Income Trust
       17  Municipal Income Trust II
       18  Municipal Income Trust III
       19  Municipal Premium Income Trust
       20  InterCapital New York Quality Municipal Securities
       21  Morgan Stanley Dean Witter Prime Income Trust
       22  InterCapital Quality Municipal Income Trust
       23  InterCapital Quality Municipal Investment Trust
       24  InterCapital Quality Municipal Securities
</TABLE>
 
                                       4
<PAGE>
    In addition, Morgan Stanley Dean Witter Services Company Inc. ("MSDW
Services"), a wholly-owned subsidiary of MSDW Advisors, serves as manager for
the following investment companies for which TCW Funds Management, Inc. is the
investment advisor (the "TCW/DW Funds"):
<TABLE>
<CAPTION>
OPEN-END FUNDS
<C>        <S>
        1  TCW/DW Emerging Markets Opportunities Trust
        2  TCW/DW Global Telecom Trust
        3  TCW/DW Income and Growth Fund
        4  TCW/DW Latin American Growth Fund
        5  TCW/DW Mid-Cap Equity Trust
        6  TCW/DW North American Government Income Trust
        7  TCW/DW Small Cap Growth Fund
        8  TCW/DW Total Return Trust
 
<CAPTION>
 
CLOSED-END FUNDS
<C>        <S>
        1  TCW/DW Term Trust 2000
        2  TCW/DW Term Trust 2002
        3  TCW/DW Term Trust 2003
</TABLE>
 
    MSDW Advisors also serves as: (i) administrator of The BlackRock Strategic
Term Trust Inc., a closed-end investment company; (ii) sub-administrator of
Templeton Global Governments Income Trust, a closed-end investment company; and
(iii) investment advisor of Offshore Dividend Growth Fund and Offshore Money
Market Fund, mutual funds established under the laws of the Cayman Islands and
available only to investors who are participants in the International Active
Assets Account program and are neither citizens nor residents of the United
States.
 
    Pursuant to an Investment Management Agreement (the "Agreement") with the
Investment Manager, the Fund has retained the Investment Manager to manage the
investment of the Fund's assets, including the placing of orders for the
purchase and sale of portfolio securities. The Investment Manager obtains and
evaluates such information and advice relating to the economy, securities
markets, and specific securities as it considers necessary or useful to
continuously manage the assets of the Fund in a manner consistent with its
investment objective and policies.
 
    Under the terms of the Agreement, in addition to managing the Fund's
investments, the Investment Manager maintains certain of the Fund's books and
records and furnishes, at its own expense, such office space, facilities,
equipment, clerical help, bookkeeping and certain legal services as the Fund may
reasonably require in the conduct of its business, including the preparation of
prospectuses, statements of additional information, proxy statements and reports
required to be filed with federal and state securities commissions (except
insofar as the participation or assistance of independent accountants and
attorneys is, in the opinion of the Investment Manager, necessary or desirable).
In addition, the Investment Manager pays the salaries of all personnel,
including officers of the Fund, who are employees of the Investment Manager. The
Investment Manager also bears the cost of telephone service, heat, light, power
and other utilities provided to the Fund. The Investment Manager has retained
MSDW Services to provide its administrative services under the Agreement.
 
    Expenses not expressly assumed by the Investment Manager under the Agreement
or by the Distributor of the Fund's shares Morgan Stanley Dean Witter
Distributors Inc. ("MSDW Distributors" or "the Distributor") (see "The
Distributor") will be paid by the Fund. These expenses will be allocated among
the four classes of shares of the Fund (each, a "Class") pro rata based on the
net assets of the Fund attributable to each Class, except as described below.
Such expenses include, but are not limited to: expenses of the Plan of
Distribution pursuant to Rule 12b-1 (the "12b-1 fee") (see "The Distributor"),
charges and expenses of any registrar, custodian, stock transfer and dividend
disbursing agent; brokerage commissions; taxes; engraving and printing of share
certificates; registration costs of the Fund and its shares under federal and
state securities laws; the cost and expense of printing, including typesetting,
and distributing Prospectuses and Statements of Additional Information of the
Fund and supplements thereto to the Fund's shareholders; all expenses of
shareholders' and Trustees' meetings and of
 
                                       5
<PAGE>
preparing, printing and mailing of proxy statements and reports to shareholders;
fees and travel expenses of trustees or members of any advisory board or
committee who are not employees of the Investment Manager or any corporate
affiliate of the Investment Manager; all expenses incident to any dividend,
withdrawal or redemption options; charges and expenses of any outside service
used for pricing of the Fund's shares; fees and expenses of legal counsel,
including counsel to the trustees who are not interested persons of the Fund or
of the Investment Manager (not including compensation or expenses of attorneys
who are employees of the Investment Manager) and independent accountants;
membership dues of industry associations; interest on Fund borrowings; postage;
insurance premiums on property or personnel (including officers and trustees) of
the Fund which inure to its benefit; extraordinary expenses (including, but not
limited to, legal claims and liabilities and litigation costs and any
indemnification relating thereto); and all other costs of the Fund's operation.
The 12b-1 fees relating to a particular Class will be allocated directly to that
Class. In addition, other expenses associated with a particular Class (except
advisory or custodial fees) may be allocated directly to that Class, provided
that such expenses are reasonably identified as specifically attributable to
that Class and the direct allocation to that Class is approved by the Trustees.
 
    As full compensation for the services and facilities furnished to the Fund
and expenses of the Fund assumed by the Investment Manager, the Fund pays the
Investment Manager monthly compensation calculated daily by applying the
following annual rates to the Fund's daily net assets: 0.60% of the portion of
the daily net assets of the Fund not exceeding $750 million and 0.55% of the
portion of the daily net assets exceeding $750 million but not exceeding $1
billion; 0.50% of the portion of the daily net assets of the Fund exceeding $1
billion but not exceeding $1.5 billion; 0.475% of the portion of the Fund's
daily net assets exceeding $1.5 billion but not exceeding $2 billion; 0.45% of
the portion of the Fund's daily net assets exceeding $2 billion but not
exceeding $3 billion; and 0.425% of the portion of the Fund's daily net assets
exceeding $3 billion. The management fee is allocated among the Classes pro rata
based on the net assets of the Fund attributable to each Class. Total
compensation accrued to the Investment Manager for the fiscal years ended
September 30, 1996, 1997 and 1998 amounted to $1,234,262, $1,578,570 and
$1,953,489, respectively.
 
    The Agreement provides that in the absence of willful misfeasance, bad
faith, gross negligence or reckless disregard of its obligations thereunder, the
Investment Manager is not liable to the Fund or any of its investors for any act
or omission by the Investment Manager or for any losses sustained by the Fund or
its investors. The Agreement in no way restricts the Investment Manager from
acting as investment manager or adviser to others.
 
    The Agreement was initially approved by the Board of Trustees on February
21, 1997 and by the shareholders of the Fund at a Special Meeting of
Shareholders held on May 21, 1997. The Agreement is substantially identical to a
prior investment management agreement which was initially approved by the Board
of Trustees on October 30, 1992 and by the shareholders of the Fund at a Special
Meeting of Shareholders held on January 12, 1993. The Agreement took effect on
May 31, 1997 upon the consummation of the merger of Dean Witter, Discover & Co.
with Morgan Stanley Group Inc. The Agreement may be terminated at any time,
without penalty, on thirty days' notice by the Board of Trustees of the Fund, by
the holders of a majority, as defined in the Investment Company Act of 1940 (the
"Act"), of the outstanding shares of the Fund, or by the Investment Manager. The
Agreement will automatically terminate in the event of its assignment (as
defined in the Act).
 
    Under its terms, the Agreement has an initial term ending April 30, 1999 and
will remain in effect from year to year thereafter, provided continuance of the
Agreement is approved at least annually by the vote of the holders of a
majority, as defined in the Act, of the outstanding shares of the Fund, or by
the Board of Trustees of the Fund; provided that in either event such
continuance is approved annually by the vote of a majority of the Trustees of
the Fund who are not parties to the Agreement or "interested persons" (as
defined in the Act) of any such party (the "Independent Trustees"), which vote
must be cast in person at a meeting called for the purpose of voting on such
approval.
 
    The following owned 5% or more of the outstanding shares of Class A on
October 31, 1998: MSDW Trust FSB, TTEE for the Robinson Nugent Inc. Profit
Sharing 401(k) Plan, P.O. Box 957, Jersey City, NJ 07303-0957--33.2%; MSDW Trust
FSB, TTEE for OBGYN of Lancaster, Harborside Financial Center,
 
                                       6
<PAGE>
Plaza 2, 7th Floor, Jersey City, NJ 07311--16.2%; Dean Witter Reynolds,
Custodian for David W. O'Neill, IRA Standard dated 10/10/96, 745 Pointe Drive,
Port Huron, MI 48060-4465--9.1%; Myra Payne Rowland, TTEE FBO M.P. Rowland Trust
AMDED 5/9/97, Attn: Tom G. Burrows, Esq., P.O. Box 541196, Merrit Island,
Florida 32954-1196--6.7%. The following owned 5% or more of the outstanding
shares of Class C on October 31, 1998: Spojnia Manor Endowment Fund, 1002
Pittston Avenue, Scranton, PA 18505-4109--6.7%. The following owned 5% or more
of the outstanding shares of Class D on October 31, 1998: Hare & Co., c/o The
Bank of New York, P.O. Box 11203, New York, NY 10286-1203--98.7%
 
    The Fund has acknowledged that the name "Morgan Stanley Dean Witter" is a
property right of DWR. The Fund has agreed that MSDW, or any corporate affiliate
of MSDW, may use or, at any time, permit others to use, the name "Morgan Stanley
Dean Witter." The Fund has also agreed that in the event the Agreement is
terminated, or if the affiliation between MSDW Advisors and its parent company
is terminated, the Fund will eliminate the name "Morgan Stanley Dean Witter"
from its name if MSDW, or any corporate affiliate of MSDW, shall so request.
 
TRUSTEES AND OFFICERS
- --------------------------------------------------------------------------------
 
    The Trustees and Executive Officers of the Fund, their principal business
occupations during the last five years and their affiliations, if any, with MSDW
Advisors and with the 85 Morgan Stanley Dean Witter Funds and the 11 TCW/DW
Funds are shown below.
 
<TABLE>
<CAPTION>
      NAME, AGE, POSITION WITH FUND AND ADDRESS                PRINCIPAL OCCUPATIONS DURING LAST FIVE YEARS
- ------------------------------------------------------  ----------------------------------------------------------
<S>                                                     <C>
Michael Bozic (57)                                      Vice Chairman of Kmart Corporation (commencing December,
Trustee                                                 1998); Director or Trustee of the Morgan Stanley Dean
c/o Kmart Corporation                                   Witter Funds; formerly Chairman and Chief Executive
3100 West Big Beaver Road                               Officer of Levitz Furniture Corporation (November,
Troy, Michigan                                          1995-November 1998) and President and Chief Executive
                                                        Officer of Hills Department Stores (May, 1991-July, 1995);
                                                        formerly variously Chairman, Chief Executive Officer,
                                                        President and Chief Operating Officer (1987-1991) of the
                                                        Sears Merchandise Group of Sears, Roebuck and Co.;
                                                        Director of Eaglemark Financial Services, Inc. and Weirton
                                                        Steel Corporation.
 
Charles A. Fiumefreddo* (65)                            Chairman, Director or Trustee, President and Chief
Chairman of the Board, President, Chief                 Executive Officer of the Morgan Stanley Dean Witter Funds;
 Executive Officer and Trustee                          Chairman, Chief Executive Officer and Trustee of the
Two World Trade Center                                  TCW/DW Funds; formerly Chairman, Chief Executive Officer
New York, New York                                      and Director of MSDW Advisors, MSDW Distributors and MSDW
                                                        Services, Executive Vice President and Director of Dean
                                                        Witter Reynolds Inc. ("DWR"), Chairman and Director of
                                                        Morgan Stanley Dean Witter Trust FSB ("MSDW Trust"), and
                                                        Director and/or officer of various MSDW subsidiaries
                                                        (until June, 1998).
</TABLE>
 
                                       7
<PAGE>
<TABLE>
<CAPTION>
      NAME, AGE, POSITION WITH FUND AND ADDRESS                PRINCIPAL OCCUPATIONS DURING LAST FIVE YEARS
- ------------------------------------------------------  ----------------------------------------------------------
<S>                                                     <C>
Edwin J. Garn (66)                                      Director or Trustee of the Morgan Stanley Dean Witter
Trustee                                                 Funds; formerly United States Senator (R-Utah) (1974-1992)
c/o Huntsman Corporation                                and Chairman, Senate Banking Committee (1980-1986);
500 Huntsman Way                                        formerly Mayor of Salt Lake City, Utah (1972-1974);
Salt Lake City, Utah                                    formerly Astronaut, Space Shuttle Discovery (April 12-19,
                                                        1985); Vice Chairman, Huntsman Corporation (since January,
                                                        1993); Director of Franklin Covey (time management
                                                        systems), John Alden Financial Corp. (health insurance),
                                                        United Space Alliance (joint venture between Lockheed
                                                        Martin and the Boeing Company) and Nuskin Asia Pacific
                                                        (multilevel marketing); member of the board of various
                                                        civic and charitable organizations.
John R. Haire (73)                                      Chairman of the Audit Committee and Director or Trustee of
Trustee                                                 the Morgan Stanley Dean Witter Funds; Chairman of the
Two World Trade Center                                  Audit Committee and Trustee of the TCW/DW Funds; formerly
New York, New York                                      Chairman of the Independent Directors or Trustees of the
                                                        Morgan Stanley Dean Witter Funds and the TCW/DW Funds
                                                        (until June, 1998); formerly President, Council for Aid to
                                                        Education (1978-1989) and Chairman and Chief Executive
                                                        Officer of Anchor Corporation, an Investment Adviser
                                                        (1964-1978).
Wayne E. Hedien (64)                                    Retired; Director or Trustee of the Morgan Stanley Dean
Trustee                                                 Witter Funds; Director of The PMI Group, Inc. (private
c/o Gordon Altman Butowsky                              mortgage insurance); Trustee and Vice Chairman of The
 Weitzen Shalov & Wein                                  Field Museum of Natural History; formerly associated with
Counsel to the Independent Trustees                     the Allstate Companies (1966-1994), most recently as
114 West 47th Street                                    Chairman of The Allstate Corporation (March,
New York, New York                                      1993-December, 1994) and Chairman and Chief Executive
                                                        Officer of its wholly-owned subsidiary, Allstate Insurance
                                                        Company (July, 1989-December, 1994); director of various
                                                        other business and charitable organizations.
Dr. Manuel H. Johnson (49)                              Senior Partner, Johnson Smick International, Inc., a
Trustee                                                 consulting firm; Co-Chairman and a founder of the Group of
c/o Johnson Smick International, Inc.                   Seven Council (G7C), an international economic commission;
1133 Connecticut Avenue, N.W.                           Director or Trustee of the  Morgan Stanley Dean Witter
Washington, D.C.                                        Funds; Trustee of the TCW/DW Funds; Director of Greenwich
                                                        Capital Markets Inc. (broker-dealer) and NVR, Inc. (home
                                                        construction); Director of NASDAQ (since June, 1995);
                                                        Chairman and Trustee of the Financial Accounting
                                                        Foundation (oversight organization for the Financial
                                                        Accounting Standards Board); formerly Vice Chairman of the
                                                        Board of Governors of the Federal Reserve System
                                                        (1986-1990) and Assistant Secretary of the U.S. Treasury
                                                        (1982-1986).
</TABLE>
 
                                       8
<PAGE>
<TABLE>
<CAPTION>
      NAME, AGE, POSITION WITH FUND AND ADDRESS                PRINCIPAL OCCUPATIONS DURING LAST FIVE YEARS
- ------------------------------------------------------  ----------------------------------------------------------
<S>                                                     <C>
Michael E. Nugent (62)                                  General Partner, Triumph Capital, L.P., a private
Trustee                                                 investment partnership; Director or Trustee of the Morgan
c/o Triumph Capital, L.P.                               Stanley Dean Witter Funds; Trustee of the TCW/DW Funds;
237 Park Avenue                                         formerly Vice President, Bankers Trust Company and BT
New York, New York                                      Capital Corporation (1984-1988); Director of various
                                                        business organizations.
Philip J. Purcell* (55)                                 Chairman of the Board of Directors and Chief Executive
Trustee                                                 Officer of MSDW, DWR and Novus Credit Services Inc.;
1585 Broadway                                           Director of MSDW Distributors; Director or Trustee of the
New York, New York                                      Morgan Stanley Dean Witter Funds; Director and/or officer
                                                        of various MSDW subsidiaries.
John L. Schroeder (68)                                  Retired; Director or Trustee of the Morgan Stanley Dean
Trustee                                                 Witter Funds; Trustee of the TCW/DW Funds; Director of
c/o Gordon Altman Butowsky                              Citizens Utilities Company; formerly Executive Vice
  Weitzen Shalov & Wein                                 President and Chief Investment Officer of the Home
Counsel to the Independent Trustees                     Insurance Company (August, 1991-September, 1995).
114 West 47th Street
New York, New York
Barry Fink (43)                                         Senior Vice President (since March, 1997) and Secretary
Vice President, Secretary                               and General Counsel (since February, 1997) and Director
 and General Counsel                                    (since July, 1998) of MSDW Advisors and MSDW Services;
Two World Trade Center                                  Senior Vice President (since March, 1997) and Assistant
New York, New York                                      Secretary and Assistant General Counsel (since February,
                                                        1997) of MSDW Distributors; Assistant Secretary of DWR
                                                        (since August, 1996); Vice President, Secretary and Gen-
                                                        eral Counsel of the Morgan Stanley Dean Witter Funds and
                                                        the TCW/DW Funds (since February, 1997); previously First
                                                        Vice President (June, 1993-February, 1997), Vice President
                                                        and Assistant Secretary and Assistant General Counsel of
                                                        MSDW Advisors and MSDW Services and Assistant Secretary of
                                                        the Morgan Stanley Dean Witter Funds and the TCW/DW Funds.
Peter M. Avelar (40)                                    Senior Vice President of MSDW Advisors; Vice President of
Vice President                                          various Morgan Stanley Dean Witter Funds.
Two World Trade Center
New York, New York
Ellen Gold (34)                                         Vice President of MSDW Advisors (since July, 1998); prior
Assistant Vice President                                thereto an Assistant Vice President (April 1997-July 1998)
Two World Trade Center                                  and Junior Portfolio manager (December 1998-April 1997).
New York, New York
Thomas F. Caloia (52)                                   First Vice President and Assistant Treasurer of MSDW
Treasurer                                               Advisors and MSDW Services; Treasurer of the Morgan
Two World Trade Center                                  Stanley Dean Witter Funds and the TCW/DW Funds.
New York, New York
</TABLE>
 
- ---------
 *Denotes Trustees who are "interested persons" of the Fund, as defined in the
  Act.
 
                                       9
<PAGE>
    In addition, Mitchell M. Merin, President, Chief Executive Officer and
Director of MSDW Advisors and MSDW Services, Chairman and Director of MSDW
Distributors and MSDW Trust, Executive Vice President and Director of DWR, and
Director of various MSDW subsidiaries, Robert M. Scanlan, President, Chief
Operating Officer and Director of MSDW Advisors and MSDW Services, Executive
Vice President of MSDW Distributors and MSDW Trust and Director of MSDW Trust,
Ronald E. Robison, Executive Vice President and Chief Administrative Officer of
MSDW Advisors and MSDW Services, Robert S. Giambrone, Senior Vice President of
MSDW Advisors, MSDW Services, MSDW Distributors and MSDW Trust and Director of
MSDW Trust, Joseph J. McAlinden, Executive Vice President and Chief Investment
Officer of MSDW Advisors and Director of MSDW Trust, Ira N. Ross, Paul D. Vance
and Ronald J. Worobel, Senior Vice President of MSDW Advisors, are Vice
Presidents of the Fund. Marilyn K. Cranney and Carsten Otto, First Vice
Presidents and Assistant General Counsels of MSDW Advisors and MSDW Services,
Frank Bruttomesso, LouAnne D. McInnis and Ruth Rossi, Vice Presidents and
Assistant General Counsels of MSDW Advisors and MSDW Services, and Todd Lebo, a
staff attorney with MSDW Advisors, are Assistant Secretaries of the Fund.
 
THE BOARD OF TRUSTEES, THE INDEPENDENT TRUSTEES, AND THE COMMITTEES
 
    The Board of Trustees consists of nine (9) trustees. These same individuals
also serve as directors or trustees for all of the Morgan Stanley Dean Witter
Funds, and are referred to in this section as Trustees. As of the date of this
Statement of Additional Information, there are a total of 85 Morgan Stanley Dean
Witter Funds, comprised of 121 portfolios. As of October 31, 1998, the Morgan
Stanley Dean Witter Funds had total net assets of approximately $109.2 billion
and more than six million shareholders.
 
    Seven Trustees (77% of the total number) have no affiliation or business
connection with MSDW Advisors or any of its affiliated persons and do not own
any stock or other securities issued by MSDW Advisors' parent company, MSDW.
These are the "disinterested" or "independent" Trustees. Four of the seven
independent Trustees are also Independent Trustees of the TCW/DW Funds.
 
    Law and regulation establish both general guidelines and specific duties for
the Independent Trustees. The Morgan Stanley Dean Witter Funds seek as
Independent Trustees individuals of distinction and experience in business and
finance, government service or academia; these are people whose advice and
counsel are in demand by others and for whom there is often competition. To
accept a position on the Funds' Boards, such individuals may reject other
attractive assignments because the Funds make substantial demands on their time.
Indeed, by serving on the Funds' Boards, certain Trustees who would otherwise be
qualified and in demand to serve on bank boards would be prohibited by law from
doing so.
 
    All of the Independent Trustees serve as members of the Audit Committee.
Three of them also serve as members of the Derivatives Committee. In addition,
three of the Trustees, including two Independent Trustees, serve as members of
the Insurance Committee. During the calendar year ended December 31, 1997, the
Audit Committee, the Derivatives Committee and the Independent Trustees held a
combined total of seventeen meetings.
 
    The Independent Trustees are charged with recommending to the full Board
approval of management, advisory and administration contracts, Rule 12b-1 plans
and distribution and underwriting agreements; continually reviewing Fund
performance; checking on the pricing of portfolio securities, brokerage
commissions, transfer agent costs and performance, and trading among Funds in
the same complex; and approving fidelity bond and related insurance coverage and
allocations, as well as other matters that arise from time to time. The
Independent Trustees are required to select and nominate individuals to fill any
Independent Trustee vacancy on the Board of any Fund that has a Rule 12b-1 plan
of distribution. Most of the Morgan Stanley Dean Witter Funds have such a plan.
 
    The Audit Committee is charged with recommending to the full Board the
engagement or discharge of the Fund's independent accountants; directing
investigations into matters within the scope of the independent accountants'
duties, including the power to retain outside specialists; reviewing with the
independent accountants the audit plan and results of the auditing engagement;
approving professional services provided by the independent accountants and
other accounting firms prior to the performance
 
                                       10
<PAGE>
of such services; reviewing the independence of the independent accountants;
considering the range of audit and non-audit fees; and reviewing the adequacy of
the Fund's system of internal controls.
 
    The Board of each Fund has formed a Derivatives Committee to approve
parameters for and monitor the activities of the Fund with respect to derivative
investments, if any, made by the Fund.
 
    Finally, the Board of each Fund has formed an Insurance Committee to review
and monitor the insurance coverage maintained by the Fund.
 
ADVANTAGES OF HAVING SAME INDIVIDUALS AS INDEPENDENT TRUSTEES FOR ALL MORGAN
STANLEY DEAN WITTER FUNDS
 
    The Independent Trustees and the Funds' management believe that having the
same Independent Trustees for each of the Morgan Stanley Dean Witter Funds
avoids the duplication of effort that would arise from having different groups
of individuals serving as Independent Trustees for each of the Funds or even of
sub-groups of Funds. They believe that having the same individuals serve as
Independent Trustees of all the Funds tends to increase their knowledge and
expertise regarding matters which affect the Fund complex generally and enhances
their ability to negotiate on behalf of each Fund with the Fund's service
providers. This arrangement also precludes the possibility of separate groups of
Independent Trustees arriving at conflicting decisions regarding operations and
management of the Funds and avoids the cost and confusion that would likely
ensue. Finally, having the same Independent Trustees serve on all Fund Boards
enhances the ability of each Fund to obtain, at modest cost to each separate
Fund, the services of Independent Trustees of the caliber, experience and
business acumen of the individuals who serve as Independent Trustees of the
Morgan Stanley Dean Witter Funds.
 
COMPENSATION OF INDEPENDENT TRUSTEES
 
    The Fund pays each Independent Trustee an annual fee of $800 plus a per
meeting fee of $50 for meetings of the Board of Trustees, the Independent
Trustees or Committees of the Board of Trustees attended by the Trustee (the
Fund pays the Chairman of the Audit Committee an additional annual fee of $750).
If a Board meeting and a meeting of the Independent Trustees or a Committee
meeting, or a meeting of the Independent Trustees and/or more than one Committee
meeting, take place on a single day, the Trustees are paid a single meeting fee
by the Fund. The Fund also reimburses such Trustees for travel and other
out-of-pocket expenses incurred by them in connection with attending such
meetings. Trustees and officers of the Fund who are or have been employed by the
Investment Manager or an affiliated company receive no compensation or expense
reimbursement from the Fund for their services as Trustee. Mr. Haire currently
serves as Chairman of the Audit Committee. Prior to June 1, 1998, Mr. Haire also
served as Chairman of the Independent Trustees, for which services the Fund paid
him an additional annual fee of $1,200.
 
    The following table illustrates the compensation paid to the Fund's
Independent Trustees by the Fund for the fiscal year ended September 30, 1998.
 
                               FUND COMPENSATION
 
<TABLE>
<CAPTION>
                                                                   AGGREGATE
                                                                 COMPENSATION
NAME OF INDEPENDENT TRUSTEE                                      FROM THE FUND
- --------------------------------------------------------------  ---------------
<S>                                                             <C>
Michael Bozic.................................................      $1,500
Edwin J. Garn.................................................       1,650
John R. Haire.................................................       2,900
Wayne E. Hedien...............................................       1,932
Dr. Manuel H. Johnson.........................................       1,600
Michael E. Nugent.............................................       1,650
John L. Schroeder.............................................       1,650
</TABLE>
 
    The following table illustrates the compensation paid to the Fund's
Independent Trustees for the calendar year ended December 31, 1997 for services
to the 84 Morgan Stanley Dean Witter Funds and, in the case of Messrs. Haire,
Johnson, Nugent and Schroeder, the 14 TCW/DW Funds that were in operation at
December 31, 1997. Mr. Haire serves as Chairman of the Audit Committee of each
Morgan Stanley Dean Witter Fund and each TCW/DW Fund and, prior to June 1, 1998,
also served as Chairman
 
                                       11
<PAGE>
of the Independent Directors or Trustees of those Funds. With respect to Messrs.
Haire, Johnson, Nugent and Schroeder, the TCW/DW Funds are included solely
because of a limited exchange privilege between those Funds and five Morgan
Stanley Dean Witter Money Market Funds. Mr. Hedien's term as Director or Trustee
of each Morgan Stanley Dean Witter Fund commenced on September 1, 1997.
 
    CASH COMPENSATION FROM MORGAN STANLEY DEAN WITTER FUNDS AND TCW/DW FUNDS
 
<TABLE>
<CAPTION>
                                                                   FOR SERVICE AS                    TOTAL CASH
                                                                    CHAIRMAN OF      FOR SERVICE    COMPENSATION
                               FOR SERVICE                          INDEPENDENT          AS              FOR
                              AS DIRECTOR OR                       DIRECTORS/TRUSTEES  CHAIRMAN OF   SERVICES TO
                               TRUSTEE AND                           AND AUDIT       INDEPENDENT         84
                                COMMITTEE        FOR SERVICE AS    COMMITTEES OF      TRUSTEES         MORGAN
                               MEMBER OF 84       TRUSTEE AND            84           AND AUDIT        STANLEY
                              MORGAN STANLEY       COMMITTEE       MORGAN STANLEY   COMMITTEES OF    DEAN WITTER
NAME OF                        DEAN WITTER        MEMBER OF 14      DEAN WITTER          14         FUNDS AND 14
INDEPENDENT TRUSTEE               FUNDS           TCW/DW FUNDS         FUNDS        TCW/DW FUNDS    TCW/DW FUNDS
- ---------------------------  ----------------   ----------------   --------------   -------------   -------------
<S>                          <C>                <C>                <C>              <C>             <C>
Michael Bozic..............      $133,602           --                 --               --            $133,602
Edwin J. Garn..............       149,702           --                 --               --             149,702
John R. Haire..............       149,702           $73,725           $157,463        $ 25,350         406,240
Wayne E. Hedien............        39,010           --                 --               --              39,010
Dr. Manuel H. Johnson......       145,702            71,125            --               --             216,827
Michael E. Nugent..........       149,702            73,725            --               --             223,427
John L. Schroeder..........       149,702            73,725            --               --             223,427
</TABLE>
 
    As of the date of this Statement of Additional Information, 57 of the Morgan
Stanley Dean Witter Funds, including the Fund, have adopted a retirement program
under which an Independent Trustee who retires after serving for at least five
years (or such lesser period as may be determined by the Board) as an
Independent Director or Trustee of any Morgan Stanley Dean Witter Fund that has
adopted the retirement program (each such Fund referred to as an "Adopting Fund"
and each such Trustee referred to as an "Eligible Trustee") is entitled to
retirement payments upon reaching the eligible retirement age (normally, after
attaining age 72). Annual payments are based upon length of service. Currently,
upon retirement, each Eligible Trustee is entitled to receive from the Adopting
Fund, commencing as of his or her retirement date and continuing for the
remainder of his or her life, an annual retirement benefit (the "Regular
Benefit") equal to 29.41% of his or her Eligible Compensation plus 0.4901667% of
such Eligible Compensation for each full month of service as an Independent
Director or Trustee of any Adopting Fund in excess of five years up to a maximum
of 58.82% after ten years of service. The foregoing percentages may be changed
by the Board.(1) "Eligible Compensation" is one-fifth of the total compensation
earned by such Eligible Trustee for service to the Adopting Fund in the five
year period prior to the date of the Eligible Trustee's retirement. Benefits
under the retirement program are not secured or funded by the Adopting Funds.
 
    The following table illustrates the retirement benefits accrued to the
Fund's Independent Trustees by the Fund for the fiscal year ended September 30,
1998 and by the 57 Morgan Stanley Dean Witter Funds (including the Fund) for the
year ended December 31, 1997, and the estimated retirement benefits for the
Fund's Independent Trustees, to commence upon their retirement, from the Fund as
of September 30, 1998 and from the 57 Morgan Stanley Dean Witter Funds as of
December 31, 1997.
 
- ---------
 
(1) An Eligible Trustee may elect alternate payments of his or her retirement
    benefits based upon the combined life expectancy of such Eligible Trustee
    and his or her spouse on the date of such Eligible Trustee's retirement. The
    amount estimated to be payable under this method, through the remainder of
    the later of the lives of such Eligible Trustee and spouse, will be the
    actuarial equivalent of the Regular Benefit. In addition, the Eligible
    Trustee may elect that the surviving spouse's periodic payment of benefits
    will be equal to either 50% or 100% of the previous periodic amount, an
    election that, respectively, increases or decreases the previous periodic
    amount so that the resulting payments will be the actuarial equivalent of
    the Regular Benefit.
 
                                       12
<PAGE>
   RETIREMENT BENEFITS FROM THE FUND AND ALL MORGAN STANLEY DEAN WITTER FUNDS
 
<TABLE>
<CAPTION>
                                    FOR ALL ADOPTING FUNDS
                                  ---------------------------                           ESTIMATED ANNUAL
                                   ESTIMATED                    RETIREMENT BENEFITS         BENEFITS
                                    CREDITED                    ACCRUED AS EXPENSES           UPON
                                     YEARS        ESTIMATED                               RETIREMENT(2)
                                   OF SERVICE     PERCENTAGE    -------------------     -----------------
                                       AT             OF                    BY ALL       FROM    FROM ALL
                                   RETIREMENT      ELIGIBLE     BY THE     ADOPTING      THE     ADOPTING
NAME OF INDEPENDENT TRUSTEE       (MAXIMUM 10)   COMPENSATION    FUND       FUNDS        FUND     FUNDS
- --------------------------------  ------------   ------------   ------     --------     ------   --------
<S>                               <C>            <C>            <C>        <C>          <C>      <C>
Michael Bozic...................       10           58.82%       $408      $ 20,499     $1,029   $ 55,026
Edwin J. Garn...................       10           58.82         614        30,878      1,029     55,026
John R. Haire...................       10           58.82         (64)(3)   (19,823)(3)  2,418    132,002
Wayne E. Hedien.................        9           50.00         589             0        875     46,772
Dr. Manuel H. Johnson...........       10           58.82         248        12,832      1,029     55,026
Michael E. Nugent...............       10           58.82         435        22,546      1,029     55,026
John L. Schroeder...............        8           49.02         816        39,350        861     46,123
</TABLE>
 
- ---------
(2) Based on current levels of compensation. Amount of annual benefits also
    varies depending on the Trustee's elections described in Footnote (1) above.
 
(3) This number reflects the effect of the extension of Mr. Haire's term as
    Director or Trustee until May 1, 1999.
 
    As of the date of this Statement of Additional Information, the aggregate
number of shares of beneficial interest of the Fund owned by the Fund's officers
and Trustees as a group was less than 1 percent of the Fund's shares of
beneficial interest outstanding.
 
INVESTMENT PRACTICES AND POLICIES
- --------------------------------------------------------------------------------
 
    CONVERTIBLE SECURITIES.  A convertible security entitles the holder to
exchange it for a fixed number of shares of common stock or other equity
security, usually of the same company, at fixed prices within a specified period
of time. As such, a convertible security entitles the holder to receive the
fixed income of a bond or the dividend preference of a preferred stock until the
holder elects to exercise the conversion privilege.
 
    A convertible security's position in a company's capital structure depends
upon its particular provisions. In the case of subordinated convertible
debentures, the holders' claims on assets and earnings are subordinated to the
claims of other creditors, and are senior to the claims of preferred and common
shareholders. In the case of convertible preferred stock, the holders' claims on
assets and earnings are subordinated to the claims of all creditors and are
senior to the claims of common shareholders.
 
    Every convertible security may be valued, on a theoretical basis, as if it
did not have a conversion privilege. Such theoretical value is determined by the
yield it provides in comparison with the yields of other securities of
comparable character and quality which do not have a conversion privilege. This
theoretical value, which will change with prevailing interest rates, the credit
standing of the issuer and other pertinent factors, is often referred to as the
"investment value," and represents the security's theoretical price support
level.
 
    "Conversion value" is the amount a convertible security would be worth in
market value if it were to be exchanged for the underlying equity security
pursuant to its conversion privilege. Conversion value fluctuates directly with
the price of the underlying equity security, usually common stock. If, because
of low prices for the common stock, the conversion value is substantially below
the investment value, the price of the convertible security is governed
principally by the factors described in the preceding paragraph. If the
conversion value rises near or above its investment value, the price of the
convertible security generally will rise above its investment value and, in
addition, will sell at some premium over its conversion value. This premium
represents the price investors are willing to pay for the privilege of
purchasing a fixed-income security with a possibility of capital appreciation
due to the conversion privilege. If this appreciation potential is not realized,
this premium may not be recovered.
 
    To the degree that the price of a convertible security rises above its
investment value because of a rise in price of the common stock, it is
influenced more by price fluctuations of the common stock and
 
                                       13
<PAGE>
less by its investment value. The price of a convertible security that is
supported principally by its conversion value will rise along with any increase
in the price of the common stock, and such price generally will decline along
with any decline in the price of the common stock except that the security will
receive additional support as its price approaches investment value. A
convertible security purchased or held at a time when its price is influenced by
its conversion value will produce a lower yield than nonconvertible senior
securities with comparable investment values. Convertible securities may be
purchased by the Fund at varying price levels above their investment values
and/or their conversion values in keeping with the Fund's investment objectives.
 
    CORPORATE FIXED-INCOME SECURITIES.  As discussed in the Prospectus, in order
to generate the current income needed to achieve its investment objective, the
Fund may invest in investment grade non-convertible fixed-income securities as
well as in such securities which are in the lower rating categories of
recognized rating agencies (Standard & Poor's Corporation and Moody's Investors
Service, Inc.) or which are not rated by such agencies. The Investment Manager
will perform its own credit analyses in addition to using recognized rating
agencies and other sources. In making such credit analyses, substantial
consideration will be given to a determination of value based upon, among other
things, anticipated cash flows, interest or dividend coverage, asset coverage,
earnings, experience of the issuer, responsiveness to changes in interest rates
and business conditions and liquidation value relative to the market price.
 
    WHEN, AS AND IF ISSUED SECURITIES.  As discussed in the Prospectus, the Fund
may purchase securities on a "when, as and if issued" basis under which the
issuance of the security depends upon the occurrence of a subsequent event, such
as approval of a merger, corporate reorganization, leveraged buyout or debt
restructuring. The commitment for the purchase of any such security will not be
recognized in the portfolio of the Fund until the Investment Manager determines
that issuance of the security is probable. At such time, the Fund will record
the transaction and, in determining its net asset value, will reflect the value
of the security daily. At such time, the Fund will also establish a segregated
account with its custodian bank in which it will maintain cash or cash
equivalents or other liquid portfolio securities equal in value to recognized
commitments for such securities. Once a segregated account has been established,
if the anticipated event does not occur and the securities are not issued, the
Fund will have lost an investment opportunity. The value of the Fund's
commitments to purchase the securities of any one issuer, together with the
value of all securities of such issuer owned by the Fund, may not exceed 5% of
the value of the Fund's total assets at the time the initial commitment to
purchase such securities is made (see "Investment Restrictions"). Subject to the
foregoing restrictions, the Fund may purchase securities on such basis without
limit. An increase in the percentage of the Fund's assets committed to the
purchase of securities on a "when, as and if issued" basis may increase the
volatility of its net asset value. The Fund may also sell securities on a "when,
as and if issued" basis provided that the issuance of the security will result
automatically from the exchange or conversion of a security owned by the Fund at
the time of sale.
 
OPTIONS AND FUTURES TRANSACTIONS
 
    The Fund may write covered call options against securities held in its
portfolio and covered put options on eligible portfolio securities and purchase
options of the same series to effect closing transactions, and may hedge against
potential changes in the market value of investments (or anticipated
investments) and facilitate the reallocation of the Fund's assets into and out
of equities and fixed-income securities by purchasing put and call options on
portfolio (or eligible portfolio) securities and engaging in transactions
involving futures contracts and options on such contracts.
 
    Call and put options on U.S. Treasury notes, bonds and bills and equity
securities are listed on Exchanges (currently the Chicago Board Options
Exchange, American Stock Exchange, New York Stock Exchange, Pacific Stock
Exchange and Philadelphia Stock Exchange) and are written in over-the-counter
transactions ("OTC Options"). Listed options are issued by the Options Clearing
Corporation ("OCC"). Ownership of a listed call option gives the Fund the right
to buy from the OCC the underlying security covered by the option at the stated
exercise price (the price per unit of the underlying security) by filing an
exercise notice prior to the expiration date of the option. The writer (seller)
of the option would
 
                                       14
<PAGE>
then have the obligation to sell to the OCC the underlying security at that
exercise price prior to the expiration date of the option, regardless of its
then current market price. Ownership of a listed put option would give the Fund
the right to sell the underlying security to the OCC at the stated exercise
price. Upon notice of exercise of the put option, the writer of the put would
have the obligation to purchase the underlying security from the OCC at the
exercise price.
 
    OPTIONS ON TREASURY BONDS AND NOTES.  Because trading interest in options
written on Treasury bonds and notes tends to center on the most recently
auctioned issues, the exchanges on which such securities trade will not continue
indefinitely to introduce options with new expirations to replace expiring
options on particular issues. Instead, the expirations introduced at the
commencement of options trading on a particular issue will be allowed to run
their course, with the possible addition of a limited number of new expirations
as the original ones expire. Options trading on each issue of bonds or notes
will thus be phased out as new options are listed on more recent issues, and
options representing a full range of expirations will not ordinarily be
available for every issue on which options are traded.
 
    OPTIONS ON TREASURY BILLS.  Because a deliverable Treasury bill changes from
week to week, writers of Treasury bill calls cannot provide in advance for their
potential exercise settlement obligations by acquiring and holding the
underlying security. However, if the Fund holds a long position in Treasury
bills with a principal amount of the securities deliverable upon exercise of the
option, the position may be hedged from a risk standpoint by the writing of a
call option. For so long as the call option is outstanding, the Fund will hold
the Treasury bills in a segregated account with its Custodian, so that they will
be treated as being covered.
 
    OTC OPTIONS.  Exchange-listed options are issued by the OCC which assures
that all transactions in such options are properly executed. OTC options are
purchased from or sold (written) to dealers or financial institutions which have
entered into direct agreements with the Fund. With OTC options, such variables
as expiration date, exercise price and premium will be agreed upon between the
Fund and the transacting dealer, without the intermediation of a third party
such as the OCC. If the transacting dealer fails to make or take delivery of the
securities underlying an option it has written, in accordance with the terms of
that option, the Fund would lose the premium paid for the option as well as any
anticipated benefit of the transaction. The Fund will engage in OTC option
transactions only with primary U.S. Government securities dealers recognized by
the Federal Reserve Bank of New York.
 
    COVERED CALL WRITING.  The Fund is permitted to write covered call options
on portfolio securities, without limit, in order to aid in achieving its
investment objective. Generally, a call option is "covered" if the Fund owns, or
has the right to acquire, without additional cash consideration (or for
additional cash consideration held for the Fund by its Custodian in a segregated
account) the underlying security subject to the option except that in the case
of call options on U.S. Treasury bills, the Fund might own U.S. Treasury bills
of a different series from those underlying the call option, but with a
principal amount and value corresponding to the exercise price and a maturity
date no later than that of the securities deliverable under the call option. A
call option is also covered if the Fund holds a call on the same security as the
underlying security of the written option, where the exercise price of the call
used for coverage is equal to or less than the exercise price of the call
written or greater than the exercise price of the call written if the
mark-to-market difference is maintained by the Fund in cash, U.S. Government
securities or other liquid portfolio securities which the Fund holds in a
segregated account maintained with its Custodian.
 
    The Fund will receive from the purchaser, in return for a call it has
written, a "premium"; i.e., the price of the option. Receipt of these premiums
may better enable the Fund to achieve a greater total return than would be
realized from holding the underlying securities alone. Moreover, the premium
received will offset a portion of the potential loss incurred by the Fund if the
securities underlying the option are ultimately sold by the Fund at a loss. The
premium received will fluctuate with varying economic market conditions. If the
market value of the portfolio securities upon which call options have been
written increases, the Fund may receive less total return from the portion of
its portfolio upon which calls have been written than it would have had such
calls not been written.
 
                                       15
<PAGE>
    As regards listed options and certain over-the-counter ("OTC") options,
during the option period, the Fund may be required, at any time, to deliver the
underlying security against payment of the exercise price on any calls it has
written (exercise of certain listed options may be limited to specific
expiration dates). This obligation is terminated upon the expiration of the
option period or at such earlier time when the writer effects a closing purchase
transaction. A closing purchase transaction is accomplished by purchasing an
option of the same series as the option previously written. However, once the
Fund has been assigned an exercise notice, the Fund will be unable to effect a
closing purchase transaction.
 
    Closing purchase transactions are ordinarily effected to realize a profit on
an outstanding call option to prevent an underlying security from being called,
to permit the sale of an underlying security or to enable the Fund to write
another call option on the underlying security with either a different exercise
price or expiration date or both. Also, effecting a closing purchase transaction
will permit the cash or proceeds from the concurrent sale of any securities
subject to the option to be used for other investments by the Fund. The Fund may
realize a net gain or loss from a closing purchase transaction depending upon
whether the amount of the premium received on the call option is more or less
than the cost of effecting the closing purchase transaction. Any loss incurred
in a closing purchase transaction may be wholly or partially offset by
unrealized appreciation in the market value of the underlying security.
Conversely, a gain resulting from a closing purchase transaction could be offset
in whole or in part or exceeded by a decline in the market value of the
underlying security.
 
    If a call option expires unexercised, the Fund realizes a gain in the amount
of the premium on the option less the commission paid. Such a gain, however, may
be offset by depreciation in the market value of the underlying security during
the option period. If a call option is exercised, the Fund realizes a gain or
loss from the sale of the underlying security equal to the difference between
the purchase price of the underlying security and the proceeds of the sale of
the security plus the premium received on the option less the commission paid.
 
    Options written by a Fund normally have expiration dates of from up to nine
months (equity securities) to eighteen months (fixed-income securities) from the
date written. The exercise price of a call option may be below, equal to or
above the current market value of the underlying security at the time the option
is written. See "Risks of Options Transactions," below.
 
    COVERED PUT WRITING.  As a writer of a covered put option, the Fund incurs
an obligation to buy the security underlying the option from the purchaser of
the put, at the option's exercise price at any time during the option period, at
the purchaser's election (certain listed put options written by the Fund will be
exercisable by the purchaser only on a specific date). A put is "covered" if the
Fund maintains, in a segregated account maintained on its behalf at the Fund's
Custodian, cash, U.S. Government securities or other liquid portfolio securities
in an amount equal to at least the exercise price of the option, at all times
during the option period. Similarly, a written put position could be covered by
the Fund by its purchase of a put option on the same security as the underlying
security of the written option, where the exercise price of the purchased option
is equal to or more than the exercise price of the put written or less than the
exercise price of the put written if the mark-to-market difference is maintained
by the Fund in cash, U.S. Government securities or other high grade debt
obligations which the Fund holds in a segregated account maintained at its
Custodian. In writing puts, the Fund assumes the risk of loss should the market
value of the underlying security decline below the exercise price of the option
(any loss being decreased by the receipt of the premium on the option written).
During the option period, the Fund may be required, at any time, to make payment
of the exercise price against delivery of the underlying security. The operation
of and limitations on covered put options in other respects are substantially
identical to those of call options.
 
    The Fund will write put options for two purposes: (1) to receive the income
derived from the premiums paid by purchasers; and (2) when the Investment
Manager wishes to purchase the security underlying the option at a price lower
than its current market price, in which case it will write the covered put at an
exercise price reflecting the lower purchase price sought. The potential gain on
a covered put option is limited to the premium received on the option (less the
commissions paid on the transaction)
 
                                       16
<PAGE>
while the potential loss equals the difference between the exercise price of the
option and the current market price of the underlying securities when the put is
exercised, offset by the premium received (less the commissions paid on the
transaction).
 
    PURCHASING CALL AND PUT OPTIONS.  The Fund may purchase listed call and put
options in amounts equalling up to 10% of its total assets. The Fund may
purchase call options only in order to close out a covered call position (see
"Covered Call Writing" above). The call purchased is likely to be on the same
securities and have the same terms as the written option. The option would
generally be acquired from the dealer or financial institution which purchased
the call written by the Fund.
 
    The Fund may purchase put options on securities which it holds (or has the
right to acquire) in its portfolio only to protect itself against a decline in
the value of the security. If the value of the underlying security were to fall
below the exercise price of the put purchased in an amount greater than the
premium paid for the option, the Fund would incur no additional loss. The Fund
may also purchase put options to close out written put positions in a manner
similar to call options closing purchase transactions. In addition, the Fund may
sell a put option which it has previously purchased prior to the sale of the
securities underlying such option. Such a sale would result in a net gain or
loss depending on whether the amount received on the sale is more or less than
the premium and other transaction costs paid on the put option when it was
purchased. Any such gain or loss could be offset in whole or in part by a change
in the market value of the underlying security. If a put option purchased by the
Fund expired without being sold or exercised, the premium would be lost.
 
    RISKS OF OPTIONS TRANSACTIONS.  During the option period, the covered call
writer has, in return for the premium on the option, given up the opportunity
for capital appreciation above the exercise price should the market price of the
underlying security increase, but has retained the risk of loss should the price
of the underlying security decline. The secured put writer also retains the risk
of loss should the market value of the underlying security decline below the
exercise price of the option less the premium received on the sale of the
option. In both cases, the writer has no control over the time when it may be
required to fulfill its obligation as a writer of the option. Once an option
writer has received an exercise notice, it cannot effect a closing purchase
transaction in order to terminate its obligation under the option and must
deliver or receive the underlying securities at the exercise price.
 
    Prior to exercise or expiration, an option position can only be terminated
by entering into a closing purchase or sale transaction. If a covered call
option writer is unable to effect a closing purchase transaction it cannot sell
the underlying security until the option expires or the option is exercised.
Accordingly, a covered call option writer may not be able to sell an underlying
security at a time when it might otherwise be advantageous to do so. A covered
put option writer who is unable to effect a closing purchase transaction would
continue to bear the risk of decline in the market price of the underlying
security until the option expires or is exercised. In addition, a covered put
writer would be unable to utilize the amount held in cash or U.S. Government or
other liquid portfolio securities as cover for the put option for other
investment purposes until the exercise or expiration of the option.
 
    The Fund's ability to close out its position as a writer of an option is
dependent upon the existence of a liquid secondary market on option exchanges.
There is no assurance that such a market will exist. However, the Fund may be
able to purchase an offsetting option which does not close out its position as a
writer but constitutes an asset of equal value to the obligation under the
option written. If the Fund is not able to either enter into a closing purchase
transaction or purchase an offsetting position, it will be required to maintain
the securities subject to the call, or the collateral underlying the put, even
though it might not be advantageous to do so, until a closing transaction can be
entered into (or the option is exercised or expires). In addition, in the event
of the bankruptcy of a broker through which the Fund engages in transactions in
options, the Fund could experience delays and/or losses in liquidating open
positions purchased or sold through the broker and/or incur a loss of all or
part of its margin deposits with the broker.
 
    Among the possible reasons for the absence of a liquid secondary market on
an Exchange are: (i) insufficient trading interest in certain options; (ii)
restrictions on transactions imposed by an
 
                                       17
<PAGE>
Exchange; (iii) trading halts, suspensions or other restrictions imposed with
respect to particular classes or series of options or underlying securities;
(iv) interruption of the normal operations on an Exchange; (v) inadequacy of the
facilities of an Exchange or the Options Clearing Corporation ("OCC") to handle
current trading volume; or (vi) a decision by one or more Exchanges to
discontinue the trading of options (or a particular class or series of options),
in which event the secondary market on that Exchange (or in that class or series
of options) would cease to exist, although outstanding options on that Exchange
that had been issued by the OCC as a result of trades on that Exchange would
generally continue to be exercisable in accordance with their terms.
 
    In the event of the bankruptcy of a broker through which the Fund engages in
transactions in options, the Fund could experience delays and/or losses in
liquidating open positions purchased or sold through the broker and/or incur a
loss of all or part of its margin deposits with the broker. Similarly, in the
event of the bankruptcy of the writer of an OTC option purchased by the Fund,
the Fund could experience a loss of all or part of the value of the option.
Transactions are entered into by the Fund only with brokers or financial
institutions deemed creditworthy by the Investment Manager.
 
    Each of the Exchanges has established limitations governing the maximum
number of call or put options on the same underlying security or futures
contract (whether or not covered) which may be written by a single investor,
whether acting alone or in concert with others (regardless of whether such
options are written on the same or different Exchanges or are held or written on
one or more accounts or through one or more brokers). An Exchange may order the
liquidation of positions found to be in violation of these limits and it may
impose other sanctions or restrictions. These position limits may restrict the
number of listed options which the Fund may write.
 
    The hours of trading for options may not conform to the hours during which
the underlying securities are traded. To the extent that the option markets
close before the markets for the underlying securities, significant price and
rate movements can take place in the underlying markets that cannot be reflected
in the option markets.
 
    FUTURES CONTRACTS.  The Fund may purchase and sell futures contracts that
are traded on U.S. commodity exchanges on such underlying securities as U.S.
Treasury bonds, notes and bills. As a futures contract purchaser, the Fund
incurs an obligation to take delivery of a specified amount of the obligation
underlying the contract at a specified time in the future for a specified price.
As a seller of a futures contract, the Fund incurs an obligation to deliver the
specified amount of the underlying obligation at a specified time in return for
an agreed upon price.
 
    Although most futures contracts call for actual delivery or acceptance of
securities, the contracts usually are closed out before the settlement date
without the making or taking of delivery. A futures contract sale is closed out
by effecting a futures contract purchase for the same aggregate amount of the
specific type of security and the same delivery date. If the sale price exceeds
the offsetting purchase price, the seller would be paid the difference and would
realize a gain. If the offsetting purchase price exceeds the sale price, the
seller would pay the difference and would realize a loss. Similarly, a futures
contract purchase is closed out by effecting a futures contract sale for the
same aggregate amount of the specific type of security and the same delivery
date. If the offsetting sale price exceeds the purchase price, the purchaser
would realize a gain, whereas if the purchase price exceeds the offsetting sale
price, the purchaser would realize a loss. There is no assurance that the Fund
will be able to enter into a closing transaction.
 
    When the Fund enters into a futures contract, it is initially required to
deposit with the Fund's Custodian, in a segregated account in the name of the
broker performing the transaction, an "initial margin" of cash or U.S.
Government securities or other liquid portfolio securities equal to
approximately 2% of the contract amount. Initial margin requirements are
established by the Exchanges on which futures contracts trade and may, from time
to time, change. In addition, brokers may establish margin deposit requirements
in excess of those required by the Exchanges.
 
                                       18
<PAGE>
    Initial margin in futures transactions is different from margin in
securities transactions in that initial margin does not involve the borrowing of
funds by a brokers' client but is, rather, a good faith deposit on the futures
contract which will be returned to the Fund upon the proper termination of the
futures contract. The margin deposits made are mark-to-market daily and the Fund
may be required to make subsequent deposits of cash or U.S. Government
securities called "variation margin," with the Fund's futures contract clearing
broker, which are reflective of price fluctuations in the futures contract.
Currently, interest rate futures contracts can be purchased on debt securities
such as U.S. Treasury Bills and Bonds, U.S. Treasury Notes with maturities
between 6 1/2 and 10 years, GNMA Certificates and Bank Certificates of Deposit.
 
    OPTIONS ON FUTURES CONTRACTS.  The Fund may purchase and write call and put
options on futures contracts which are traded on an Exchange and enter into
closing transactions with respect to such options to terminate an existing
position. An option on a futures contract gives the purchaser the right (in
return for the premium paid), to assume a position in a futures contract (a long
position if the option is a call and a short position if the option is a put) at
a specified exercise price at any time during the term of the option. Upon
exercise of the option, the delivery of the futures position by the writer of
the option to the holder of the option is accompanied by delivery of the
accumulated balance in the writer's futures margin account, which represents the
amount by which the market price of the futures contract at the time of exercise
exceeds, in the case of a call, or is less than, in the case of a put, the
exercise price of the option on the futures contract.
 
    The Fund will purchase and write options on futures contracts for identical
purposes to those set forth above for the purchase of a futures contract
(purchase of a call option or sale of a put option) and the sale of a futures
contract (purchase of a put option or sale of a call option), or to close out a
long or short position in futures contracts. If, for example, the Investment
Manager wished to protect against an increase in interest rates and the
resulting negative impact on the value of a portion of its fixed-income
portfolio, it might write a call option on an interest rate futures contract,
the underlying security of which correlates with the portion of the portfolio
the Investment Manager seeks to hedge. Any premiums received in the writing of
options on futures contracts may, of course, augment the total return of the
Fund and thereby provide a further hedge against losses resulting from price
declines in portions of the Fund's portfolio.
 
    The writer of an option on a futures contract is required to deposit initial
and variation margin pursuant to requirements similar to those applicable to
futures contracts. Premiums received from the writing of an option on a futures
contract are included in initial margin deposits.
 
    LIMITATIONS ON FUTURES CONTRACTS AND OPTIONS ON FUTURES.  The Fund may not
enter into futures contracts or purchase related options thereon if, immediately
thereafter, the amount committed to margin plus the amount paid for premiums for
unexpired options on futures contracts exceeds 5% of the value of the Fund's
total assets, after taking into account unrealized gains and unrealized losses
on such contracts it has entered into, provided, however, that in the case of an
option that is in-the-money (the exercise price of the call (put) option is less
(more) than the market price of the underlying security) at the time of
purchase, the in-the-money amount may be excluded in calculating the 5%.
However, there is no overall limitation on the percentage of the Fund's assets
which may be subject to a hedge position. In addition, in accordance with the
regulations of the Commodity Futures Trading Commission ("CFTC") under which the
Fund is exempted from registration as a commodity pool operator, the Fund may
only enter into futures contracts and options on futures contracts transactions
for purposes of hedging a part or all of its portfolio. If the CFTC changes its
regulations so that the Fund would be permitted to write options on futures
contracts for purposes other than hedging the Fund's investments without CFTC
registration, the Fund may engage in such transactions for those purposes.
Except as described above, there are no other limitations on the use of futures
and options thereon by the Fund.
 
    RISKS OF TRANSACTIONS IN FUTURES CONTRACTS AND RELATED OPTIONS.  As stated
in the Prospectus, the Fund may sell a futures contract to protect against the
decline in the value of securities held by the Fund. However, it is possible
that the futures market may advance and the value of securities held in the
 
                                       19
<PAGE>
portfolio of the Fund may decline. If this occurred, the Fund would lose money
on the futures contract and also experience a decline in value of its portfolio
securities. However, while this could occur for a very brief period or to a very
small degree, over time the value of a diversified portfolio will tend to move
in the same direction as the futures contracts.
 
    If the Fund purchases a futures contract to hedge against the increase in
value of securities it intends to buy, and the value of such securities
decreases, then the Investment Manager may determine not to invest in the
securities as planned and will realize a loss on the futures contract that is
not offset by a reduction in the price of the securities.
 
    In order to assure that the Fund is entering into transactions in futures
contracts for hedging purposes as such is defined by the Commodities Futures
Trading Commission either: 1) a substantial majority (i.e., approximately 75%)
of all anticipatory hedge transactions (transactions in which the Fund does not
own at the time of the transaction, but expects to acquire, the securities
underlying the relevant futures contract) involving the purchase of futures
contracts will be completed by the purchase of securities which are the subject
of the hedge or 2) the underlying value of all long positions in futures
contracts will not exceed the total value of: a) all short-term debt obligations
held by the Fund; b) cash held by the Fund; c) cash proceeds due to the Fund on
investments within thirty days; d) the margin deposited on the contracts; and e)
any unrealized appreciation in the value of the contracts.
 
    If the Fund maintains a short position in a futures contract or has sold a
call option on a futures contract, it will cover this position by holding, in a
segregated account maintained at its Custodian, cash, U.S. Government securities
or other high grade debt obligations equal in value (when added to any initial
or variation margin on deposit) to the market value of the securities underlying
the futures contract or the exercise price of the option. Such a position may
also be covered by owning the securities underlying the futures contract, or by
holding a call option permitting the Fund to purchase the same contract at a
price no higher than the price at which the short position was established.
 
    In addition, if the Fund holds a long position in a futures contract or has
sold a put option on a futures contract, it will hold cash, U.S. Government
securities or other liquid portfolio securities equal to the purchase price of
the contract or the exercise price of the put option (less the amount of initial
or variation margin on deposit) in a segregated account maintained for the Fund
by its Custodian. Alternatively, the Fund could cover its long position by
purchasing a put option on the same futures contract with an exercise price as
high or higher than the price of the contract held by the Fund.
 
    Exchanges limit the amount by which the price of a futures contract may move
on any day. If the price moves equal the daily limit on successive days, then it
may prove impossible to liquidate a futures position until the daily limit moves
have ceased. In the event of adverse price movements, the Fund would continue to
be required to make daily cash payments of variation margin on open futures
positions. In such situations, if the Fund has insufficient cash, it may have to
sell portfolio securities to meet daily variation margin requirements at a time
when it may be disadvantageous to do so. In addition, the Fund may be required
to take or make delivery of the instruments underlying futures contracts it
holds at a time when it is disadvantageous to do so. The inability to close out
options and futures positions could also have an adverse impact on the Fund's
ability to effectively hedge its portfolio. In addition, in the event of the
bankruptcy of a broker through which the Fund engages in transactions in futures
or options thereon, the Fund could experience delays and/or losses in
liquidating open positions purchased or sold through the broker and/or incur a
loss of all or part of its margin deposits with the broker.
 
    In the event of the bankruptcy of a broker through which the Fund engages in
transactions in futures or options thereon, the Fund could experience delays
and/or losses in liquidating open positions purchased or sold through the broker
and/or incur a loss of all or part of its margin deposits with the broker.
Similarly, in the event of the bankruptcy of the writer of an OTC option
purchased by the Fund, the Fund could experience a loss of all or part of the
value of the option. Transactions are entered into by the Fund only with brokers
or financial institutions deemed creditworthy by the Investment Manager.
 
    While the futures contracts and options transactions to be engaged in by the
Fund for the purpose of hedging the Fund's portfolio securities are not
speculative in nature, there are risks inherent in the use
 
                                       20
<PAGE>
of such instruments. One such risk which may arise in employing futures
contracts to protect against the price volatility of portfolio securities is
that the prices of securities and indexes subject to futures contracts (and
thereby the futures contract prices) may correlate imperfectly with the behavior
of the cash prices of the Fund's portfolio securities. Another such risk is that
prices of futures contracts may not move in tandem with the changes in
prevailing interest rates against which the Fund seeks a hedge. A correlation
may also be distorted by the fact that the futures market is dominated by
short-term traders seeking to profit from the difference between a contract or
security price objective and their cost of borrowed funds. Such distortions are
generally minor and would diminish as the contract approached maturity.
 
    There may exist an imperfect correlation between the price movements of
futures contracts purchased by the Fund and the movements in the prices of the
securities which are the subject of the hedge. If participants in the futures
market elect to close out their contracts through offsetting transactions rather
than meet margin deposit requirements, distortions in the normal relationship
between the debt securities and futures markets could result. Price distortions
could also result if investors in futures contracts opt to make or take delivery
of underlying securities rather than engage in closing transactions due to the
resultant reduction in the liquidity of the futures market. In addition, due to
the fact that, from the point of view of speculators, the deposit requirements
in the futures markets are less onerous than margin requirements in the cash
market, increased participation by speculators in the futures market could cause
temporary price distortions. Due to the possibility of price distortions in the
futures market and because of the imperfect correlation between movements in the
prices of securities and movements in the prices of futures contracts, a correct
forecast of interest rate trends by the Investment Manager may still not result
in a successful hedging transaction.
 
    There is no assurance that a liquid secondary market will exist for futures
contracts and related options in which the Fund may invest. In the event a
liquid market does not exist, it may not be possible to close out a futures
position, and in the event of adverse price movements, the Fund would continue
to be required to make daily cash payments of variation margin. In addition,
limitations imposed by an exchange or board of trade on which futures contracts
are traded may compel or prevent the Fund from closing out a contract which may
result in reduced gain or increased loss to the Fund. The absence of a liquid
market in futures contracts might cause the Fund to make or take delivery of the
underlying securities at a time when it may be disadvantageous to do so.
 
    The extent to which the Fund may enter into transactions involving futures
contracts and options thereon may be limited by the Internal Revenue Code's
requirements for qualification as a regulated investment company and the Fund's
intention to qualify as such (see "Dividends, Distributions and Taxes" in the
Prospectus).
 
    Compared to the purchase or sale of futures contracts, the purchase of call
or put options on futures contracts involves less potential risk to the Fund
because the maximum amount at risk is the premium paid for the options (plus
transaction costs). However, there may be circumstances when the purchase of a
call or put option on a futures contract would result in a loss to the Fund
notwithstanding that the purchase or sale of a futures contract would not result
in a loss, as in the instance where there is no movement in the prices of the
futures contract or underlying securities.
 
    The Investment Manager has substantial experience in the use of the
investment techniques described above under the heading "Options and Futures
Transactions," which techniques require skills different from those needed to
select the portfolio securities underlying various options and futures
contracts.
 
LENDING OF PORTFOLIO SECURITIES
 
    Consistent with applicable regulatory requirements, the Fund may lend its
portfolio securities to brokers, dealers and other financial institutions,
provided that such loans are callable at any time by the Fund (subject to notice
provisions described below), and are at all times secured by cash or cash
equivalents, which are maintained in a segregated account pursuant to applicable
regulations and that are equal to at least the market value, determined daily,
of the loaned securities. The advantage of such loans is that the Fund continues
to receive the income on the loaned securities while at the same time
 
                                       21
<PAGE>
earning interest on the cash amounts deposited as collateral, which will be
invested in short-term obligations. The Fund will not lend its portfolio
securities if such loans are not permitted by the laws or regulations of any
state in which its shares are qualified for sale and will not lend more than 25%
of the value of its total assets.
 
    A loan may be terminated by the borrower on one business day's notice, or by
the Fund on two business days' notice. If the borrower fails to deliver the
loaned securities within two days after receipt of notice, the Trust could use
the collateral to replace the securities while holding the borrower liable for
any excess of replacement cost over collateral. As with any extensions of
credit, there are risks of delay in recovery and in some cases even loss of
rights in the collateral should the borrower of the securities fail financially.
However, these loans of portfolio securities will only be made to firms deemed
by the Fund's management to be creditworthy and when the income which can be
earned from such loans justifies the attendant risks. Upon termination of the
loan, the borrower is required to return the securities to the Fund. Any gain or
loss in the market price during the loan period would inure to the Fund. The
creditworthiness of firms to which the Fund lends its portfolio securities will
be monitored on an ongoing basis by the Investment Manager pursuant to
procedures adopted and reviewed, on an ongoing basis, by the Trustees of the
Fund.
 
    When voting or consent rights which accompany loaned securities pass to the
borrower, the Fund will follow the policy of calling the loaned securities, to
be delivered within one day after notice, to permit the exercise of such rights
if the matters involved would have a material effect on the Fund's investment in
such loaned securities. The Fund will pay reasonable finder's, administrative
and custodial fees in connection with a loan of its securities. However, the
Fund did not lend any of its portfolio securities during the fiscal year ended
September 30, 1998 and it has no intention of doing so in the foreseeable
future.
 
REPURCHASE AGREEMENTS
 
    When cash may be available for only a few days, it may be invested by the
Fund in repurchase agreements until such time as it may otherwise be invested or
used for payments of obligations of the Fund. These agreements, which may be
viewed as a type of secured lending by the Fund, typically involve the
acquisition by the Fund of debt securities from a selling financial institution
such as a bank, savings and loan association or broker-dealer. The agreement
provides that the Fund will sell back to the institution, and that the
institution will repurchase, the underlying security ("collateral"), which is
held by the Fund's Custodian, at a specified price and at a fixed time in the
future, usually not more than seven days from the date of purchase. The Fund
will receive interest from the institution until the time when the repurchase is
to occur. Although such date is deemed by the Fund to be the maturity date of a
repurchase agreement, the maturities of securities subject to repurchase
agreements are not subject to any limits and may exceed one year. While
repurchase agreements involve certain risks not associated with direct
investments in debt securities, the Fund follows procedures designed to minimize
such risks. These procedures include effecting repurchase transactions only with
large, well-capitalized and well-established financial institutions, whose
financial condition will be continually monitored by the Investment Manager
subject to procedures established by the Trustees. In addition, the value of the
collateral underlying the repurchase agreement will always be at least equal to
the repurchase price, including any accrued interest earned on the repurchase
agreement. In the event of a default or bankruptcy by a selling financial
institution, the Fund will seek to liquidate such collateral. However, the
exercising of the Fund's right to liquidate such collateral could involve
certain costs or delays and, to the extent that proceeds from any sale upon a
default of the obligation to repurchase were less than the repurchase price, the
Fund could suffer a loss. It is the current policy of the Fund not to invest in
repurchase agreements that do not mature within seven days if any such
investment, together with any other illiquid assets held by the Fund, amounts to
more than 10% of its total assets. The Fund's investments in repurchase
agreements may at times be substantial when, in the view of the Investment
Manager, liquidity or other considerations warrant. The Fund did not enter into
any repurchase agreements during the fiscal year ended September 30, 1998, in an
amount greater than 5% of its net assets.
 
                                       22
<PAGE>
ZERO COUPON SECURITIES
 
    A portion of the securities purchased by the Fund may be "zero coupon"
Treasury securities. These are U.S. Treasury bills, notes and bonds which have
been stripped of their unmatured interest coupons and receipts or which are
certificates representing interests in such stripped debt obligations and
coupons. "Zero coupon" securities are purchased at a discount from their face
amount, giving the purchaser the right to receive their full value at maturity.
A zero coupon security pays no interest to its holder during its life. Its value
to an investor consists of the difference between its face value at the time of
maturity and the price for which it was acquired, which is generally an amount
significantly less than its face value (sometimes referred to as a "deep
discount" price).
 
    The interest earned on such securities is, implicitly, automatically
compounded and paid out at maturity. While such compounding at a constant rate
eliminates the risk of receiving lower yields upon reinvestment of interest if
prevailing interest rates decline, the owner of a zero coupon security will be
unable to participate in higher yields upon reinvestment of interest received if
prevailing interest rates rise. For this reason, zero coupon securities are
subject to substantially greater market price fluctuations during periods of
changing prevailing interest rates than are comparable debt securities which
make current distributions of interest. Current federal tax law requires that a
holder (such as the Fund) of a zero coupon security accrue a portion of the
discount at which the security was purchased as income each year even though the
Fund receives no interest payments in cash on the security during the year.
 
    Currently the only U.S. Treasury security issued without coupons is the
Treasury bill. However, in the last few years a number of banks and brokerage
firms have separated ("stripped") the principal portions from the coupon
portions of the U.S. Treasury bonds and notes and sold them separately in the
form of receipts of certificates representing undivided interests in these
instruments (which instruments are generally held by a bank in a custodial or
trust account).
 
WARRANTS
 
    The Fund may invest up to 5% of its net assets in warrants, but not more
than 2% of such assets in warrants not listed on either the New York or American
Stock Exchange. However, the acquisition of warrants attached to other
securities is not subject to this limitation. For the fiscal year ended
September 30, 1998, the Fund's investments in warrants did not exceed 5% of its
net assets.
 
FOREIGN SECURITIES
 
    The Fund may invest in securities of foreign companies. However, the Fund
will not invest more than 10% of the value of its total assets, at the time of
purchase, in foreign securities (other than securities of Canadian issuers
registered under the Securities Exchange Act of 1934 or American Depository
Receipts, on which there is no such limit). Investments in certain Canadian
issuers may be speculative due to certain political risks and may be subject to
substantial price fluctuations. Foreign securities investments may be affected
by changes in currency rates or exchange control regulations, changes in
governmental administration or economic or monetary policy (in the United States
and abroad) or changed circumstances in dealings between nations. Costs may be
incurred in connection with conversions between various currencies held by the
Fund.
 
    The Fund may invest in securities of foreign companies. Dividends paid by
foreign issuers may be subject to withholding and other foreign taxes which may
decrease the net return on such investments as compared to dividends paid to the
Fund by domestic corporations. It should be noted that there may be less
publicly available information about foreign issuers than about domestic
issuers, and foreign issuers are not subject to uniform accounting, auditing and
financial reporting standards and requirements comparable to those of domestic
issuers. Securities of some foreign issuers are less liquid and more volatile
than securities of comparable domestic issuers and foreign brokerage commissions
are generally higher than in the United States. Foreign securities markets may
also be less liquid, more volatile and less subject to government supervision
than those in the United States. The Fund may be affected either unfavorably or
favorably by fluctuations in the relative rates of exchange as between the
currencies of different nations and exchange control regulations. Investments in
foreign countries could be affected by other factors not present in the United
States, including expropriation, confiscatory taxation and potential
difficulties in enforcing contractual obligations. Securities purchased on
foreign
 
                                       23
<PAGE>
exchanges will be held in custody by a foreign branch of a domestic bank. During
the fiscal year ended September 30, 1998, the Fund did not purchase any foreign
securities in an amount greater than 5% of its net assets.
 
PORTFOLIO TURNOVER
 
    The Fund may sell portfolio securities without regard to the length of time
they have been held whenever such sale will, in the Investment Manager's
opinion, strengthen the Fund's position and contribute to its investment
objective. As a result, the Fund's portfolio turnover rate may exceed 100%. A
100% turnover rate would occur, for example, if 100% of the securities held in
the Fund's portfolio (excluding all securities whose maturities at acquisition
were one year or less) were sold and replaced within one year. During the fiscal
years ended September 30, 1997 and 1998, the Fund's portfolio turnover rate was
182.33% and 95%, respectively.
 
INVESTMENT RESTRICTIONS
- --------------------------------------------------------------------------------
 
    In addition to the investment restrictions enumerated in the Prospectus, the
investment restrictions listed below have been adopted by the Fund as
fundamental policies, except as otherwise indicated. Under the Act, a
fundamental policy may not be changed without the vote of a majority of the
outstanding voting securities of the Fund, as defined in the Act. Such a
majority is defined as the lesser of (a) 67% or more of the shares present at a
meeting of shareholders, if the holders of 50% of the outstanding shares of the
Fund are present or represented by proxy or (b) more than 50% of the outstanding
shares of the Fund.
 
    The Fund may not:
 
         1. Invest in securities of any issuer if, to the knowledge of the Fund,
    any officer, or trustee/ director of the Fund or of the Investment Manager
    owns more than 1/2 of 1% of the outstanding securities of such issuer, and
    such officers and trustees/directors who own more than 1/2 of 1% own in the
    aggregate more than 5% of the outstanding securities of such issuer.
 
         2. Purchase or sell real estate or interests therein, although the Fund
    may purchase securities of issuers which engage in real estate operations
    and securities secured by real estate or interests therein.
 
         3. Purchase or sell commodities except that the Fund may purchase
    financial futures contracts and related options thereon.
 
         4. Purchase oil, gas or other mineral leases, rights or royalty
    contracts, or exploration or development programs, except that the Fund may
    invest in the securities of companies which operate, invest in, or sponsor
    such programs.
 
         5. Purchase securities of other investment companies, except in
    connection with a merger, consolidation, reorganization or acquisition of
    assets.
 
         6. Pledge its assets or assign or otherwise encumber them except to
    secure permitted borrowings. (For the purpose of this restriction,
    collateral arrangements with respect to the writing of options and
    collateral arrangements with respect to initial and variation margin for
    futures are not deemed to be pledges of assets and such arrangements are not
    deemed to be the issuance of a senior security as set forth in restriction
    (7).)
 
         7. Issue senior securities as defined in the Act except insofar as the
    Fund may be deemed to have issued a senior security by reason of: (a)
    entering into any repurchase agreement; (b) borrowing money in accordance
    with restrictions described above and in the Prospectus; (c) purchasing any
    securities on a when-issued or delayed delivery basis; or (d) lending
    portfolio securities.
 
         8. Make loans of money or securities, except: (a) by the purchase of
    debt obligations in which the Fund may invest consistent with its investment
    objectives and policies; (b) by investment in repurchase agreements; or (c)
    by lending its portfolio securities.
 
                                       24
<PAGE>
         9. Make short sales of securities or maintain a short position, unless
    at all times when a short position is open it either owns an equal amount of
    such securities or owns securities which, without payment of any further
    consideration, are convertible into or exchangeable for securities of the
    same issue as, and equal in amount to, the securities sold short.
 
        10. Purchase securities on margin, except for such short-term loans as
    are necessary for the clearance of portfolio securities. The deposit or
    payment by the Fund of initial or variation margin in connection with
    futures contracts or related options thereon is not considered the purchase
    of a security on margin.
 
        11. Engage in the underwriting of securities, except insofar as the Fund
    may be deemed an underwriter under the Securities Act of 1933 in disposing
    of a portfolio security.
 
        12. Invest for the purpose of exercising control or management of any
    other issuer.
 
    If a percentage restriction is adhered to at the time of investment, a later
increase or decrease in percentage resulting from a change in values of
portfolio securities or amount of total or net assets will not be considered a
violation of any of the foregoing restrictions.
 
    Notwithstanding any other investment policy or restriction, the Fund may
seek to achieve its investment objective by investing all or substantially all
of its assets in another investment company having substantially the same
investment objective and policies as the Fund.
 
PORTFOLIO TRANSACTIONS AND BROKERAGE
- --------------------------------------------------------------------------------
 
    Subject to the general supervision of the Trustees, the Investment Manager
is responsible for decisions to buy and sell securities and futures contracts
for the Fund, the selection of brokers and dealers to effect the transactions
and the negotiation of brokerage commissions, if any. Purchases and sales of
securities on a stock exchange are effected through brokers who charge a
commission for their services. In the over-the-counter market, securities are
generally traded on a "net" basis with dealers acting as principal for their own
accounts without a stated commission, although the price of the security usually
includes a profit to the dealer. Option and futures transactions will usually be
effected through a broker and a commission will be charged.
 
    The Fund also expects that securities will be purchased at times in
underwritten offerings where the price includes a fixed amount of compensation,
generally referred to as the underwriter's concession or discount. On occasion,
the Fund may also purchase certain money market instruments directly from an
issuer, in which case no commissions or discounts are paid.
 
    The Investment Manager currently serves as investment manager to a number of
clients, including other investment companies, and may in the future act as
investment manager or adviser to others. It is the practice of the Investment
Manager to cause purchase and sale transactions to be allocated among the Fund
and others whose assets it manages in such manner as it deems equitable. In
making such allocations among the Fund and other client accounts, various
factors may be considered, including are the respective investment objectives,
the relative size of portfolio holdings of the same or comparable securities,
the availability of cash for investment, the size of investment commitments
generally held and the opinions of the persons responsible for managing the
portfolios of the Fund and other client accounts. In the case of certain initial
and secondary public offerings, the Investment Manager utilizes a pro-rata
allocation process based on the size of the Morgan Stanley Dean Witter Funds
involved and the number of shares available from the public offering.
 
    The aggregate amount of brokerage commissions paid by the Fund during the
fiscal years ended September 30, 1996, 1997 and 1998 was $162,895, $204,554 and
$140,951, respectively.
 
    The policy of the Fund regarding purchases and sales of securities and
futures contracts for its portfolio is that primary consideration will be given
to obtaining the most favorable prices and efficient execution of transactions.
In seeking to implement the Fund's policies, the Investment Manager effects
transactions with those brokers and dealers who the Investment Manager believes
provide the most favorable prices and are capable of providing efficient
executions. If the Investment Manager believes
 
                                       25
<PAGE>
such price and execution are obtainable from more than one broker or dealer, it
may give consideration to placing portfolio transactions with those brokers and
dealers who also furnish research and other services to the Fund or the
Investment Manager. Such services may include, but are not limited to, any one
or more of the following: information as to the availability of securities for
purchase or sale; statistical or factual information or opinions pertaining to
investment; wire services; and appraisals or evaluations of portfolio
securities.
 
    The information and services received by the Investment Manager from brokers
and dealers may be of benefit to the Investment Manager in the management of
accounts of some of its other clients and may not in all cases benefit the Fund
directly. While the receipt of such information and services is useful in
varying degrees and would generally reduce the amount of research or services
otherwise performed by the Investment Manager and thereby reduce its expenses,
it is of indeterminable value and the management fee paid to the Investment
Manager is not reduced by any amount that may be attributable to the value of
such services. During the fiscal year ended September 30, 1998, the Fund paid
$133,355 in brokerage commissions in connection with transactions in the
aggregate amount of $56,782,549 to brokers because of research services
provided.
 
    Pursuant to an order of the Securities and Exchange Commission, the Fund may
effect principal transactions in certain money market instruments with DWR. The
Fund will limit its transactions with DWR to U.S. Government and Government
Agency Securities, Bank Money Instruments (I.E., Certificates of Deposit and
Bankers' Acceptances) and Commercial Paper. Such transactions will be effected
with DWR only when the price available from DWR is better than that available
from other dealers.
 
   
    Consistent with the policy described above, brokerage transactions in
securities listed on exchanges or admitted to unlisted trading privileges may be
effected through DWR, Morgan Stanley & Co. Incorporated ("MS & Co.") and other
affiliated brokers and dealers. In order for an affiliated broker or dealer to
effect portfolio transactions for the Fund, the commissions, fees or other
remuneration received by the affiliated broker or dealer must be reasonable and
fair compared to the commissions, fees or other remuneration paid to other
brokers in connection with comparable transactions involving similar securities
being purchased or sold on an exchange during a comparable period of time. This
standard would allow the affiliated broker or dealer to receive no more than the
remuneration which would be expected to be received by an unaffiliated broker in
a commensurate arm's-length transaction. Furthermore, the Trustees of the Fund,
including a majority of the Trustees who are not "interested" Trustees, have
adopted procedures which are reasonably designed to provide that any
commissions, fees or other remuneration paid to an affiliated broker or dealer
are consistent with the foregoing standard. During the fiscal years ended
September 30, 1996, 1997 and 1998, the Fund paid a total of $11,922, $1,258 and
$1,750, respectively, in brokerage commissions to DWR. The Fund does not reduce
the management fee it pays to the Investment Manager by any amount of the
brokerage commissions it may pay to an affiliated broker or dealer. During the
fiscal year ended September 30, 1998, the brokerage commissions paid to DWR
represented approximately 1.24% of the total brokerage commissions paid by the
Fund during the year and were paid on account of transactions having an
aggregate dollar value equal to approximately 1.45% of the aggregate dollar
value of all portfolio transactions of the Fund during the year for which
commissions were paid. During the period June 1, 1997 through September 30, 1997
and during the fiscal year ended September 30, 1998, the Fund paid a total of $0
and $2,500, respectively, in brokerage commissions to MS & Co., which
broker-dealer became an affiliate of the Investment Manager on May 31, 1997 upon
consummation of the merger of Dean Witter, Discover & Co. with Morgan Stanley
Group Inc. During the fiscal year ended September 30, 1998, the brokerage
commissions paid to MS & Co. represented approximately 1.77% of the total
brokerage commissions paid by the Fund during the year and were paid on account
of transactions having an aggregate dollar value equal to approximately 1.38% of
the aggregate dollar value of all portfolio transactions of the Fund during the
year for which commissions were paid.
    
 
    During the fiscal year ended September 30, 1998, the Fund purchased
convertible preferred stock issued by Merrill Lynch & Co. Inc. At September 30,
1998, the Fund held convertible preferred stock issued by Merrill Lynch & Co.
Inc. with a market value of $7,312,500 and $1,106,250. This issuer was
 
                                       26
<PAGE>
among the ten brokers or the ten dealers which executed transactions for or with
the Fund in the largest dollar amounts during the fiscal year ended September
30, 1998.
 
THE DISTRIBUTOR
- --------------------------------------------------------------------------------
 
    As discussed in the Prospectus, shares of the Fund are distributed by Morgan
Stanley Dean Witter Distributors Inc. (the "Distributor"). The Distributor has
entered into a selected dealer agreement with DWR, which through its own sales
organization sells shares of the Fund. In addition, the Distributor may enter
into selected dealer agreements with other selected broker-dealers. The
Distributor, a Delaware corporation, is a wholly-owned subsidiary of MSDW. The
Trustees of the Fund, including a majority of the Trustees who are not, and were
not at the time they voted, interested persons of the Fund, as defined in the
Act (the "Independent Trustees"), approved, at their meeting held on June 30,
1997, the current Distribution Agreement appointing the Distributor as exclusive
distributor of the Fund's shares and providing for the Distributor to bear
distribution expenses not borne by the Fund. By its terms, the Distribution
Agreement had an initial term ending April 30, 1998 and will remain in effect
from year to year thereafter if approved by the Trustees. At their meeting held
on April 30, 1998, the Trustees of the Fund, including a majority of the
Independent Trustees, approved the continuation of the Distribution Agreement
until April 30, 1999.
 
    The Distributor bears all expenses it may incur in providing services under
the Distribution Agreement. Such expenses include the payment of commissions for
sales of the Fund's shares and incentive compensation to Morgan Stanley Dean
Witter Financial Advisors and other selected broker-dealer representatives. The
Distributor also pays certain expenses in connection with the distribution of
the Fund's shares, including the costs of preparing, printing and distributing
advertising or promotional materials, and the costs of printing and distributing
prospectuses and supplements thereto used in connection with the offering and
sale of the Fund's shares. The Fund bears the costs of initial typesetting,
printing and distribution of prospectuses and supplements thereto to
shareholders. The Fund also bears the costs of registering the Fund and its
shares under federal securities laws and pays filing fees in accordance with
state securities laws. The Fund and the Distributor have agreed to indemnify
each other against certain liabilities, including liabilities under the
Securities Act of 1933, as amended. Under the Distribution Agreement, the
Distributor uses its best efforts in rendering services to the Fund, but in the
absence of willful misfeasance, bad faith, gross negligence or reckless
disregard of its obligations, the Distributor is not liable to the Fund or any
of its shareholders for any error of judgment or mistake of law or for any act
or omission or for losses sustained by the Fund or its shareholders.
 
PLAN OF DISTRIBUTION
 
    The Fund has adopted a Plan of Distribution pursuant to Rule 12b-1 under the
Act (the "Plan") pursuant to which each Class, other than Class D, pays the
Distributor compensation accrued daily and payable monthly at the following
annual rates: 0.25% and 1.0% of the average daily net assets of Class A and
Class C, respectively, and, with respect to Class B, 1.0 % of the lesser of: (a)
the average daily aggregate gross sales of the Fund's Class B shares since the
inception of the Fund (not including reinvestments of dividends or capital gains
distributions), less the average daily aggregate net asset value of the Fund's
Class B shares redeemed since the Fund's inception upon which a contingent
deferred sales charge has been imposed or upon which such charge has been
waived, or (b) the average daily net assets of Class B. The Distributor also
receives the proceeds of front-end sales charges and of contingent deferred
sales charges imposed on certain redemptions of shares, which are separate and
apart from payments made pursuant to the Plan (see "Purchase of Fund Shares" in
the Prospectus). The Distributor has informed the Fund that it and/or DWR
received (a) approximately $144,945, $191,981 and $379,822 in contingent
deferred sales charges from Class B for the fiscal years ended September 30,
1996, 1997 and 1998, respectively, (b) no contingent deferred sales charges were
received from Class A for the fiscal years ended September 30, 1997 and 1998,
respectively, (c) approximately $1 and $2,747 in contingent deferred sales
charges from Class C for the fiscal years ended September, 1997 and 1998,
respectively, and (d) approximately $1,643 and $27,570 in front-end sales
charges from Class A for the fiscal years ended September 30, 1997 and 1998,
respectively, none of which was retained by the Distributor.
 
                                       27
<PAGE>
    The Distributor has informed the Fund that the entire fee payable by Class A
and a portion of the fees payable by each of Class B and Class C each year
pursuant to the Plan equal to 0.25% of such Class's average daily net assets are
currently each characterized as a "service fee" under the Rules of the
Association of the National Association of Securities Dealers, Inc. (of which
the Distributor is a member). The "service fee" is a payment made for personal
service and/or the maintenance of shareholder accounts. The remaining portion of
the Plan fees payable by a Class, if any, is characterized as an "asset-based
sales charge" as such is defined by the aforementioned Rules of the Association.
 
    The Plan was adopted by a majority vote of the Board of Trustees, including
all of the Trustees of the Fund who are not "interested persons" of the Fund (as
defined in the Act) and who have no direct or indirect financial interest in the
operation of the Plan (the "Independent 12b-1 Trustees"), cast in person at a
meeting called for the purpose of voting on the Plan, on April 17, 1996 and by
the shareholders holding a majority, as defined in the Act, of the outstanding
voting securities of the Fund at an Annual Meeting of Shareholders of the Fund
held on December 29, 1986.
 
    At their meeting held on October 30, 1992, the Trustees of the Fund,
including all of the Independent 12b-1 Trustees, approved certain amendments to
the Plan which took effect in January, 1993 and were designed to reflect the
fact that upon an internal reorganization the share distribution activities
theretofore performed for the Fund by DWR were assumed by the Distributor and
DWR's sales activities are now being performed pursuant to the terms of a
selected dealer agreement between the Distributor and DWR. The amendments
provide that payments under the Plan will be made to the Distributor rather than
to DWR as before the amendment, and that the Distributor in turn is authorized
to make payments to DWR, its affiliates or other selected broker-dealers (or
direct that the Fund pay such entities directly). The Distributor is also
authorized to retain part of such fee as compensation for its own
distribution-related expenses. At their meeting held on April 28, 1993, the
Trustees, including a majority of the Independent 12b-1 Trustees, approved
certain technical amendments to the Plan in connection with amendments adopted
by the National Association of Securities Dealers, Inc. to its Rules of the
Association. At their meeting held on October 26, 1995, the Trustees of the
Fund, including all of the Independent 12b-1 Trustees, approved an amendment to
the Plan to permit payments to be made under the Plan with respect to certain
distribution expenses incurred in connection with the distribution of shares,
including personal services to shareholders with respect to holdings of such
shares, of an investment company whose assets are acquired by the Fund in a
tax-free reorganization. At their meeting held on June 30, 1997, the Trustees,
including a majority of the Independent 12b-1 Trustees, approved amendments to
the Plan to reflect the multiple-class structure for the Fund, which took effect
on July 28, 1997.
 
    Under the Plan and as required by Rule 12b-1, the Trustees receive and
review promptly after the end of each calendar quarter a written report provided
by the Distributor of the amounts expended under the Plan and the purpose for
which such expenditures were made. Class B shares of the Fund accrued amounts
payable to the Distributor under the Plan, during the fiscal year ended
September 30, 1998, of $3,206,390. This amount is equal to 1.0% of the average
daily net assets of Class B for the fiscal year and was calculated pursuant to
clause (b) of the compensation formula under the Plan. This amount is treated by
the Fund as an expense in the year it is accrued. For the fiscal year ended
September 30, 1998, Class A and Class C shares of the Fund accrued payments
under the Plan amounting to $1,753 and $20,751, respectively, which amounts are
equal to 0.24% and 1.0% of the average daily net assets of Class A and Class C,
respectively, for the fiscal year.
 
    The Plan was adopted in order to permit the implementation of the Fund's
method of distribution. Under this distribution method the Fund offers four
Classes of shares, each with a different distribution arrangement as set forth
in the Prospectus.
 
    With respect to Class A shares, DWR compensates its Financial Advisors by
paying them, from proceeds of the front-end sales charge, commissions for the
sale of Class A shares, currently a gross sales credit of up to 5.0% of the
amount sold (except as provided in the following sentence) and an annual
residual commission, currently a residual of up to 0.25% of the current value of
the respective accounts for which they are the Financial Advisors or dealers of
record in all cases. On orders of $1 million or more (for which no sales charge
was paid) or net asset value purchases by employer
 
                                       28
<PAGE>
sponsored 401(k) and other plans qualified under Section 401(a) of the Internal
Revenue Code ("Qualified Retirement Plans") for which Morgan Stanley Dean Witter
Trust FSB ("MSDW Trust") serves as Trustee or DWR's Retirement Plan Services
serves as recordkeeper pursuant to a written Recordkeeping Services Agreement,
the Investment Manager compensates DWR's Financial Advisors by paying them, from
its own funds, a gross sales credit of 1.0% of the amount sold.
 
    With respect to Class B shares, DWR compensates its Financial Advisors by
paying them, from its own funds, commissions for the sale of Class B shares,
currently a gross sales credit of up to 5.0% of the amount sold (except as
provided in the following sentence) and an annual residual commission, currently
a residual of up to 0.25% of the current value (not including reinvested
dividends or distributions) of the amount sold in all cases. In the case of
Class B shares purchased on or after July 28, 1997 by Qualified Retirement Plans
for which MSDW Trust serves as Trustee of DWR's Retirement Plan Services serves
as recordkeeper pursuant to a written Recordkeeping Services Agreement, DWR
compensates its Financial Advisors by paying them, from its own funds, a gross
sales credit of 3.0% of the amount sold.
 
    With respect to Class C shares, DWR compensates its Financial Advisors by
paying them, from its own funds, commissions for the sale of Class C shares,
currently a gross sales credit of up to 1.0% of the amount sold and an annual
residual commission, currently a residual of up to 1.0% of the current value of
the respective accounts for which they are the Financial Advisors of record.
 
    With respect to Class D shares other than shares held by participants in the
MSDW Advisors mutual fund asset allocation program, the Investment Manager
compensates DWR's Financial Advisors by paying them, from its own funds,
commissions for the sale of Class D shares, currently a gross sales credit of up
to 1.0% of the amount sold. There is a chargeback of 100% of the amount paid if
the Class D shares are redeemed in the first year and a chargeback of 50% of the
amount paid if the Class D shares are redeemed in the second year after
purchase. The Investment Manager also compensates DWR's Financial Advisors by
paying them, from its own funds, an annual residual commission, currently a
residual of up to 0.10% of the current value of the respective accounts for
which they are the Financial Advisors of record (not including accounts of
participants in the MSDW Advisors mutual fund asset allocation program).
 
    The gross sales credit is a charge which reflects commissions paid by DWR to
its Financial Advisors and DWR's Fund-associated distribution-related expenses,
including sales compensation, and overhead and other branch office
distribution-related expenses including (a) the expenses of operating DWR's
branch offices in connection with the sale of Fund shares, including lease
costs, the salaries and employee benefits of operations and sales support
personnel, utility costs, communications costs and the costs of stationery and
supplies, (b) the costs of client sales seminars, (c) travel expenses of mutual
fund sales coordinators to promote the sale of Fund shares and (d) other
expenses relating to branch promotion of Fund sales. The distribution fee that
the Distributor receives from the Fund under the Plan, in effect, offsets
distribution expenses incurred under the Plan on behalf of the Fund and, in the
case of Class B shares, opportunity costs, such as the gross sales credit and an
assumed interest charge thereon ("carrying charge"). In the Distributor's
reporting of the distribution expenses to the Fund, in the case of Class B
shares, such assumed interest (computed at the "broker's call rate") has been
calculated on the gross credit as it is reduced by amounts received by the
Distributor under the Plan and any contingent deferred sales charges received by
the Distributor upon redemption of shares of the Fund. No other interest charge
is included as a distribution expense in the Distributor's calculation of its
distribution costs for this purpose. The broker's call rate is the interest rate
charged to securities brokers on loans secured by exchange-listed securities.
 
    The Fund is authorized to reimburse expenses incurred or to be incurred in
promoting the distribution of the Fund's Class A and Class C shares and in
servicing shareholder accounts. Reimbursement will be made through payments at
the end of each month. The amount of each monthly payment may in no event exceed
an amount equal to a payment at the annual rate of 0.25%, in the case of Class
A, and 1.0%, in the case of Class C, of the average net assets of the respective
Class during the month. No interest or other financing charges, if any, incurred
on any distribution expenses on behalf of Class A and
 
                                       29
<PAGE>
Class C will be reimbursable under the Plan. With respect to Class A, in the
case of all expenses other than expenses representing the service fee, and, with
respect to Class C, in the case of all expenses other than expenses representing
a gross sales credit or a residual to Morgan Stanley Dean Witter Financial
Advisors and other selected broker-dealer representatives, such amounts shall be
determined at the beginning of each calendar quarter by the Trustees, including
a majority of the Independent 12b-1 Trustees. Expenses representing the service
fee (for Class A) or a gross sales credit or a residual to Morgan Stanley Dean
Witter Financial Advisors and other selected broker-dealer representatives (for
Class C) may be reimbursed without prior determination. In the event that the
Distributor proposes that monies shall be reimbursed for other than such
expenses, then in making quarterly determinations of the amounts that may be
reimbursed by the Fund, the Distributor will provide and the Trustees will
review a quarterly budget of projected distribution expenses to be incurred on
behalf of the Fund, together with a report explaining the purposes and
anticipated benefits of incurring such expenses. The Trustees will determine
which particular expenses, and the portions thereof, that may be borne by the
Fund, and in making such a determination shall consider the scope of the
Distributor's commitment to promoting the distribution of the Fund's Class A and
Class C shares.
 
    Each Class paid 100% of the amounts accrued under the Plan with respect to
that Class for the fiscal year ended September 30, 1998 to the Distributor. The
Distributor and DWR estimate that they have spent, pursuant to the Plan,
$180,244,722 on behalf of Class B since the inception of the Fund. It is
estimated that this amount was spent in approximately the following ways: (i)
1.86% ($3,347,448) -- advertising and promotional expenses; (ii) 0.33%
($596,833) printing of prospectuses for distribution to other than current
shareholders; and (iii) 97.81% ($176,300,441) -- other expenses, including the
gross sales credit and the carrying charge, of which 28.39% ($50,039,462)
represents carrying charges, 28.95% ($51,038,730) represents commission credits
to DWR branch offices and other selected broker-dealers for payments of
commissions to Morgan Stanley Dean Witter Financial Advisors and other selected
broker-dealer representatives, and 41.83% ($73,750,341) represents overhead and
other branch office distribution-related expenses. The amounts accrued by Class
A and Class C for distribution during the fiscal year ended September 30, 1998
were for expenses which relate to compensation of sales personnel and associated
overhead expenses.
 
    In the case of Class B shares, at any given time, the expenses of
distributing shares of the Fund may be more or less than the total of (i) the
payments made by the Fund pursuant to the Plan and (ii) the proceeds of
contingent deferred sales charges paid by investors upon redemption of shares.
The Distributor has advised the Fund that in the case of Class B shares the
excess distribution expenses, including the carrying charge designed to
approximate the opportunity costs incurred by DWR which arise from it having
advanced monies without having received the amount of any sales charges imposed
at the time of sale of the Fund's Class B shares, totalled $80,592,295 as of
September 30, 1998. Because there is no requirement under the Plan that the
Distributor be reimbursed for all distribution expenses with respect to Class B
shares or any requirement that the Plan be continued from year to year, this
excess amount does not constitute a liability of the Fund. Although there is no
legal obligation for the Fund to pay expenses incurred in excess of payments
made to the Distributor under the Plan and the proceeds of contingent deferred
sales charges paid by investors upon redemption of shares, if for any reason the
Plan is terminated, the Trustees will consider at that time the manner in which
to treat such expenses. Any cumulative expenses incurred, but not yet recovered
through distribution fees or contingent deferred sales charges, may or may not
be recovered through future distribution fees or contingent deferred sales
charges.
 
    No interested person of the Fund nor any Trustee of the Fund who is not an
interested person of the Fund, as defined in the Act, has any direct financial
interest in the operation of the Plan except to the extent that the Distributor,
MSDW Advisors, MSDW Services, DWR or certain of their employees may be deemed to
have such an interest as a result of benefits derived from the successful
operation of the Plan or as a result of receiving a portion of the amounts
expended thereunder by the Fund.
 
    Under its terms, the Plan had an initial term ending December 31, 1985 and
will continue from year to year thereafter, provided such continuance is
approved annually by a vote of the Trustees in the manner described above. The
most recent continuance of the Plan for one year, until April 30, 1999, was
 
                                       30
<PAGE>
approved by the Trustees of the Fund, including a majority of the Independent
12b-1 Trustees, at a meeting of the Trustees held on April 30, 1998. Prior to
approving the continuation of the Plan, the Trustees requested and received from
the Distributor and reviewed all the information which they deemed necessary to
arrive at an informed determination. In making their determination to continue
the Plan, the Trustees considered: (1) the Fund's experience under the Plan and
whether such experience indicates that the Plan is operating as anticipated; (2)
the benefits the Fund had obtained, was obtaining and would be likely to obtain
under the Plan; and (3) what services had been provided and were continuing to
be provided under the Plan to the Fund and its shareholders. Based upon their
review, the Trustees of the Fund, including each of the Independent 12b-1
Trustees, determined that continuation of the Plan would be in the best interest
of the Fund and would have a reasonable likelihood of continuing to benefit the
Fund and its shareholders. In the Trustees' quarterly review of the Plan, they
will consider its continued appropriateness and the level of compensation
provided therein.
 
    The Plan may not be amended to increase materially the amount to be spent
for the services described therein without approval by the shareholders of the
affected Class or Classes of the Fund, and all material amendments to the Plan
must also be approved by the Trustees in the manner described above. The Plan
may be terminated at any time, without payment of any penalty, by vote of a
majority of the Independent 12b-1 Trustees or by a vote of a majority of the
outstanding voting securities of the Fund (as defined in the Act) on not more
than thirty days' written notice to any other party to the Plan. So long as the
Plan is in effect, the election and nomination of Independent 12b-1 Trustees
shall be committed to the discretion of the Independent 12b-1 Trustees.
 
DETERMINATION OF NET ASSET VALUE
 
    As stated in the Prospectus, short-term securities with remaining maturities
of sixty days or less at the time of purchase are valued at amortized cost,
unless the Trustees determine such does not reflect the securities' market
value, in which case these securities will be valued at their fair value as
determined by the Trustees. Other short-term debt securities will be valued on a
mark-to-market basis until such time as they reach a remaining maturity of sixty
days, whereupon they will be valued at amortized cost using their value on the
61st day unless the Trustees determine such does not reflect the securities'
market value, in which case these securities will be valued at their fair value
as determined by the Trustees. Listed options on debt securities are valued at
the latest sale price on the exchange on which they are listed unless no sales
of such options have taken place that day, in which case they will be valued at
the mean between their latest bid and asked prices. Unlisted options on debt
securities and all options on equity securities are valued at the mean between
their latest bid and asked prices. Futures are valued at the latest sale price
on the commodities exchange on which they trade unless the Trustees determine
such price does not reflect their market value, in which case they will be
valued at their fair value as determined by the Trustees. All other securities
and other assets are valued at their fair value as determined in good faith
under procedures established by and under the supervision of the Trustees.
 
    Generally, trading in foreign securities, as well as corporate bonds, United
States government securities and money market instruments, is substantially
completed each day at various times prior to the close of the New York Stock
Exchange. The values of such securities used in computing the net asset value of
the Fund's shares are determined as of such times. Foreign currency exchange
rates are also generally determined prior to the close of the New York Stock
Exchange. Occasionally, events which may affect the values of such securities
and such exchange rates may occur between the times at which they are determined
and the close of the New York Stock Exchange and will therefore not be reflected
in the computation of the Fund's net asset value. If events that may affect the
value of such securities occur during such period, then these securities may be
valued at their fair value as determined in good faith under procedures
established by and under the supervision of the Trustees.
 
    The net asset value per share for each Class of shares of the Fund is
determined once daily at 4:00 p.m. New York time (or, on days when the New York
Stock Exchange closes prior to 4:00 p.m., at such earlier time), on each day
that the New York Stock Exchange is open and on each other day in which there is
a sufficient degree of trading in the Fund's investments to affect the net asset
value, except
 
                                       31
<PAGE>
that the net asset value may not be computed on a day on which no orders to
purchase, or tenders to sell or redeem, Fund shares have been received, by
taking the value of all assets of the Fund, subtracting its liabilities,
dividing by the number of shares outstanding and adjusting to the nearest cent.
The New York Stock Exchange currently observes the following holidays: New
Year's Day; Reverend Dr. Martin Luther King, Jr. Day; President's Day; Good
Friday; Memorial Day; Independence Day; Labor Day; Thanksgiving Day; and
Christmas Day.
 
PURCHASE OF FUND SHARES
- --------------------------------------------------------------------------------
 
    As discussed in the Prospectus, the Fund offers four Classes of shares as
follows:
 
INITIAL SALES CHARGE ALTERNATIVE--CLASS A SHARES
 
    Class A shares are sold to investors with an initial sales charge that
declines to zero for larger purchases; however, Class A shares sold without an
initial sales charge are subject to a contingent deferred sales charge ("CDSC")
of 1.0% if redeemed within one year of purchase, except in the circumstances
discussed in the Prospectus.
 
    RIGHT OF ACCUMULATION.  As discussed in the Prospectus, investors may
combine the current value of shares purchased in separate transactions for
purposes of benefitting from the reduced sales charges available for purchases
of shares of the Fund totalling at least $25,000 in net asset value. For
example, if any person or entity who qualifies for this privilege holds Class A
shares of the Fund and/or other Morgan Stanley Dean Witter Funds that are
multiple class funds ("Morgan Stanley Dean Witter Multi-Class Funds") or shares
of other Morgan Stanley Dean Witter Funds sold with a front-end sales charge
purchased at a price including a front-end sales charge having a current value
of $5,000, and purchases $20,000 of additional shares of the Fund, the sales
charge applicable to the $20,000 purchase would be 4.75% of the offering price.
 
    The Distributor must be notified by the selected broker-dealer or the
shareholder at the time a purchase order is placed that the purchase qualifies
for the reduced charge under the Right of Accumulation. Similar notification
must be made in writing by the selected broker-dealer or shareholder when such
an order is placed by mail. The reduced sales charge will not be granted if: (a)
such notification is not furnished at the time of the order; or (b) a review of
the records of the Distributor or Morgan Stanley Dean Witter Trust FSB (the
"Transfer Agent") fails to confirm the investor's represented holdings.
 
    LETTER OF INTENT.  As discussed in the Prospectus, reduced sales charges are
available to investors who enter into a written Letter of Intent providing for
the purchase, within a thirteen-month period, of Class A shares of the Fund from
the Distributor or from a single Selected Broker-Dealer.
 
    A Letter of Intent permits an investor to establish a total investment goal
to be achieved by any number of purchases over a thirteen-month period. Each
purchase of Class A shares made during the period will receive the reduced sales
commission applicable to the amount represented by the goal, as if it were a
single purchase. A number of shares equal in value to 5% of the dollar amount of
the Letter of Intent will be held in escrow by the Transfer Agent, in the name
of the shareholder. The initial purchase under a Letter of Intent must be equal
to at least 5% of the stated investment goal.
 
    The Letter of Intent does not obligate the investor to purchase, nor the
Fund to sell, the indicated amount. In the event the Letter of Intent goal is
not achieved within the thirteen-month period, the investor is required to pay
the difference between the sales charge otherwise applicable to the purchases
made during this period and sales charges actually paid. Such payment may be
made directly to the Distributor or, if not paid, the Distributor is authorized
by the shareholder to liquidate a sufficient number of his or her escrowed
shares to obtain such difference.
 
    If the goal is exceeded and purchases pass the next sales charge level, the
sales charge on the entire amount of the purchase that results in passing that
level and on subsequent purchases will be subject to further reduced sales
charges in the same manner as set forth above under "Right of Accumulation," but
there will be no retroactive reduction of sales charges on previous purchases.
For
 
                                       32
<PAGE>
the purpose of determining whether the investor is entitled to a further reduced
sales charge applicable to purchases at or above a sales charge level which
exceeds the stated goal of a Letter of Intent, the cumulative current net asset
value of any shares owned by the investor in any other Morgan Stanley Dean
Witter Funds held by the shareholder which were previously purchased at a price
including a front-end sales charge (including shares of the Fund and other
Morgan Stanley Dean Witter Funds acquired in exchange for those shares, and
including in each case shares acquired through reinvestment of dividends and
distributions) will be added to the cost or net asset value of shares of the
Fund owned by the investor. However, shares of "Exchange Funds" (see
"Shareholder Services--Exchange Privilege") and the purchase of shares of other
Morgan Stanley Dean Witter Funds will not be included in determining whether the
stated goal of a Letter of Intent has been reached.
 
    At any time while a Letter of Intent is in effect, a shareholder may, by
written notice to the Distributor, increase the amount of the stated goal. In
that event, only shares purchased during the previous 90-day period and still
owned by the shareholder will be included in the new sales charge reduction. The
5% escrow and minimum purchase requirements will be applicable to the new stated
goal. Investors electing to purchase shares of the Fund pursuant to a Letter of
Intent should carefully read such Letter of Intent.
 
CONTINGENT DEFERRED SALES CHARGE ALTERNATIVE--CLASS B SHARES
 
    Class B shares are sold without an initial sales charge but are subject to a
CDSC payable upon most redemptions within six years after purchase. As stated in
the Prospectus, a CDSC will be imposed on any redemption by an investor if after
such redemption the current value of the investor's Class B shares of the Fund
is less than the dollar amount of all payments by the shareholder for the
purchase of Class B shares during the preceding six years (or, in the case of
shares held by certain Qualified Retirement Plans, three years). However, no
CDSC will be imposed to the extent that the net asset value of the shares
redeemed does not exceed: (a) the current net asset value of shares purchased
more than six years (or, in the case of shares held by certain Qualified
Retirement Plans, three years) prior to the redemption, plus (b) the current net
asset value of shares purchased through reinvestment of dividends or
distributions of the Fund or another Morgan Stanley Dean Witter Fund (see
"Shareholder Services-- Targeted Dividends"), plus (c) the current net asset
value of shares acquired in exchange for (i) shares of Morgan Stanley Dean
Witter front-end sales charge funds, or (ii) shares of other Morgan Stanley Dean
Witter Funds for which shares of front-end sales charge funds have been
exchanged (see "Shareholder Services--Exchange Privilege"), plus (d) increases
in the net asset value of the investor's shares above the total amount of
payments for the purchase of Fund shares made during the preceding six (three)
years. The CDSC will be paid to the Distributor. In addition, no CDSC will be
imposed on redemptions of shares which are attributable to reinvestment of
dividends or distributions from, or the proceeds of, certain Unit Investment
Trusts.
 
    In determining the applicability of the CDSC to each redemption, the amount
which represents an increase in the net asset value of the investor's shares
above the amount of the total payments for the purchase of shares within the
last six years (or, in the case of shares held by certain Qualified Retirement
Plans, three years) will be redeemed first. In the event the redemption amount
exceeds such increase in value, the next portion of the amount redeemed will be
the amount which represents the net asset value of the investor's shares
purchased more than six (three) years prior to the redemption and/or shares
purchased through reinvestment of dividends or distributions and/or shares
acquired in exchange for shares of Morgan Stanley Dean Witter front-end sales
charge funds, or for shares of other Morgan Stanley Dean Witter funds for which
shares of front-end sales charge funds have been exchanged. A portion of the
amount redeemed which exceeds an amount which represents both such increase in
value and the value of shares purchased more than six years (or, in the case of
shares held by certain Qualified Retirement Plans, three years) prior to the
redemption and/or shares purchased through reinvestment of dividends or
distributions and/or shares acquired in the above-described exchanges will be
subject to a CDSC.
 
                                       33
<PAGE>
    The amount of the CDSC, if any, will vary depending on the number of years
from the time of payment for the purchase of Class B shares of the Fund until
the time of redemption of such shares. For purposes of determining the number of
years from the time of any payment for the purchase of shares, all payments made
during a month will be aggregated and deemed to have been made on the last day
of the month. The following table sets forth the rates of the CDSC applicable to
most Class B shares of the Fund:
 
<TABLE>
<CAPTION>
                              YEAR SINCE
                               PURCHASE                                  CDSC AS A PERCENTAGE OF
                             PAYMENT MADE                                    AMOUNT REDEEMED
- ----------------------------------------------------------------------  --------------------------
<S>                                                                     <C>
First.................................................................               5.0%
Second................................................................               4.0%
Third.................................................................               3.0%
Fourth................................................................               2.0%
Fifth.................................................................               2.0%
Sixth.................................................................               1.0%
Seventh and thereafter................................................             None
</TABLE>
 
    The following table sets forth the rates of the CDSC applicable to Class B
shares of the Fund purchased on or after July 28, 1997 by Qualified Retirement
Plans for which MSDW Trust serves as Trustee or DWR's Retirement Plan Services
serves as recordkeeper pursuant to a written Record-keeping Services Agreement:
 
<TABLE>
<CAPTION>
                              YEAR SINCE
                               PURCHASE                                  CDSC AS A PERCENTAGE OF
                             PAYMENT MADE                                    AMOUNT REDEEMED
- ----------------------------------------------------------------------  --------------------------
<S>                                                                     <C>
First.................................................................               2.0%
Second................................................................               2.0%
Third.................................................................               1.0%
Fourth and thereafter.................................................             None
</TABLE>
 
    In determining the rate of the CDSC, it will be assumed that a redemption is
made of shares held by the investor for the longest period of time within the
applicable six-year or three-year period. This will result in any such CDSC
being imposed at the lowest possible rate. The CDSC will be imposed, in
accordance with the table shown above, on any redemptions within six years (or,
in the case of shares held by certain Qualified Retirement Plans, three years)
of purchase which are in excess of these amounts and which redemptions do not
qualify for waiver of the CDSC, as described in the Prospectus.
 
LEVEL LOAD ALTERNATIVE--CLASS C SHARES
 
    Class C shares are sold without a sales charge but are subject to a CDSC of
1.0% on most redemptions made within one year after purchase, except in the
circumstances discussed in the Prospectus.
 
NO LOAD ALTERNATIVE--CLASS D SHARES
 
    Class D shares are offered without any sales charge on purchase or
redemption. Class D shares are offered only to those persons meeting the
qualifications set forth in the Prospectus.
 
SHAREHOLDER SERVICES
- --------------------------------------------------------------------------------
 
    Upon the purchase of shares of the Fund, a Shareholder Investment Account is
opened for the investor on the books of the Fund and maintained by the Transfer
Agent. This is an open account in which shares owned by the investor are
credited by the Transfer Agent in lieu of issuance of a share certificate. If a
share certificate is desired, it must be requested in writing for each
transaction. Certificates are issued only for full shares and may be redeposited
in the account at any time. There is no charge to the investor for issuance of a
certificate. Whenever a shareholder instituted transaction takes place in the
Shareholder Investment Account, the shareholder will be mailed a confirmation of
the transaction from the Fund or from DWR or other selected broker-dealer.
 
                                       34
<PAGE>
    AUTOMATIC INVESTMENT OF DIVIDENDS AND DISTRIBUTIONS.  As stated in the
Prospectus, all income dividends and capital gains distributions are
automatically paid in full and fractional shares of the applicable Class of the
Fund, unless the shareholder requests that they be paid in cash. Each purchase
of shares of the Fund is made upon the condition that the Transfer Agent is
thereby automatically appointed as agent of the investor to receive all
dividends and capital gains distributions on shares owned by the investor. Such
dividends and distributions will be paid, at the net asset value per share, in
shares of the applicable Class of the Fund (or in cash if the shareholder so
requests) as of the close of business on the record date. At any time an
investor may request the Transfer Agent, in writing, to have subsequent
dividends and/or capital gains distributions paid to him or her in cash rather
than shares. To assure sufficient time to process the change, such request
should be received by the Transfer Agent at least five business days prior to
the record date of the dividend or distribution. In the case of recently
purchased shares for which registration instructions have not been received on
the record date, cash payments will be made to DWR or other selected
broker-dealer, and will be forwarded to the shareholder, upon the receipt of
proper instructions. It has been and remains the Fund's policy and practice
that, if checks for dividends or distributions paid in cash remain uncashed, no
interest will accrue on amounts represented by such uncashed checks.
 
    TARGETED DIVIDENDS.-SM-  In states where it is legally permissible,
shareholders may also have all income dividends and capital gains distributions
automatically invested in shares of any Class of an open-end Morgan Stanley Dean
Witter Fund other than Morgan Stanley Dean Witter Convertible Securities Trust
or in another Class of Morgan Stanley Dean Witter Convertible Securities Trust.
Such investment will be made as described above for automatic investment in
shares of the applicable Class of the Fund, at the net asset value per share of
the selected Morgan Stanley Dean Witter Fund as of the close of business on the
payment date of the dividend or distribution and will begin to earn dividends,
if any, in the selected Morgan Stanley Dean Witter Fund the next business day.
To participate in the Targeted Dividends program, shareholders should contact
their Morgan Stanley Dean Witter Financial Advisor or other selected
broker-dealer representative or the Transfer Agent. Shareholders of the Fund
must be shareholders of the selected Class of the Morgan Stanley Dean Witter
Fund targeted to receive investments from dividends at the time they enter the
Targeted Dividends program. Investors should review the prospectus of the
targeted Morgan Stanley Dean Witter Fund before entering the program.
 
    EASYINVEST.-SM-  Shareholders may subscribe to EasyInvest, an automatic
purchase plan which provides for any amount from $100 to $5,000 to be
transferred automatically from a checking or savings account or following
redemption of shares of a Morgan Stanley Dean Witter money market fund, on a
semi-monthly, monthly or quarterly basis, to the Transfer Agent for investment
in shares of the Fund. Shares purchased through EasyInvest will be added to the
shareholder's existing account at the net asset value calculated the same
business day the transfer of funds is effected (subject to any applicable sales
charges). Shares of the Morgan Stanley Dean Witter money market funds redeemed
in connection with EasyInvest are redeemed on the business day preceeding the
transfer of funds. For further information or to subscribe to EasyInvest,
shareholders should contact their Morgan Stanley Dean Witter Financial Advisor
or other selected broker-dealer representative or the Transfer Agent.
 
    INVESTMENT OF DIVIDENDS OR DISTRIBUTIONS RECEIVED IN CASH.  Any shareholder
who receives a cash payment representing a dividend or distribution may invest
such dividend or distribution in shares of the applicable Class at the net asset
value, without the imposition of a CDSC upon redemption, by returning the check
or the proceeds to the Transfer Agent within thirty days after the payment date.
If the shareholder returns the proceeds of a dividend or distribution, such
funds must be accompanied by a signed statement indicating that the proceeds
constitute a dividend or distribution to be invested. Such investment will be
made at the net asset value per share next determined after receipt of the
proceeds by the Transfer Agent.
 
    SYSTEMATIC WITHDRAWAL PLAN.  As discussed in the Prospectus, a systematic
withdrawal plan (the "Withdrawal Plan") is available for shareholders whose
shares of Morgan Stanley Dean Witter Funds have an aggregate value of $10,000 or
more. Shares of any Fund from which redemptions will be made pursuant to the
Plan must have a value of $1,000 or more (referred to as a "SWP Fund"). The
required
 
                                       35
<PAGE>
share values are determined on the date the shareholder establishes the
Withdrawal Plan. The Withdrawal Plan provides for monthly, quarterly,
semi-annual or annual payments in any amount not less than $25, or in any whole
percentage of the value of the SWP Funds' shares, on an annualized basis. Any
applicable Contingent Deferred Sales Charge ("CDSC") will be imposed on shares
redeemed under the Withdrawal Plan (see "Purchase of Fund Shares"), except that
the CDSC, if any, will be waived on redemptions under the Withdrawal Plan of up
to 12% annually of the value of each SWP Fund account, based on the share values
next determined after the shareholder establishes the Withdrawal Plan.
Redemptions for which this CDSC waiver policy applies may be in amounts up to 1%
per month, 3% per quarter, 6% semi-annually or 12% annually. Under this CDSC
waiver policy, amounts withdrawn each period will be paid by first redeeming
shares not subject to a CDSC because the shares were purchased by the
reinvestment of dividends or capital gains distributions, the CDSC period has
elapsed or some other waiver of the CDSC applies. If shares subject to a CDSC
must be redeemed, shares held for the longest period of time will be redeemed
first and continuing with shares held the next longest period of time until
shares held the shortest period of time are redeemed. Any shareholder
participating in the Withdrawal Plan will have sufficient shares redeemed from
his or her account so that the proceeds (net of any applicable CDSC) to the
shareholder will be the designated monthly, quarterly, semi-annual or annual
amount.
 
    A shareholder may suspend or terminate participation in the Withdrawal Plan
at any time. A shareholder who has suspended participation may resume payments
under the Withdrawal Plan, without requiring a new determination of the account
value for the 12% CDSC waiver. The Withdrawal Plan may be terminated or revised
at any time by the Fund.
 
    Prior to adding an additional SWP Fund to an existing Withdrawal Plan, the
required $10,000/$1,000 share values must be met, to be calculated on the date
the shareholder adds the additional SWP Fund. However, the addition of a new SWP
Fund will not change the account value for the 12% CDSC waiver for the SWP Funds
already participating in the Withdrawal Plan.
 
    The Transfer Agent acts as agent for the shareholder in tendering to the
Fund for redemption sufficient full and fractional shares to provide the amount
of the periodic withdrawal payment designated in the application. The shares
will be redeemed at their net asset value determined, at the shareholder's
option, on the tenth or twenty-fifth day (or next following business day) of the
relevant month, quarter, or semi-annual or annual period and normally a check
for the proceeds will be mailed by the Transfer Agent, or amounts credited to a
shareholder's Dean Witter Reynolds Inc. or other selected broker-dealer
brokerage account, or amounts deposited electronically into the shareholder's
bank account via the Automated Clearing House, within five business days after
the date of redemption.
 
    Withdrawal Plan payments should not be considered as dividends, yields or
income. If periodic withdrawal plan payments continuously exceed net investment
income and net capital gains, the shareholder's original investment will be
correspondingly reduced and ultimately exhausted. Each withdrawal constitutes a
redemption of shares and any gain or loss realized must be recognized for
federal income tax purposes. Although a shareholder may make additional
investments while participating in the Withdrawal Plan, withdrawals made
concurrently with purchases of additional shares are inadvisable because of
sales charges applicable to purchases or redemptions of shares (see "Purchase of
Fund Shares" in the Prospectus).
 
    Any shareholder who wishes to have payments under the Withdrawal Plan made
to a third party or sent to an address other than the one listed on the account
must send complete written instructions to the Transfer Agent to enroll in the
Withdrawal Plan. The shareholder's signature on such instructions must be
guaranteed by an eligible guarantor acceptable to the Transfer Agent
(shareholders should contact the Transfer Agent for a determination as to
whether a particular institution is such an eligible guarantor). A shareholder
may, at any time, change the amount and interval of withdrawal payments through
his or her Morgan Stanley Dean Witter Financial Advisor or other selected
broker-dealer representative or by written notification to the Transfer Agent.
In addition, the party and/or the address to which checks are mailed may be
changed by written notification to the Transfer Agent, with signature guarantees
required in the manner described above. The shareholder may also terminate the
With-
 
                                       36
<PAGE>
drawal Plan at any time by written notice to the Transfer Agent. In the event of
such termination, the account will be continued as a regular Shareholder
Investment Account. The shareholder may also redeem all or part of the shares
held in the Withdrawal Plan account (see "Redemptions and Repurchases" in the
Prospectus) at any time.
 
    DIRECT INVESTMENTS THROUGH TRANSFER AGENT.  As discussed in the Prospectus,
shareholders may make additional investments in any Class of shares of the Fund
for which they qualify at any time by sending a check in any amount, not less
than $100, payable to Morgan Stanley Dean Witter Convertible Securities Trust,
and indicating the selected Class, directly to the Fund's Transfer Agent. In the
case of Class A shares, after deduction of any applicable sales charge, the
balance will be applied to the purchase of Fund shares, and, in the case of
shares of the other Classes, the entire amount will be applied to the purchase
of Fund shares, at the net asset value per share next computed after receipt of
the check or purchase payment by the Transfer Agent. The shares so purchased
will be credited to the investor's account.
 
    TAX-SHELTERED RETIREMENT PLANS.  Retirement plans are available for use by
corporations, the self-employed, Individual Retirement Accounts and Custodial
Accounts under Section 403(b)(7) of the Internal Revenue Code. Adoption of such
plans should be on advice of legal counsel or tax advisor.
 
    For further information regarding plan administration, custodial fees and
other details, investors should contact their Morgan Stanley Dean Witter
Financial Advisor or other selected broker-dealer representative or the Transfer
Agent.
 
EXCHANGE PRIVILEGE
 
    As discussed in the Prospectus, the Fund makes available to its shareholders
an Exchange Privilege whereby shareholders of each Class of shares of the Fund
may exchange their shares for shares of the same Class of shares of any other
Morgan Stanley Dean Witter Multi-Class Fund without the imposition of any
exchange fee. Shares may also be exchanged for shares of any of the following
funds: Morgan Stanley Dean Witter Short-Term U.S. Treasury Trust, Morgan Stanley
Dean Witter Limited Term Municipal Trust, Morgan Stanley Dean Witter Short-Term
Bond Fund and five Morgan Stanley Dean Witter Funds which are money market funds
(the foregoing eight funds are hereinafter referred to as the "Exchange Funds").
Class A shares may also be exchanged for shares of Morgan Stanley Dean Witter
Multi-State Municipal Series Trust and Morgan Stanley Dean Witter Hawaii
Municipal Trust, which are Morgan Stanley Dean Witter Funds sold with a
front-end sales charge ("FSC Funds"). Exchanges may be made after the shares of
the Fund acquired by purchase (not by exchange or dividend reinvestment) have
been held for thirty days. There is no waiting period for exchanges of shares
acquired by exchange or dividend reinvestment. An exchange will be treated for
federal income tax purposes the same as a repurchase or redemption of shares, on
which the shareholder may realize a capital gain or loss.
 
    Any new account established through the Exchange Privilege will have the
same registration and cash dividend or dividend reinvestment plan as the present
account, unless the Transfer Agent receives written notification to the
contrary. For telephone exchanges, the exact registration of the existing
account and the account number must be provided.
 
    Any shares held in certificate form cannot be exchanged but must be
forwarded to the Transfer Agent and deposited into the shareholder's account
before being eligible for exchange. (Certificates mailed in for deposit should
not be endorsed.)
 
    As described below, and in the Prospectus under the caption "Purchase of
Fund Shares," a CDSC may be imposed upon a redemption, depending on a number of
factors, including the number of years from the time of purchase until the time
of redemption or exchange ("holding period"). When shares of a Morgan Stanley
Dean Witter Multi-Class Fund are exchanged for shares of an Exchange Fund, the
exchange is executed at no charge to the shareholder, without the imposition of
the CDSC at the time of the exchange. During the period of time the shareholder
remains in the Exchange Fund (calculated from the last day of the month in which
the Exchange Fund shares were acquired), the holding period or "year since
purchase payment made" is frozen. When shares are redeemed out of the Exchange
Fund, they
 
                                       37
<PAGE>
will be subject to a CDSC which would be based upon the period of time the
shareholder held shares in a Morgan Stanley Dean Witter Multi-Class Fund.
However, in the case of shares exchanged into an Exchange Fund on or after April
23, 1990, upon a redemption of shares which results in a CDSC being imposed, a
credit (not to exceed the amount of the CDSC) will be given in an amount equal
to the Exchange Fund 12b-1 distribution fees incurred on or after that date
which are attributable to those shares. Shareholders acquiring shares of an
Exchange Fund pursuant to this exchange privilege may exchange those shares back
into a Morgan Stanley Dean Witter Multi-Class Fund from the Exchange Fund, with
no CDSC being imposed on such exchange. The holding period previously frozen
when shares were first exchanged for shares of the Exchange Fund resumes on the
last day of the month in which shares of a Morgan Stanley Dean Witter
Multi-Class Fund are reacquired. A CDSC is imposed only upon an ultimate
redemption, based upon the time (calculated as described above) the shareholder
was invested in a Morgan Stanley Dean Witter Multi-Class Fund. In the case of
exchanges of Class A shares which are subject to a CDSC, the holding period also
includes the time (calculated as described above) the shareholder was invested
in a FSC Fund.
 
    When shares initially purchased in a Morgan Stanley Dean Witter Multi-Class
Fund are exchanged for shares of a Morgan Stanley Dean Witter Multi-Class Fund,
shares of a FSC Fund, or shares of an Exchange Fund, the date of purchase of the
shares of the fund exchanged into, for purposes of the CDSC upon redemption,
will be the last day of the month in which the shares being exchanged were
originally purchased. In allocating the purchase payments between funds for
purposes of the CDSC, the amount which represents the current net asset value of
shares at the time of the exchange which were (i) purchased more than one, three
or six years (depending on the CDSC schedule applicable to the shares) prior to
the exchange, (ii) originally acquired through reinvestment of dividends or
distributions and (iii) acquired in exchange for shares of FSC Funds, or for
shares of other Morgan Stanley Dean Witter Funds for which shares of FSC Funds
have been exchanged (all such shares called "Free Shares"), will be exchanged
first. After an exchange, all dividends earned on shares in an Exchange Fund
will be considered Free Shares. If the exchanged amount exceeds the value of
such Free Shares, an exchange is made, on a block-by-block basis, of non-Free
Shares held for the longest period of time (except that, with respect to Class B
shares, if shares held for identical periods of time but subject to different
CDSC schedules are held in the same Exchange Privilege account, the shares of
that block that are subject to a lower CDSC rate will be exchanged prior to the
shares of that block that are subject to a higher CDSC rate). Shares equal to
any appreciation in the value of non-Free Shares exchanged will be treated as
Free Shares, and the amount of the purchase payments for the non-Free Shares of
the fund exchanged into will be equal to the lesser of (a) the purchase payments
for, or (b) the current net asset value of, the exchanged non-Free Shares. If an
exchange between funds would result in exchange of only part of a particular
block of non-Free Shares, then shares equal to any appreciation in the value of
the block (up to the amount of the exchange) will be treated as Free Shares and
exchanged first, and the purchase payment for that block will be allocated on a
pro rata basis between the non-Free Shares of that block to be retained and the
non-Free Shares to be exchanged. The prorated amount of such purchase payment
attributable to the retained non-Free Shares will remain as the purchase payment
for such shares, and the amount of purchase payment for the exchanged non-Free
Shares will be equal to the lesser of (a) the prorated amount of the purchase
payment for, or (b) the current net asset value of, those exchanged non-Free
Shares. Based upon the procedures described in the Prospectus under the caption
"Purchase of Fund Shares," any applicable CDSC will be imposed upon the ultimate
redemption of shares of any fund, regardless of the number of exchanges since
those shares were originally purchased.
 
    With respect to the redemption or repurchase of shares of the Fund, the
application of proceeds to the purchase of new shares in the Fund or any other
of the funds and the general administration of the Exchange Privilege, the
Transfer Agent acts as agent for the Distributor and for the shareholder's
selected broker-dealer, if any, in the performance of such functions. With
respect to exchanges, redemptions or repurchases, the Transfer Agent shall be
liable for its own negligence and not for the default or negligence of its
correspondents or for losses in transit. The Fund shall not be liable for any
default or negligence of the Transfer Agent, the Distributor or any selected
broker-dealer.
 
                                       38
<PAGE>
    The Distributor and any selected broker-dealer have authorized and appointed
the Transfer Agent to act as their agent in connection with the application of
proceeds of any redemption of Fund shares to the purchase of shares of any other
fund and the general administration of the Exchange Privilege. No commission or
discounts will be paid to the Distributor or any selected broker-dealer for any
transactions pursuant to this Exchange Privilege.
 
    Exchanges are subject to the minimum investment requirement and any other
conditions imposed by each fund. (The minimum initial investment for the
Exchange Privilege account of each Class is $5,000 for Morgan Stanley Dean
Witter Liquid Asset Fund Inc., Morgan Stanley Dean Witter Tax-Free Daily Income
Trust, Morgan Stanley Dean Witter California Tax-Free Daily Income Trust and
Morgan Stanley Dean Witter New York Municipal Money Market Trust, although those
funds may, in their discretion, accept initial investments of as low as $1,000.
The minimum initial investment for the Exchange Privilege account of each Class
is $10,000 for Morgan Stanley Dean Witter Short-Term U.S. Treasury Trust,
although that fund, in its discretion, may accept initial purchases of as low as
$5,000. The minimum initial investment for the Exchange Privilege account of
each Class is $5,000 for Morgan Stanley Dean Witter Special Value Fund. The
minimum initial investment for the Exchange Privilege account of each Class of
all other Morgan Stanley Dean Witter Funds for which the Exchange Privilege is
available is $1,000.) Upon exchange into an Exchange Fund, the shares of that
fund will be held in a special Exchange Privilege Account separately from
accounts of those shareholders who have acquired their shares directly from that
fund. As a result, certain services normally available to shareholders of those
funds, including the check writing feature, will not be available for funds held
in that account.
 
    The Fund and each of the other Morgan Stanley Dean Witter Funds may limit
the number of times this Exchange Privilege may be exercised by any investor
within a specified period of time. Also, the Exchange Privilege may be
terminated or revised at any time by the Fund and/or any of the Morgan Stanley
Dean Witter Funds for which shares of the Fund have been exchanged, upon such
notice as may be required by applicable regulatory agencies (presently sixty
days' prior written notice for termination or material revision), provided that
six months' prior written notice of termination will be given to the
shareholders who hold shares of Exchange Funds pursuant to the Exchange
Privilege, and provided further that the Exchange Privilege may be terminated or
materially revised without notice at times (a) when the New York Stock Exchange
is closed for other than customary weekends and holidays, (b) when trading on
that Exchange is restricted, (c) when an emergency exists as a result of which
disposal by the Fund of securities owned by it is not reasonably practicable or
it is not reasonably practicable for the Fund fairly to determine the value of
its net assets, (d) during any other period when the Securities and Exchange
Commission by order so permits (provided that applicable rules and regulations
of the Securities and Exchange Commission shall govern as to whether the
conditions prescribed in (b) or (c) exist) or (e) if the Fund would be unable to
invest amounts effectively in accordance with its investment objective, policies
and restrictions.
 
    For further information regarding the Exchange Privilege, shareholders
should contact their Morgan Stanley Dean Witter Financial Advisor or other
selected broker-dealer representative or the Transfer Agent.
 
REDEMPTIONS AND REPURCHASES
- --------------------------------------------------------------------------------
 
    REDEMPTION.  As stated in the Prospectus, shares of each Class of the Fund
can be redeemed for cash at any time at the net asset value per share next
determined; however, such redemption proceeds will be reduced by the amount of
any applicable CDSC. If shares are held in a shareholder's account without a
share certificate, a written request for redemption to the Fund's Transfer Agent
at P.O. Box 983, Jersey City, NJ 07303 is required. If certificates are held by
the shareholder, the shares may be redeemed by surrendering the certificates
with a written request for redemption. The share certificate, or an accompanying
stock power, and the request for redemption, must be signed by the shareholder
or shareholders exactly as the shares are registered. Each request for
redemption, whether or not accompanied by a share certificate, must be sent to
the Fund's Transfer Agent, which will redeem the shares at
 
                                       39
<PAGE>
their net asset value next computed (see "Purchase of Fund Shares" in the
Prospectus) after it receives the request, and certificate, if any, in good
order. Any redemption request received after such computation will be redeemed
at the next determined net asset value. The term "good order" means that the
share certificate, if any, and request for redemption are properly signed,
accompanied by any documentation required by the Transfer Agent, and bear
signature guarantees when required by the Fund or the Transfer Agent. If
redemption is requested by a corporation, partnership, trust or fiduciary, the
Transfer Agent may require that written evidence of authority acceptable to the
Transfer Agent be submitted before such request is accepted.
 
    Whether certificates are held by the shareholder or shares are held in a
shareholder's account, if the proceeds are to be paid to any person other than
the record owner, or if the proceeds are to be paid to a corporation (other than
the Distributor or a selected broker-dealer for the account of the shareholder),
partnership, trust or fiduciary, or sent to the shareholder at an address other
than the registered address, signatures must be guaranteed by an eligible
guarantor acceptable to the Transfer Agent (shareholders should contact the
Transfer Agent for a determination as to whether a particular institution is
such an eligible guarantor). A stock power may be obtained from any dealer or
commercial bank. The Fund may change the signature guarantee requirements from
time to time upon notice to shareholders, which may be by means of a supplement
to the prospectus or a new prospectus.
 
    REPURCHASE.  As stated in the Prospectus, DWR and other selected
broker-dealers are authorized to repurchase shares represented by a share
certificate which is delivered to any of their offices. Shares held in a
shareholder's account without a share certificate may also be repurchased by DWR
and other selected broker-dealers upon the telephonic request of the
shareholder. The repurchase price is the net asset value next computed after
such purchase order is received by DWR or other selected broker-dealer reduced
by any applicable CDSC.
 
    PAYMENT FOR SHARES REDEEMED OR REPURCHASED.  As discussed in the Prospectus,
payment for shares of any Class presented for repurchase or redemption will be
made by check within seven days after receipt by the Transfer Agent of the
certificate and/or written request in good order. Such payment may be postponed
or the right of redemption suspended at times (a) when the New York Stock
Exchange is closed for other than customary weekends and holidays, (b) when
trading on that Exchange is restricted, (c) when an emergency exists as a result
of which disposal by the Fund of securities owned by it is not reasonably
practicable or it is not reasonably practicable for the Fund fairly to determine
the value of its net assets, or (d) during any other period when the Securities
and Exchange Commission by order so permits; provided that applicable rules and
regulations of the Securities and Exchange Commission shall govern as to whether
the conditions prescribed in (b) or (c) exist. If the shares to be redeemed have
recently been purchased by check (including a certified or bank cashier's
check), payment of redemption proceeds may be delayed for the minimum time
needed to verify that the check used for investment has been honored (not more
than fifteen days from the time of receipt of the check by the Transfer Agent).
It has been and remains the Fund's policy and practice that, if checks for
dividends or distributions paid in cash remain uncashed, no interest will accrue
on amounts represented by such uncashed checks. Shareholders maintaining margin
accounts with DWR or another selected broker-dealer are referred to their Morgan
Stanley Dean Witter Financial Advisor or other selected broker-dealer
representative regarding restrictions on redemption of shares of the Fund
pledged in the margin account.
 
    TRANSFERS OF SHARES.  In the event a shareholder requests a transfer of
shares to a new registration, such shares will be transferred without sales
charge at the time of transfer. With regard to the status of shares which are
either subject to the CDSC or free of such charge (and with regard to the length
of time shares subject to the charge have been held), any transfer involving
less than all of the shares in an account will be made on a pro rata basis (that
is, by transferring shares in the same proportion that the transferred shares
bear to the total shares in the account immediately prior to the transfer). The
transferred shares will continue to be subject to any applicable CDSC as if they
had not been so transferred.
 
                                       40
<PAGE>
    REINSTATEMENT PRIVILEGE.  As discussed in the Prospectus, a shareholder who
has had his or her shares redeemed or repurchased and has not previously
exercised this reinstatement privilege may within 35 days after the date of
redemption or repurchase reinstate any portion or all of the proceeds of such
redemption or repurchase in shares of the Fund in the same Class at the net
asset value next determined after the reinstatement request, together with such
proceeds, is received by the Transfer Agent.
 
    Exercise of the reinstatement privilege will not affect the federal income
tax treatment of any gain or loss realized upon redemption or repurchase, except
that if the redemption or repurchase resulted in a loss and reinstatement is
made in shares of the Fund, some or all of the loss, depending on the amount
reinstated, will not be allowed as a deduction for federal income tax purposes
but will be applied to adjust the cost basis of the shares acquired upon
reinstatement.
 
DIVIDENDS, DISTRIBUTIONS AND TAXES
- --------------------------------------------------------------------------------
 
    As discussed in the Prospectus, the Fund will determine either to distribute
or to retain all or part of any net long-term capital gains in any year for
reinvestment. If any such gains are retained, the Fund will pay federal income
tax thereon, and will notify shareholders that following an election by the
Fund, the shareholders will be required to include such undistributed gains in
determining their taxable income and may claim their share of the tax paid by
the Fund as a credit against their individual federal income tax.
 
    Because the Fund intends to distribute all of its net investment income and
capital gains to shareholders and otherwise continue to qualify as a regulated
investment company under Subchapter M of the Internal Revenue Code, it is not
expected that the Fund will be required to pay any federal income tax.
Shareholders will normally have to pay federal income taxes, and any state
income taxes, on the dividends and distributions they receive from the Fund.
Such dividends and distributions, to the extent that they are derived from net
investment income or short-term capital gains, are taxable to the shareholder as
ordinary income regardless of whether the shareholder receives such payments in
additional shares or in cash. Any dividends declared in the last calendar
quarter of any year to shareholder of record for that period which are paid in
the following calendar year prior to February 1 will be deemed received by the
shareholder in the prior calendar year.
 
    Gains or losses on the sales of securities by the Fund will be long-term
capital gains or losses if the securities have been held by the Fund for more
than twelve months. Gains or losses on the sale of securities held for twelve
months or less will be short-term capital gains or losses.
 
    Distributions of net long-term capital gains, if any, are taxable to
shareholders as long-term capital gains regardless of how long a shareholder has
held the Fund's shares and regardless of whether the distribution is received in
additional shares or in cash. Capital gains distributions are not eligible for
the dividends received deduction. The Taxpayer Relief Act reduced the maximum
tax rate on long term capital gains from 28% to 20%; however, it also lengthened
the required holding period to obtain the lower rate from more than 12 months to
more than 18 months. However, the IRS Restructuring and Reform Act of 1998
reduces the holding period requirement for the lower capital gain rate to more
than twelve months for transactions occurring after January 1, 1998. The lower
rates do not apply to collectibles and certain other assets. The lower rates do
not apply to collectibles and certain other assets. Additionally, the maximum
capital gain rate for assets that are held more than five years and that are
acquired after December 31, 2000 is 18%.
 
    The Fund has qualified and intends to remain qualified as a regulated
investment company under Subchapter M of the Internal Revenue Code of 1986 (the
"Code"). If so qualified, the Fund will not be subject to federal income tax on
its net investment income and net short-term capital gains, if any, realized
during any fiscal year in which it distributes such income and capital gains to
its shareholders.
 
    Any dividend or capital gains distribution received by a shareholder from
any investment company will have the effect of reducing the net asset value of
the shareholder's stock in that company by the
 
                                       41
<PAGE>
exact amount of the dividend or capital gains distribution. Furthermore, capital
gains distributions and dividends are subject to federal income taxes. If the
net asset value of the shares should be reduced below a shareholder's cost as a
result of the payment of dividends or the distribution of realized long-term
capital gains, such payment or distribution would be in part a return of the
shareholder's investment to the extent of such reduction below the shareholder's
cost, but nonetheless would be fully taxable at either ordinary or capital gain
rates. Therefore, an investor should consider the tax implications of purchasing
Fund shares immediately prior to a dividend or distribution record date.
 
    Dividend payments will be eligible for the federal dividends received
deduction available to the Fund's corporate shareholders only to the extent the
aggregate dividends received by the Fund would be eligible for the deduction if
the Fund were the shareholder claiming the dividends received deduction. The
amount of dividends paid by the Fund which may qualify for the dividends
received deduction is limited to the aggregate amount of qualifying dividends
which the Fund derives from its portfolio investments which the Fund has held to
a minimum period, usually 46 days within a 90-day period beginning 45 days
before the ex-dividend date of each qualifying dividend. Shareholders must meet
a similar holding period requirement with respect to their shares to claim the
dividends received deduction with respect to any distribution of qualifying
dividends. Any distributions made by the Fund will not be eligible for the
dividends received deduction with respect to shares which are held by the
shareholder for 45 days or less. Any long-term capital gain distributions will
also not be eligible for the dividends received deduction. The ability to take
the dividends received deduction will also be limited in the case of a Fund
shareholder which incurs or continues indebtedness which is directly
attributable to its investment in the Fund.
 
    After the end of the year, shareholders will be sent full information on
their dividends and capital gains distributions for tax purposes, including
information as to the portion taxable as ordinary income, the portion taxable as
long-term capital gains and the portion eligible for the dividends received
deduction. To avoid being subject to a 31% federal backup withholding tax on
taxable dividends, capital gains distributions and the proceeds of redemptions
and repurchases, shareholders' taxpayer identification numbers must be furnished
and certified as to their accuracy.
 
    Shareholders are urged to consult their attorneys or tax advisors regarding
specific questions as to federal, state or local taxes.
 
PERFORMANCE INFORMATION
- --------------------------------------------------------------------------------
 
   
    As discussed in the Prospectus, from time to time the Fund may quote its
"yield" and/or its "total return" in advertisements and sales literature. These
figures are computed separately for Class A, Class B, Class C and Class D
shares. Yield is calculated for any 30-day period as follows: the amount of
interest income for each security in the Fund's portfolio is determined in
accordance with regulatory requirements; the total for the entire portfolio
constitutes the Fund's gross income for the period. Expenses accrued during the
period are subtracted to arrive at "net investment income" of each Class. The
resulting amount is divided by the product of the maximum offering price per
share on the last day of the period multiplied by the average number of shares
of the applicable Class outstanding during the period that were entitled to
dividends. This amount is added to 1 and raised to the sixth power. 1 is then
subtracted from the result and the difference is multiplied by 2 to arrive at
the annualized yield. For the 30-day period ended September 30, 1998, the yield,
calculated pursuant to the formula described above, was 7.56%, 7.19%, 7.20% and
8.25% for Class A, Class B, Class C and Class D, respectively.
    
 
    The Fund's "average annual total return" represents an annualization of the
Fund's total return over a particular period and is computed by finding the
annual percentage rate which will result in the ending redeemable value of a
hypothetical $1,000 investment made at the beginning of a one, five or ten year
period, or for the period from the date of commencement of the Fund's
operations, if shorter than any of the foregoing. The ending redeemable value is
reduced by any CDSC at the end of the one, five or ten year or other period. For
the purpose of this calculation, it is assumed that all dividends and
distributions are reinvested. The formula for computing the average annual total
return involves a percentage
 
                                       42
<PAGE>
obtained by dividing the ending redeemable value by the amount of the initial
investment, taking a root of the quotient (where the root is equivalent to the
number of years in the period) and subtracting 1 from the result. The average
annual total return of Class B for the fiscal year ended September 30, 1998, for
the five years ended September 30, 1998 and for the ten years ended September
30, 1998 was -18.14%, 7.46% and 8.43%, respectively. The average annual total
returns of Class A for the fiscal year ended September 30, 1998 and for the
period July 28, 1997 (inception of the Class) through September 30, 1998 were
- -17.93% and -10.90%, respectively. The average annual total returns of Class C
for the fiscal year ended September 30, 1998 and for the period July 28, 1997
(inception of the Class) through September 30, 1998 were -14.89% and -7.45%,
respectively. The average annual total returns of Class D for the fiscal year
ended September 30, 1998 and for the period July 28, 1997 (inception of the
Class) through September 30, 1998 were -13.19% and -6.52%, respectively.
 
    In addition to the foregoing, the Fund may advertise its total return for
each Class over different periods of time by means of aggregate, average,
year-by-year or other types of total return figures. Such calculations may or
may not reflect the imposition of the maximum front-end sales charge for Class A
or the deduction of the CDSC for each of Class B and Class C which, if
reflected, would reduce the performance quoted. For example, the average annual
total return of the Fund may be calculated in the manner described above, but
without deduction for any applicable contingent deferred sales charge. Based on
this calculation, the average annual total return of Class B for the fiscal year
ended September 30, 1998, for the five years ended September 30, 1998 and for
the ten years ended September 30, 1998 was -14.01%, 7.76% and 8.43%,
respectively. Based on this calculation, the average annual total returns of
Class A for the fiscal year ended September 30, 1998 and for the period July 28,
1997 through September 30, 1998 were -13.38% and -6.72%, respectively, the
average annual total returns of Class C for the fiscal year ended September 30,
1998 and for the period July 28, 1997 through September 30, 1998 were -14.07%
and -7.45%, respectively, and the average annual total returns of Class D for
the fiscal year ended September 30, 1998 and for the period July 28, 1997
through September 30, 1998 were -13.19% and -6.52%, respectively.
 
    In addition, the Fund may compute its aggregate total return for each Class
for specified periods by determining the aggregate percentage rate which will
result in the ending value of a hypothetical $1,000 investment made at the
beginning of the period. For the purpose of this calculation, it is assumed that
all dividends and distributions are reinvested. The formula for computing
aggregate total return involves a percentage obtained by dividing the ending
value (without the reduction for any sales charge) by the initial $1,000
investment and subtracting 1 from the result. Based on the foregoing
calculation, the total return for Class B for the fiscal year ended September
30, 1998, for the five years ended September 30, 1998 and for the ten years
ended September 30, 1998 was -14.01%, 45.28% and 124.68%, respectively. Based on
the foregoing calculations, the total returns for Class A for the fiscal year
ended September 30, 1998 and for the period July 28, 1997 through September 30,
1998 were -13.38% and -7.84%, respectively, the total returns of Class C for the
fiscal year ended September 30, 1998 and for the period July 28, 1997 through
September 30, 1998 were -14.07% and -8.69%, respectively, and the total returns
of Class D for the fiscal year ended September 30, 1998 and for the period July
28, 1997 through September 30, 1998 were -13.19% and -7.62%, respectively.
 
    The Fund may also advertise the growth of hypothetical investments of
$10,000, $50,000 and $100,000 in each Class of shares of the Fund by adding 1 to
the Fund's aggregate total return to date (expressed as a decimal and without
taking into account the extent of any applicable CDSC) and multiplying by
$9,475, $48,000 and $97,000 in the case of Class A (investments of $10,000,
$50,000 and $100,000 adjusted for the initial sales charge) or by $10,000,
$50,000 and $100,000 in the case of each of Class B, Class C and Class D, as the
case may be. Investments of $10,000, $50,000 or $100,000 in
 
                                       43
<PAGE>
each Class at inception of the Class would have grown (declined) to the
following amounts at September 30, 1998:
 
<TABLE>
<CAPTION>
                                                    INVESTMENT AT INCEPTION OF:
                                  INCEPTION      ----------------------------------
CLASS                               DATE         $10,000     $ 50,000     $100,000
- ------------------------------  -------------    --------    ---------    ---------
<S>                             <C>              <C>         <C>          <C>
Class A.......................     7/28/97       $ 8,732     $ 44,237     $ 89,395
Class B.......................    10/31/85        25,762      128,810      257,620
Class C.......................     7/28/97         9,131       45,655       91,310
Class D.......................     7/28/97         9,239       46,195       92,390
</TABLE>
 
    The Fund may advertise, from time to time, its performance relative to
certain performance rankings and indices compiled by independent organizations.
 
DESCRIPTION OF SHARES OF THE FUND
- --------------------------------------------------------------------------------
 
    As discussed in the Prospectus, the shareholders of the Fund are entitled to
a full vote for each full share held. All of the Trustees have been elected by
the shareholders of the Fund, most recently at a Special Meeting of Shareholders
held on May 21, 1997. The Trustees themselves have the power to alter the number
and the terms of office of the Trustees, and they may at any time lengthen or
shorten their own terms or make their terms of unlimited duration and appoint
their own successors, provided that always at least a majority of the Trustees
has been elected by the shareholders of the Fund. Under certain circumstances,
the Trustees may be removed by action of the Trustees. The shareholders also
have the right, under certain circumstances, to remove the Trustees. The voting
rights of shareholders are not cumulative, so that holders of more than 50
percent of the shares voting can, if they choose, elect all Trustees being
selected, while the holders of the remaining shares would be unable to elect any
Trustees.
 
    The Declaration of Trust permits the Trustees to authorize the creation of
additional series of shares (the proceeds of which would be invested in
separate, independently managed portfolios) and additional classes of shares
within any series. The Trustees have not authorized any such additional series
or classes of shares other than as set forth in the Prospectus.
 
    The Declaration of Trust further provides that no Trustee, officer, employee
or agent of the Fund is liable to the Fund or to a shareholder, nor is any
Trustee, officer, employee or agent liable to any third persons in connection
with the affairs of the Fund, except as such liability may a rise from his/her
or its own bad faith, willful misfeasance, gross negligence, or reckless
disregard of his duties. It also provides that all third persons shall look
solely to the Fund property for satisfaction of claims arising in connection
with the affairs of the Fund. With the exceptions stated, the Declaration of
Trust provides that a Trustee, officer, employee or agent is entitled to be
indemnified against all liability in connection with the affairs of the Fund.
 
    The Fund is authorized to issue an unlimited number of shares of beneficial
interest. The Trust shall be of unlimited duration, subject to the provisions in
the Declaration of Trust concerning termination by action of the shareholders or
the Trustees.
 
CUSTODIAN AND TRANSFER AGENT
- --------------------------------------------------------------------------------
 
    The Bank of New York, 90 Washington Street, New York, New York 10286 is the
Custodian of the Fund's assets. Any of the Fund's cash balances with the
Custodian in excess of $100,000 are unprotected by federal deposit insurance.
Such balances may, at times, be substantial.
 
    Morgan Stanley Dean Witter Trust FSB ("MSDW Trust"), Harborside Financial
Center, Plaza Two, Jersey City, New Jersey 07311 is the Transfer Agent of the
Fund's shares and Dividend Disbursing Agent for payment of dividends and
distributions on Fund shares and Agent for shareholders under various investment
plans described herein. Morgan Stanley Dean Witter Trust FSB is an affiliate of
Morgan Stanley Dean Witter Advisors Inc., the Fund's Investment Manager and
Morgan Stanley Dean Witter Distributors Inc., the Fund's Distributor. As
Transfer Agent and Dividend Disbursing Agent, MSDW Trust's
 
                                       44
<PAGE>
responsibilities include maintaining shareholder accounts; including providing
subaccounting and recordkeeping services for certain retirement accounts;
disbursing cash dividends and reinvesting dividends; processing account
registration changes; handling purchase and redemption transactions; mailing
prospectuses and reports; mailing and tabulating proxies; processing share
certificate transactions; and maintaining shareholder records and lists. For
these services MSDW Trust receives a per shareholder account fee from the Fund.
 
INDEPENDENT ACCOUNTANTS
- --------------------------------------------------------------------------------
 
    PricewaterhouseCoopers LLP serves as the independent accountants of the
Fund. The independent accountants are responsible for auditing the annual
financial statements of the Fund.
 
REPORTS TO SHAREHOLDERS
- --------------------------------------------------------------------------------
 
    The Fund will send to shareholders, at least semi-annually, reports showing
the Fund's portfolio and other information. An annual report, containing
financial statements audited by independent accountants, will be sent to
shareholders each year.
 
    The Fund's fiscal year ends on September 30. The financial statements of the
Fund must be audited at least once a year by independent accountants whose
selection is made annually by the Fund's Trustees.
 
LEGAL COUNSEL
- --------------------------------------------------------------------------------
 
    Barry Fink, Esq., who is an officer and the General Counsel of the
Investment Manager, is an officer and the General Counsel of the Fund.
 
EXPERTS
- --------------------------------------------------------------------------------
 
    The annual financial statements of the Fund for the year ended September 30,
1998 included in this Statement of Additional Information and incorporated by
reference in the Prospectus have been so included and incorporated in reliance
on the report of PricewaterhouseCoopers LLP, independent accountants, given on
the authority of said firm as experts in auditing and accounting.
 
REGISTRATION STATEMENT
- --------------------------------------------------------------------------------
 
    This Statement of Additional Information and the Prospectus do not contain
all of the information set forth in the Registration Statement the Fund has
filed with the Securities and Exchange Commission. The complete Registration
Statement may be obtained from the Securities and Exchange Commission upon
payment of the fee prescribed by the rules and regulations of the Commission.
 
                                       45
<PAGE>
 
MORGAN STANLEY DEAN WITTER CONVERTIBLE SE-
CURITIES TRUST
REPORT OF INDEPENDENT ACCOUNTANTS
 
TO THE SHAREHOLDERS AND TRUSTEES
OF MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
 
In our opinion, the accompanying statement of assets and liabilities, including
the portfolio of investments, and the related statements of operations and of
changes in net assets and the financial highlights present fairly, in all
material respects, the financial position of Morgan Stanley Dean Witter
Convertible Securities Trust (the "Fund"), formerly Dean Witter Convertible
Securities Trust, at September 30, 1998, the results of its operations for the
year then ended, the changes in its net assets for each of the two years in the
period then ended and the financial highlights for each of the periods
presented, in conformity with generally accepted accounting principles. These
financial statements and financial highlights (hereafter referred to as
"financial statements") are the responsibility of the Fund's management; our
responsibility is to express an opinion on these financial statements based on
our audits. We conducted our audits of these financial statements in accordance
with generally accepted auditing standards which require that we plan and
perform the audit to obtain reasonable assurance about whether the financial
statements are free of material misstatement. An audit includes examining, on a
test basis, evidence supporting the amounts and disclosures in the financial
statements, assessing the accounting principles used and significant estimates
made by management, and evaluating the overall financial statement presentation.
We believe that our audits, which included confirmation of securities at
September 30, 1998 by correspondence with the custodian, provide a reasonable
basis for the opinion expressed above.
 
PricewaterhouseCoopers LLP
1177 AVENUE OF THE AMERICAS
NEW YORK, NEW YORK 10036
NOVEMBER 9, 1998
 
                      1998 FEDERAL TAX NOTICE (UNAUDITED)
 
       During the fiscal year ended September 30, 1998, 40.38% of the
       income dividends qualified for the dividends received deduction
       available to corporations.
 
                                       46
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                                   COUPON   MATURITY
THOUSANDS                                                                                    RATE      DATE       VALUE
- ---------------------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                              <C>      <C>       <C>
           CONVERTIBLE BONDS (54.8%)
           AEROSPACE (1.3%)
$  4,000   Spacehab, Inc. - 144A**........................................................   8.00%   10/15/07  $  3,580,920
                                                                                                               ------------
           ASSISTED LIVING SERVICES (2.3%)
   2,500   ARV Assisted Living, Inc.......................................................   6.75    04/01/06     1,447,500
   3,000   Assisted Living Concepts, Inc. - 144A**........................................   5.625   05/01/03     2,460,000
   3,100   Emeritus Corp. - 144A**........................................................   6.25    01/01/06     2,169,442
                                                                                                               ------------
                                                                                                                  6,076,942
                                                                                                               ------------
           AUTO PARTS (3.6%)
   2,000   Magna International, Inc. (Canada) - 144A**....................................   4.875   02/15/05     2,007,500
   3,450   Mark IV Industries, Inc. - 144A**..............................................   4.75    11/01/04     2,868,882
   5,750   MascoTech, Inc.................................................................   4.50    12/15/03     4,894,688
                                                                                                               ------------
                                                                                                                  9,771,070
                                                                                                               ------------
           BIOTECHNOLOGY (0.7%)
   2,000   Chiron Corp....................................................................   1.90    11/17/00     1,879,440
                                                                                                               ------------
           BOOKS/MAGAZINE (1.0%)
   2,835   Nelson (Thomas), Inc...........................................................   5.75    11/30/99     2,814,361
                                                                                                               ------------
           BROADCASTING (1.8%)
   5,000   Scandinavian Broadcasting System SA (Luxembourg) - 144A**......................   7.00    12/01/04     4,889,400
                                                                                                               ------------
           BUSINESS SERVICES (0.8%)
   2,000   Data Processing Resource Corp. - 144A**........................................   5.25    04/01/05     2,027,500
                                                                                                               ------------
           CABLE/CELLULAR (3.6%)
   6,500   Tele-Communications International, Inc.........................................   4.50    02/15/06     6,266,000
   8,770   U.S. Cellular Corp.............................................................   0.00    06/15/15     3,332,775
                                                                                                               ------------
                                                                                                                  9,598,775
                                                                                                               ------------
           CLOTHING/SHOE/ACCESSORY CHAINS (4.0%)
   3,600   Genesco Inc. - 144A**..........................................................   5.50    04/15/05     2,085,948
   6,500   Saks Holdings, Inc.............................................................   5.50    09/15/06     6,435,000
   3,000   The Sports Authority, Inc......................................................   5.25    09/15/01     2,250,240
                                                                                                               ------------
                                                                                                                 10,771,188
                                                                                                               ------------
           COMPUTER SOFTWARE (0.2%)
   1,000   Tecnomatix Technologies Ltd. (Israel) - 144A**.................................   5.25    08/15/04       588,750
                                                                                                               ------------
           CONTRACT DRILLING (0.8%)
   9,000   Pride International, Inc.......................................................   0.00    04/24/18     2,230,740
                                                                                                               ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       47
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                                   COUPON   MATURITY
THOUSANDS                                                                                    RATE      DATE       VALUE
- ---------------------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                              <C>      <C>       <C>
           DIVERSIFIED FINANCIAL SERVICES (0.0%)
$  1,000   Cityscape Financial Corp. (a)..................................................   6.00%   05/01/06  $     10,000
   1,000   Cityscape Financial Corp. (Eurobond) (a).......................................   6.00    05/01/06        20,000
   2,500   Cityscape Financial Corp. - 144A** (a).........................................   6.00    05/01/06        25,000
                                                                                                               ------------
                                                                                                                     55,000
                                                                                                               ------------
           E.D.P. SERVICES (2.6%)
   2,250   Affiliated Computer Services - 144A**..........................................   4.00    03/15/05     2,131,088
   3,500   May & Speh, Inc................................................................   5.25    04/01/03     4,898,985
                                                                                                               ------------
                                                                                                                  7,030,073
                                                                                                               ------------
           ELECTRONIC DISTRIBUTORS (0.9%)
   4,000   Reptron Electronics, Inc.......................................................   6.75    08/01/04     2,400,000
                                                                                                               ------------
           ENGINEERING & CONSTRUCTION (1.2%)
   4,000   Emcor Group, Inc...............................................................   5.75    04/01/05     3,302,480
                                                                                                               ------------
           FINANCE (1.0%)
   3,000   Financial Federal Corp. - 144A**...............................................   4.50    05/01/05     2,748,750
                                                                                                               ------------
           FUNERAL SERVICES (1.2%)
   3,000   Equity Corporation International - 144A**......................................   4.50    12/31/04     3,320,040
                                                                                                               ------------
           HEALTH CARE MISCELLANEOUS (0.8%)
   2,175   Pharmaceutical Marketing Services, Inc.........................................   6.25    02/01/03     1,898,166
     150   Pharmaceutical Marketing Services, Inc. (Eurobond).............................   6.25    02/01/03       131,250
                                                                                                               ------------
                                                                                                                  2,029,416
                                                                                                               ------------
           HOME BUILDING (0.7%)
   4,300   Lennar Corp....................................................................   0.00    07/29/18     1,827,371
                                                                                                               ------------
           HOTELS/RESORTS (1.1%)
   5,150   Sholodge, Inc..................................................................   7.50    05/01/04     2,974,125
                                                                                                               ------------
           MAJOR U.S. TELECOMMUNICATIONS (2.8%)
   2,500   Bell Atlantic Financial Service - 144A**
             (exchangeable into Cable & Wireless Communications common stock).............   4.25    09/15/05     2,425,775
   5,000   Bell Atlantic Financial Service - 144A**
             (exchangeable into Telecom Corporation of New Zealand common stock)..........   5.75    04/01/03     5,080,350
   1,750   SA Telecommunications, Inc. - 144A** (a).......................................  10.00    08/15/06        52,500
                                                                                                               ------------
                                                                                                                  7,558,625
                                                                                                               ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       48
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                                   COUPON   MATURITY
THOUSANDS                                                                                    RATE      DATE       VALUE
- ---------------------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                              <C>      <C>       <C>
           MANAGED HEALTH CARE (7.5%)
$  3,000   Concentra Managed Care, Inc. - 144A**..........................................   4.50%   03/15/03  $  1,925,640
   4,000   Continucare Corp. - 144A**.....................................................   8.00    10/31/02     2,020,000
   2,000   Greenery Rehabilitation Group, Inc.............................................   8.75    04/01/15     2,035,000
   3,500   Healthsouth Corp. - 144A**.....................................................   3.25    04/01/03     2,660,000
   7,960   Phymatrix Corp.................................................................   6.75    06/15/03     3,024,800
   4,660   Quantum Health Resources, Inc..................................................   4.75    10/01/00     4,189,992
   5,000   Tenet Healthcare, Inc..........................................................   6.00    12/01/05     4,179,700
                                                                                                               ------------
                                                                                                                 20,035,132
                                                                                                               ------------
           MEDICAL ELECTRONICS (1.0%)
   3,600   ThermoTrex Corp................................................................   3.25    11/01/07     2,611,728
                                                                                                               ------------
           MEDICAL SPECIALTIES (0.7%)
   2,500   Fuisz Technologies Ltd.........................................................   7.00    10/15/04     1,864,850
                                                                                                               ------------
           NEWSPAPERS (1.0%)
   6,500   Hollinger, Inc. (Canada).......................................................   0.00    10/05/13     2,652,845
                                                                                                               ------------
           OFFICE EQUIPMENT & SUPPLIES (1.1%)
   4,700   Danka Business Systems, PLC (United Kingdom)...................................   6.75    04/01/02     3,057,961
                                                                                                               ------------
           OILFIELD SERVICES/EQUIPMENT (1.2%)
   1,900   Halter Marine Group, Inc. - 144A**.............................................   4.50    09/15/04     1,391,750
   3,000   Key Energy Group, Inc. - 144A**................................................   5.00    09/15/04     1,871,520
                                                                                                               ------------
                                                                                                                  3,263,270
                                                                                                               ------------
           OTHER CONSUMER SERVICES (0.5%)
   1,500   Interim Services Inc...........................................................   4.50    06/01/05     1,290,600
                                                                                                               ------------
           OTHER METALS/MINERALS (0.4%)
   1,250   Crown Resources Corp. (Eurobond)...............................................   5.75    08/27/01       950,000
                                                                                                               ------------
           REAL ESTATE INVESTMENT TRUST (3.2%)
   3,640   Capstar Hotel Corp.............................................................   4.75    10/15/04     2,432,503
   2,650   Centertrust Retail Properties (Series A).......................................   7.50    01/15/01     2,492,656
   4,000   Macerich Company (Eurobond)....................................................   7.25    12/15/02     3,750,000
                                                                                                               ------------
                                                                                                                  8,675,159
                                                                                                               ------------
           RESTAURANTS (1.0%)
   2,500   Hometown Buffet, Inc...........................................................   7.00    12/01/02     2,568,275
                                                                                                               ------------
           SCIENTIFIC INSTRUMENTS (0.8%)
   2,500   Thermo Instrument Systems, Inc. - 144A**.......................................   4.50    10/15/03     2,083,600
                                                                                                               ------------
           SEMICONDUCTOR EQUIPMENT (0.5%)
   2,000   Integrated Process Equipment Corp. - 144A**....................................   6.25    09/15/04     1,242,500
                                                                                                               ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       49
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                                   COUPON   MATURITY
THOUSANDS                                                                                    RATE      DATE       VALUE
- ---------------------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                              <C>      <C>       <C>
           SHOE MANUFACTURING (2.0%)
$    625   Nine West Group, Inc...........................................................   5.50%   07/15/03  $    413,181
   7,500   Nine West Group, Inc. - 144A**.................................................   5.50    07/15/03     4,958,175
                                                                                                               ------------
                                                                                                                  5,371,356
                                                                                                               ------------
           TELECOMMUNICATIONS (1.2%)
   4,000   Global Telesystems Group, Inc..................................................   5.75    07/01/10     3,134,240
                                                                                                               ------------
           WIRELESS COMMUNICATION (0.3%)
   1,500   Cellstar Corp..................................................................   5.00    10/15/02       778,020
                                                                                                               ------------
 
           TOTAL CONVERTIBLE BONDS
           (IDENTIFIED COST $175,039,563)....................................................................   147,054,502
                                                                                                               ------------
</TABLE>
 
<TABLE>
<CAPTION>
NUMBER OF
 SHARES
- ---------
<C>        <S>                                                                                      <C>
           CONVERTIBLE PREFERRED STOCKS (32.3%)
           APPAREL (1.7%)
 111,000   Warnaco Group, Inc. $3.00..............................................................     4,440,000
                                                                                                    ------------
           AUTO PARTS (0.9%)
 120,000   BTI Capital Trust $3.25 - 144A**.......................................................     2,400,000
                                                                                                    ------------
           AUTOMOBILE - RENTALS (0.7%)
  40,000   Budget Group Capital Trust $3.125 - 144A**.............................................     1,832,520
                                                                                                    ------------
           BIOTECHNOLOGY (0.7%)
  63,500   Gensia Sicor, Inc. $3.75...............................................................     1,841,500
                                                                                                    ------------
           BROADCASTING (3.5%)
 111,000   Metromedia International Group, Inc. $3.625............................................     2,525,250
  97,000   Sinclair Broadcasting Group, Inc. $3.00................................................     4,656,000
 242,000   Triathlon Broadcasting Co. $0.945......................................................     2,299,000
                                                                                                    ------------
                                                                                                       9,480,250
                                                                                                    ------------
           CABLE/CELLULAR (0.7%)
  35,000   MediaOne Group, Inc. $3.63
             (exchangeable into AirTouch Communications, Inc. common stock).......................     1,955,625
                                                                                                    ------------
           COMPUTER SOFTWARE (1.8%)
  51,000   Microsoft Corp. (Series A) $2.196......................................................     4,915,125
                                                                                                    ------------
           CONTAINERS/PACKAGING (0.5%)
  33,100   Owens-Illinois, Inc. $2.375............................................................     1,183,325
                                                                                                    ------------
           DRUG STORE CHAIN (1.2%)
  40,000   CVS Automatic Common Exchange Security Trust $4.23.....................................     3,260,000
                                                                                                    ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       50
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
NUMBER OF
 SHARES                                                                                                VALUE
- ----------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                                      <C>
           ELECTRONIC DISTRIBUTORS (0.6%)
  50,000   Pioneer Standard Electronics, Inc. $3.375 - 144A**.....................................  $  1,700,000
                                                                                                    ------------
           FINANCE (1.1%)
  33,400   Insignia Financing, Inc. $3.25.........................................................     1,275,479
  50,000   United Rentals Trust I $3.25 - 144A**..................................................     1,787,500
                                                                                                    ------------
                                                                                                       3,062,979
                                                                                                    ------------
           HEALTHCARE PRODUCTS & SERVICES (0.9%)
  60,000   Owens & Minor Trust I $2.687 - 144A**..................................................     2,268,780
                                                                                                    ------------
 
           HOME BUILDING (0.9%)
 300,000   Kaufman & Broad Home Corp. $0.825......................................................     2,512,500
                                                                                                    ------------
 
           INVESTMENT BANKERS/BROKERS/SERVICES (3.1%)
  50,000   Merrill Lynch & Co., Inc. $2.39
             (exchangeable into IMC Global, Inc. common stock)....................................     1,106,250
 100,000   Merrill Lynch & Co., Inc. $4.087
             (exchangeable into SunAmerica, Inc. common stock)....................................     7,312,500
                                                                                                    ------------
                                                                                                       8,418,750
                                                                                                    ------------
           MAJOR U.S. TELECOMMUNICATIONS (1.2%)
  73,400   Loral Space & Communications Ltd. (Series C) $3.00 (Bermuda)...........................     3,312,175
                                                                                                    ------------
           MANAGED HEALTH CARE (0.2%)
  50,000   Sun Financing I $1.75 - 144A**.........................................................       606,250
                                                                                                    ------------
           MOBILE HOME & RECREATION (0.8%)
  50,000   Fleetwood Capital Trust $3.00 - 144A**.................................................     2,162,500
                                                                                                    ------------
           NEWSPAPERS (0.7%)
  80,000   Tribune Co. $1.75
             (exchangeable into The Learning Company, Inc. common stock)..........................     1,730,000
                                                                                                    ------------
           NON - U.S. BANKS (1.1%)
 110,000   National Australia Bank, Ltd. $1.969 (Australia) (Units) ++............................     2,928,750
                                                                                                    ------------
           OIL & GAS PRODUCTION (1.2%)
 100,000   Belco Oil & Gas Corp $1.625............................................................     1,650,000
 100,000   Chesapeake Energy Corp. $3.50 - 144A**.................................................     1,600,000
                                                                                                    ------------
                                                                                                       3,250,000
                                                                                                    ------------
           OTHER CONSUMER SERVICES (0.8%)
  80,000   Cendant Corp. $3.75....................................................................     2,000,000
                                                                                                    ------------
 
           RAILROADS (1.0%)
  60,000   Union Pacific Capital Trust $3.125 - 144A**............................................     2,700,000
                                                                                                    ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       51
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
NUMBER OF
 SHARES                                                                                                VALUE
- ----------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                                      <C>
           REAL ESTATE INVESTMENT TRUST (3.8%)
  80,000   Equity Office Properties Trust (Series B) $2.625 - 144A**..............................  $  3,265,040
 161,600   FelCor Lodging Trust, Inc. (Series A) $1.95............................................     3,474,400
  85,000   Reckson Associates Reality Corp. (Series A) $1.906.....................................     1,859,375
  36,600   Rouse Co. (Series B) $3.00.............................................................     1,647,000
                                                                                                    ------------
                                                                                                      10,245,815
                                                                                                    ------------
           TELECOMMUNICATIONS (3.2%)
  52,800   EchoStar Communications Corp. (Series C) $3.375........................................     2,838,000
  80,000   NEXTLINK Communications, Inc. $3.25 - 144A**...........................................     2,920,000
  80,000   Winstar Communications, Inc. (Series D) $3.50..........................................     2,840,000
                                                                                                    ------------
                                                                                                       8,598,000
                                                                                                    ------------
 
           TOTAL CONVERTIBLE PREFERRED STOCKS
           (IDENTIFIED COST $106,379,162).........................................................    86,804,844
                                                                                                    ------------
 
           COMMON STOCKS (3.8%)
           APPAREL (1.9%)
 285,000   Tropical Sportswear International Corp.*...............................................     5,236,875
                                                                                                    ------------
           ASSISTED LIVING SERVICES (0.4%)
  90,000   Emeritus Corp.*........................................................................       973,125
                                                                                                    ------------
           CASINO/GAMBLING (0.7%)
 690,000   Alliance Gaming Corp.*.................................................................     1,746,562
                                                                                                    ------------
           NEWSPAPERS (0.7%)
 126,132   Hollinger International, Inc. (Class A)................................................     1,813,148
                                                                                                    ------------
           WHOLESALE DISTRIBUTOR (0.1%)
  55,000   AVTEAM, Inc. (Class A)*................................................................       309,375
                                                                                                    ------------
 
           TOTAL COMMON STOCKS
           (IDENTIFIED COST $9,020,182)...........................................................    10,079,085
                                                                                                    ------------
</TABLE>
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                           COUPON         MATURITY
THOUSANDS                                                                            RATE            DATE
- ---------                                                                          ---------   -----------------
<C>        <S>                                                                     <C>         <C>                <C>
           SHORT-TERM INVESTMENTS (7.5%)
           U.S. GOVERNMENT AGENCIES (b) (7.4%)
$ 19,900   Federal Home Loan Mortgage Corp. (AMORTIZED COST $19,897,133).........  5.16-5.46%  10/01/98-10/05/98    19,897,133
                                                                                                                  ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       52
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
PORTFOLIO OF INVESTMENTS SEPTEMBER 30, 1998, CONTINUED
 
<TABLE>
<CAPTION>
PRINCIPAL
AMOUNT IN                                                                           COUPON         MATURITY
THOUSANDS                                                                            RATE            DATE            VALUE
- ------------------------------------------------------------------------------------------------------------------------------
<C>        <S>                                                                     <C>         <C>                <C>
           REPURCHASE AGREEMENT (0.1%)
$    355   The Bank of New York (dated 09/30/98; proceeds $354,687) (c)
             (IDENTIFIED COST $354,638)..........................................  5.00%           10/01/98       $    354,638
                                                                                                                  ------------
 
           TOTAL SHORT-TERM INVESTMENTS
           (IDENTIFIED COST $20,251,771)........................................................................    20,251,771
                                                                                                                  ------------
</TABLE>
 
<TABLE>
<S>                                                                                         <C>     <C>
TOTAL INVESTMENTS
(IDENTIFIED COST $310,690,678) (D)........................................................   98.4 %   264,190,202
 
OTHER ASSETS IN EXCESS OF LIABILITIES.....................................................    1.6       4,301,871
                                                                                            ------  -------------
 
NET ASSETS................................................................................  100.0 % $ 268,492,073
                                                                                            ------  -------------
                                                                                            ------  -------------
</TABLE>
 
- ---------------------
 
 *   Non-income producing security.
**   Resale is restricted to qualified institutional investors.
++   Consists of more than one class of securities traded together as a unit;
     stocks with attached warrants.
(a)  Non-income producing security; bond in default.
(b)  Securities were purchased on a discount basis. The interest rates shown
     have been adjusted to reflect a money market equivalent yield.
(c)  Collateralized by $53,202 U.S. Treasury Bond 7.25% due 05/15/16 valued at
     $67,332 and $229,690 U.S. Treasury Bond 7.25% due 08/15/22 valued at
     $294,398.
(d)  The aggregate cost for federal income tax purposes approximates identified
     cost. The aggregate gross unrealized appreciation is $10,687,190 and the
     aggregate gross unrealized depreciation is $57,187,666, resulting in net
     unrealized depreciation of $46,500,476.
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       53
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL STATEMENTS
 
STATEMENT OF ASSETS AND LIABILITIES
SEPTEMBER 30, 1998
 
<TABLE>
<S>                                                                                            <C>
ASSETS:
Investments in securities, at value
  (identified cost $310,690,678).............................................................  $ 264,190,202
Receivable for:
    Interest.................................................................................      3,134,097
    Investments sold.........................................................................      1,755,307
    Dividends................................................................................        162,178
    Shares of beneficial interest sold.......................................................        135,771
Prepaid expenses and other assets............................................................         57,958
                                                                                               -------------
     TOTAL ASSETS............................................................................    269,435,513
                                                                                               -------------
LIABILITIES:
Payable for:
    Shares of beneficial interest repurchased................................................        344,704
    Plan of distribution fee.................................................................        224,685
    Investment management fee................................................................        136,426
    Dividends and distributions to shareholders..............................................        113,379
Accrued expenses and other payables..........................................................        124,246
                                                                                               -------------
     TOTAL LIABILITIES.......................................................................        943,440
                                                                                               -------------
     NET ASSETS..............................................................................  $ 268,492,073
                                                                                               -------------
                                                                                               -------------
COMPOSITION OF NET ASSETS:
Paid-in-capital..............................................................................  $ 417,814,399
Net unrealized depreciation..................................................................    (46,500,476)
Accumulated undistributed net investment income..............................................      7,406,854
Accumulated net realized loss................................................................   (110,228,704)
                                                                                               -------------
     NET ASSETS..............................................................................  $ 268,492,073
                                                                                               -------------
                                                                                               -------------
CLASS A SHARES:
Net Assets...................................................................................       $962,774
Shares Outstanding (UNLIMITED AUTHORIZED, $.01 PAR VALUE)....................................         77,356
     NET ASSET VALUE PER SHARE...............................................................         $12.45
                                                                                               -------------
                                                                                               -------------
 
     MAXIMUM OFFERING PRICE PER SHARE,
       (NET ASSET VALUE PLUS 5.54% OF NET ASSET VALUE).......................................         $13.14
                                                                                               -------------
                                                                                               -------------
CLASS B SHARES:
Net Assets...................................................................................   $263,442,796
Shares Outstanding (UNLIMITED AUTHORIZED, $.01 PAR VALUE)....................................     21,153,152
     NET ASSET VALUE PER SHARE...............................................................         $12.45
                                                                                               -------------
                                                                                               -------------
CLASS C SHARES:
Net Assets...................................................................................     $2,390,061
Shares Outstanding (UNLIMITED AUTHORIZED, $.01 PAR VALUE)....................................        192,321
     NET ASSET VALUE PER SHARE...............................................................         $12.43
                                                                                               -------------
                                                                                               -------------
CLASS D SHARES:
Net Assets...................................................................................     $1,696,442
Shares Outstanding (UNLIMITED AUTHORIZED, $.01 PAR VALUE)....................................        136,321
     NET ASSET VALUE PER SHARE...............................................................         $12.44
                                                                                               -------------
                                                                                               -------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       54
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL STATEMENTS, CONTINUED
 
STATEMENT OF OPERATIONS
FOR THE YEAR ENDED SEPTEMBER 30, 1998
 
<TABLE>
<S>                                                                                             <C>
NET INVESTMENT INCOME:
 
INCOME
Interest......................................................................................  $ 13,785,021
Dividends.....................................................................................     6,422,146
                                                                                                ------------
 
     TOTAL INCOME.............................................................................    20,207,167
                                                                                                ------------
 
EXPENSES
Plan of distribution fee (Class A shares).....................................................         1,753
Plan of distribution fee (Class B shares).....................................................     3,206,390
Plan of distribution fee (Class C shares).....................................................        20,751
Investment management fee.....................................................................     1,953,489
Transfer agent fees and expenses..............................................................       415,843
Registration fees.............................................................................        92,766
Shareholder reports and notices...............................................................        75,540
Professional fees.............................................................................        52,102
Custodian fees................................................................................        30,770
Trustees' fees and expenses...................................................................        18,968
Other.........................................................................................        12,912
                                                                                                ------------
 
     TOTAL EXPENSES...........................................................................     5,881,284
                                                                                                ------------
 
     NET INVESTMENT INCOME....................................................................    14,325,883
                                                                                                ------------
 
NET REALIZED AND UNREALIZED GAIN (LOSS):
Net realized gain.............................................................................     7,466,716
Net change in unrealized appreciation.........................................................   (67,299,872)
                                                                                                ------------
 
     NET LOSS.................................................................................   (59,833,156)
                                                                                                ------------
 
NET DECREASE..................................................................................  $(45,507,273)
                                                                                                ------------
                                                                                                ------------
</TABLE>
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       55
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL STATEMENTS, CONTINUED
 
STATEMENT OF CHANGES IN NET ASSETS
 
<TABLE>
<CAPTION>
                                                                        FOR THE YEAR        FOR THE YEAR
                                                                           ENDED                ENDED
                                                                     SEPTEMBER 30, 1998  SEPTEMBER 30, 1997*
- ------------------------------------------------------------------------------------------------------------
<S>                                                                  <C>                 <C>
 
INCREASE (DECREASE) IN NET ASSETS:
 
OPERATIONS:
Net investment income..............................................  $      14,325,883   $       11,712,892
Net realized gain..................................................          7,466,716           28,783,111
Net change in unrealized appreciation..............................        (67,299,872 )         17,490,793
                                                                     ------------------  -------------------
 
     NET INCREASE (DECREASE).......................................        (45,507,273 )         57,986,796
                                                                     ------------------  -------------------
 
DIVIDENDS TO SHAREHOLDERS FROM NET INVESTMENT INCOME:
Class A shares.....................................................            (35,198 )               (423 )
Class B shares.....................................................        (12,386,068 )        (10,913,049 )
Class C shares.....................................................            (89,566 )             (5,412 )
Class D shares.....................................................           (116,323 )               (216 )
                                                                     ------------------  -------------------
 
     TOTAL DIVIDENDS...............................................        (12,627,155 )        (10,919,100 )
                                                                     ------------------  -------------------
Net increase from transactions in shares of beneficial interest....          9,302,728           35,921,871
                                                                     ------------------  -------------------
 
     NET INCREASE (DECREASE).......................................        (48,831,700 )         82,989,567
 
NET ASSETS:
Beginning of period................................................        317,323,773          234,334,206
                                                                     ------------------  -------------------
 
     END OF PERIOD
    (INCLUDING UNDISTRIBUTED NET INVESTMENT INCOME OF $7,406,854
    AND $5,708,126, RESPECTIVELY)..................................  $     268,492,073   $      317,323,773
                                                                     ------------------  -------------------
                                                                     ------------------  -------------------
</TABLE>
 
- ---------------------
 
 *   Class A, Class C and Class D shares were issued July 28, 1997.
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       56
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998
 
1. ORGANIZATION AND ACCOUNTING POLICIES
 
Morgan Stanley Dean Witter Convertible Securities Trust (the "Fund"), formerly
Dean Witter Convertible Securities Trust, is registered under the Investment
Company Act of 1940, as amended (the "Act"), as a diversified, open-end
management investment company. The Fund's investment objective is to seek a high
level of total return on its assets through a combination of current income and
capital appreciation. The Fund was organized as a Massachusetts business trust
on May 21, 1985 and commenced operations on October 31, 1985. On July 28, 1997,
the Fund commenced offering three additional classes of shares, with the then
current shares designated as Class B shares.
 
The Fund offers Class A shares, Class B shares, Class C shares and Class D
shares. The four classes are substantially the same except that most Class A
shares are subject to a sales charge imposed at the time of purchase and some
Class A shares, and most Class B shares and Class C shares are subject to a
contingent deferred sales charge imposed on shares redeemed within one year, six
years and one year, respectively. Class D shares are not subject to a sales
charge. Additionally, Class A shares, Class B shares and Class C shares incur
distribution expenses.
 
The preparation of financial statements in accordance with generally accepted
accounting principles requires management to make estimates and assumptions that
affect the reported amounts and disclosures. Actual results could differ from
those estimates.
 
The following is a summary of significant accounting policies:
 
A. VALUATION OF INVESTMENTS --  (1) an equity security listed or traded on the
New York, American or other domestic or foreign stock exchange is valued at its
latest sale price on that exchange prior to the time when assets are valued; if
there were no sales that day, the security is valued at the latest bid price (in
cases where a security is traded on more than one exchange, the security is
valued on the exchange designated as the primary market pursuant to procedures
adopted by the Trustees); (2) all other portfolio securities for which
over-the-counter market quotations are readily available are valued at the
latest available bid price prior to the time of valuation; (3) when market
quotations are not readily available, including circumstances under which it is
determined by Morgan Stanley Dean Witter Advisors Inc. (the "Investment
Manager"), formerly Dean Witter InterCapital Inc., that sale and bid prices are
not reflective of a security's market value, portfolio securities are valued at
their fair value as determined in good faith under procedures established by and
under the general supervision of the Trustees (valuation of debt securities for
which market quotations are not readily available may be based upon current
market prices of securities which are comparable in coupon,
 
                                       57
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998, CONTINUED
 
rating and maturity or an appropriate matrix utilizing similar factors); (4)
certain portfolio securities may be valued by an outside pricing service
approved by the Trustees. The pricing service may utilize a matrix system
incorporating security quality, maturity and coupon as the evaluation model
parameters, and/or research and evaluations by its staff, including review of
broker-dealer market price quotations, if available, in determining what it
believes is the fair valuation of the portfolio securities valued by such
pricing service; and (5) short-term debt securities having a maturity date of
more than sixty days at time of purchase are valued on a mark-to-market basis
until sixty days prior to maturity and thereafter at amortized cost based on
their value on the 61st day. Short-term debt securities having a maturity date
of sixty days or less at the time of purchase are valued at amortized cost.
 
B. ACCOUNTING FOR INVESTMENTS --  Security transactions are accounted for on the
trade date (date the order to buy or sell is executed). Realized gains and
losses on security transactions are determined by the identified cost method.
Dividend income and other distributions are recorded on the ex-dividend date.
Discounts are accreted over the life of the respective securities. Interest
income is accrued daily.
 
C. MULTIPLE CLASS ALLOCATIONS --  Investment income, expenses (other than
distribution fees), and realized and unrealized gains and losses are allocated
to each class of shares based upon the relative net asset value on the date such
items are recognized. Distribution fees are charged directly to the respective
class.
 
D. FEDERAL INCOME TAX STATUS --  It is the Fund's policy to comply with the
requirements of the Internal Revenue Code applicable to regulated investment
companies and to distribute all of its taxable income to its shareholders.
Accordingly, no federal income tax provision is required.
 
E. DIVIDENDS AND DISTRIBUTIONS TO SHAREHOLDERS --  The Fund records dividends
and distributions to its shareholders on the record date. The amount of
dividends and distributions from net investment income and net realized capital
gains are determined in accordance with federal income tax regulations which may
differ from generally accepted accounting principles. These "book/tax"
differences are either considered temporary or permanent in nature. To the
extent these differences are permanent in nature, such amounts are reclassified
within the capital accounts based on their federal tax-basis treatment;
temporary differences do not require reclassification. Dividends and
distributions which exceed net investment income and net realized capital gains
for financial reporting purposes but not for tax purposes are reported as
dividends in excess of net investment
 
                                       58
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998, CONTINUED
 
income or distributions in excess of net realized capital gains. To the extent
they exceed net investment income and net realized capital gains for tax
purposes, they are reported as distributions of paid-in-capital.
 
2. INVESTMENT MANAGEMENT AGREEMENT
 
Pursuant to an Investment Management Agreement with the Investment Manager, the
Fund pays the Investment Manager a management fee, accrued daily and payable
monthly, by applying the following annual rates to the Fund's net assets
determined as of the close of each business day: 0.60% to the portion of the
daily net assets not exceeding $750 million; 0.55% to the portion of the daily
net assets exceeding $750 million but not exceeding $1 billion; 0.50% to the
portion of daily net assets exceeding $1 billion but not exceeding $1.5 billion;
0.475% to the portion of daily net assets exceeding $1.5 billion but not
exceeding $2 billion; 0.45% to the portion of daily net assets exceeding $2
billion but not exceeding $3 billion; and 0.425% to the portion of daily net
assets exceeding $3 billion.
 
Under the terms of the Agreement, in addition to managing the Fund's
investments, the Investment Manager maintains certain of the Fund's books and
records and furnishes, at its own expense, office space, facilities, equipment,
clerical, bookkeeping and certain legal services and pays the salaries of all
personnel, including officers of the Fund who are employees of the Investment
Manager. The Investment Manager also bears the cost of telephone services, heat,
light, power and other utilities provided to the Fund.
 
3. PLAN OF DISTRIBUTION
 
Shares of the Fund are distributed by Morgan Stanley Dean Witter Distributors
Inc. (the "Distributor"), an affiliate of the Investment Manager. The Fund has
adopted a Plan of Distribution (the "Plan") pursuant to Rule 12b-1 under the
Act. The Plan provides that the Fund will pay the Distributor a fee which is
accrued daily and paid monthly at the following annual rates: (i) Class A -- up
to 0.25% of the average daily net assets of Class A; (ii) Class B -- 1.0% of the
lesser of: (a) the average daily aggregate gross sales of the Class B shares
since the inception of the Fund (not including reinvestment of dividend or
capital gain distributions) less the average daily aggregate net asset value of
the Class B shares redeemed since the Fund's inception upon which a contingent
deferred sales charge has been imposed or waived; or (b) the average daily net
assets of Class B; and (iii) Class C -- up to 1.0% of the average daily net
assets of Class C. In the case of Class A shares, amounts paid under the Plan
are paid to the Distributor for services provided. In the
 
                                       59
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998, CONTINUED
 
case of Class B and Class C shares, amounts paid under the Plan are paid to the
Distributor for (1) services provided and the expenses borne by it and others in
the distribution of the shares of these Classes, including the payment of
commissions for sales of these Classes and incentive compensation to, and
expenses of, Morgan Stanley Dean Witter Financial Advisors and others who engage
in or support distribution of the shares or who service shareholder accounts,
including overhead and telephone expenses; (2) printing and distribution of
prospectuses and reports used in connection with the offering of these shares to
other than current shareholders; and (3) preparation, printing and distribution
of sales literature and advertising materials. In addition, the Distributor may
utilize fees paid pursuant to the Plan, in the case of Class B shares, to
compensate Dean Witter Reynolds Inc. ("DWR"), an affiliate of the Investment
Manager and Distributor, and other selected broker-dealers for their opportunity
costs in advancing such amounts, which compensation would be in the form of a
carrying charge on any unreimbursed expenses.
 
In the case of Class B shares, provided that the Plan continues in effect, any
cumulative expenses incurred by the Distributor but not yet recovered may be
recovered through the payment of future distribution fees from the Fund pursuant
to the Plan and contingent deferred sales charges paid by investors upon
redemption of Class B shares. Although there is no legal obligation for the Fund
to pay expenses incurred in excess of payments made to the Distributor under the
Plan and the proceeds of contingent deferred sales charges paid by investors
upon redemption of shares, if for any reason the Plan is terminated, the
Trustees will consider at that time the manner in which to treat such expenses.
The Distributor has advised the Fund that such excess amounts, including
carrying charges, totaled $80,592,295 at September 30, 1998.
 
In the case of Class A shares and Class C shares, expenses incurred pursuant to
the Plan in any calendar year in excess of 0.25% or 1.0% of the average daily
net assets of Class A or Class C, respectively, will not be reimbursed by the
Fund through payments in any subsequent year, except that expenses representing
a gross sales credit to Morgan Stanley Dean Witter Financial Advisors or other
selected broker-dealer representatives may be reimbursed in the subsequent
calendar year. For the year ended September 30, 1998, the distribution fee was
accrued for Class A shares and Class C shares at the annual rate of 0.24% and
1.0%, respectively.
 
The Distributor has informed the Fund that for the year ended September 30,
1998, it received contingent deferred sales charges from certain redemptions of
the Fund's Class B shares and Class C
 
                                       60
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998, CONTINUED
 
shares of $379,822 and $2,747, respectively and received $27,570 in front-end
sales charges from sales of the Fund's Class A shares. The respective
shareholders pay such charges which are not an expense of the Fund.
 
4. SECURITY TRANSACTIONS AND TRANSACTIONS WITH AFFILIATES
 
The cost of purchases and proceeds from sales of portfolio securities, excluding
short-term investments, for the year ended September 30, 1998 aggregated
$309,447,939 and $276,502,691, respectively.
 
For the year ended September 30, 1998, the Fund incurred brokerage commissions
of $1,750 with DWR for portfolio transactions executed on behalf of the Fund.
 
For the year ended September 30, 1998, the Fund incurred brokerage commissions
of $2,500 with Morgan Stanley & Co., Inc., an affiliate of the Investment
Manager and Distributor, for portfolio transactions executed on behalf of the
Fund.
 
Morgan Stanley Dean Witter Trust FSB, an affiliate of the Investment Manager and
Distributor, is the Fund's transfer agent. At September 30, 1998, the Fund had
transfer agent fees and expenses payable of approximately $5,000.
 
The Fund has an unfunded noncontributory defined benefit pension plan covering
all independent Trustees of the Fund who will have served as independent
Trustees for at least five years at the time of retirement. Benefits under this
plan are based on years of service and compensation during the last five years
of service. Aggregate pension costs for the year ended September 30, 1998
included in Trustees' fees and expenses in the Statement of Operations amounted
to $5,407. At September 30, 1998, the Fund had an accrued pension liability of
$50,640 which is included in accrued expenses in the Statement of Assets and
Liabilities.
 
5. FEDERAL INCOME TAX STATUS
 
During the year ended September 30, 1998, the Fund utilized approximately
$8,751,000 of its net capital loss carryover. At September 30, 1998, the Fund
had a net capital loss carryover of approximately $108,866,000 of which
$46,135,000 will be available through September 30, 1999 and $62,731,000 will be
available through September 30, 2000 to offset future capital gains to the
extent provided by regulations.
 
                                       61
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
NOTES TO FINANCIAL STATEMENTS SEPTEMBER 30, 1998, CONTINUED
 
Capital losses incurred after October 31 ("post-October" losses) within the
taxable year are deemed to arise on the first business day of the Fund's next
taxable year. The Fund incurred and will elect to defer net capital losses of
approximately $1,307,000 during fiscal 1998.
 
As of September 30, 1998, the Fund had temporary book/tax differences primarily
attributable to post-October losses and permanent book/tax differences primarily
attributable to an expired capital loss carryover. To reflect reclassifications
arising from the permanent differences, paid-in-capital was charged and
accumulated net realized loss was credited $27,580,247.
 
6. SHARES OF BENEFICIAL INTEREST
 
Transactions in shares of beneficial interest were as follows:
 
<TABLE>
<CAPTION>
                                                                           FOR THE YEAR                  FOR THE YEAR
                                                                              ENDED                         ENDED
                                                                        SEPTEMBER 30, 1998           SEPTEMBER 30, 1997*
                                                                   ----------------------------   --------------------------
                                                                     SHARES          AMOUNT         SHARES         AMOUNT
                                                                   -----------   --------------   -----------   ------------
<S>                                                                <C>           <C>              <C>           <C>
CLASS A SHARES
Sold.............................................................       92,001   $    1,349,947         3,315   $     48,759
Reinvestment of dividends........................................        2,116           29,640            14            215
Redeemed.........................................................      (20,090)        (277,904)      --             --
                                                                   -----------   --------------   -----------   ------------
Net increase -- Class A..........................................       74,027        1,101,683         3,329         48,974
                                                                   -----------   --------------   -----------   ------------
 
CLASS B SHARES
Sold.............................................................    4,637,313       68,850,146     8,039,107    107,873,505
Reinvestment of dividends........................................      701,336        9,971,800       650,270      8,859,176
Redeemed.........................................................   (5,189,588)     (75,033,986)   (6,107,632)   (81,482,825)
                                                                   -----------   --------------   -----------   ------------
Net increase -- Class B..........................................      149,061        3,787,960     2,581,745     35,249,856
                                                                   -----------   --------------   -----------   ------------
 
CLASS C SHARES
Sold.............................................................      213,545        3,143,678        41,818        612,076
Reinvestment of dividends........................................        4,574           64,248           303          4,520
Redeemed.........................................................      (66,982)        (917,694)         (937)       (13,788)
                                                                   -----------   --------------   -----------   ------------
Net increase -- Class C..........................................      151,137        2,290,232        41,184        602,808
                                                                   -----------   --------------   -----------   ------------
 
CLASS D SHARES
Sold.............................................................      201,768        2,963,943         1,366         20,017
Reinvestment of dividends........................................          103            1,480            14            216
Redeemed.........................................................      (66,930)        (842,570)      --             --
                                                                   -----------   --------------   -----------   ------------
Net increase -- Class D..........................................      134,941        2,122,853         1,380         20,233
                                                                   -----------   --------------   -----------   ------------
Net increase in Fund.............................................      509,166   $    9,302,728     2,627,638   $ 35,921,871
                                                                   -----------   --------------   -----------   ------------
                                                                   -----------   --------------   -----------   ------------
</TABLE>
 
- ---------------------
 
 *   For Class A, C and D, for the period July 28, 1997 (issue date) through
     September 30, 1997.
 
                                       62
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL HIGHLIGHTS
 
Selected ratios and per share data for a share of beneficial interest
outstanding throughout each period:
 
<TABLE>
<CAPTION>
                                                     FOR THE YEAR ENDED SEPTEMBER 30
                  ------------------------------------------------------------------------------------------------------
                   1998++    1997*++     1996      1995       1994      1993      1992       1991      1990      1989
- ------------------------------------------------------------------------------------------------------------------------
 
<S>               <C>        <C>       <C>       <C>        <C>       <C>       <C>        <C>       <C>       <C>
CLASS B SHARES
 
PER SHARE
OPERATING
PERFORMANCE:
 
Net asset value,
 beginning of
 period.......... $   15.07  $  12.72  $  11.67  $   10.75  $  10.62  $   8.92  $    8.67  $   7.65  $   9.68  $    8.63
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
 
Net investment
 income..........      0.65      0.60      0.55       0.60      0.42      0.37       0.34      0.37      0.46       0.48
 
Net realized and
 unrealized gain
 (loss)..........     (2.70)     2.31      1.12       0.82      0.11      1.67       0.15      1.05     (2.06)      1.20
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
 
Total from
 investment
 operations......     (2.05)     2.91      1.67       1.42      0.53      2.04       0.49      1.42     (1.60)      1.68
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
 
Less dividends
 from net
 investment
 income..........     (0.57)    (0.56)    (0.62)     (0.50)    (0.40)    (0.34)     (0.24)    (0.40)    (0.43)     (0.63)
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
 
Net asset value,
 end of period... $   12.45  $  15.07  $  12.72  $   11.67  $  10.75  $  10.62  $    8.92  $   8.67  $   7.65  $    9.68
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
                  ---------  --------  --------  ---------  --------  --------  ---------  --------  --------  ---------
 
TOTAL INVESTMENT
RETURN+..........    (14.01)%    23.38%    14.70%     13.68%     5.02%    23.22%      5.69%    18.93%   (16.93)%     20.20%
 
RATIOS TO AVERAGE
NET ASSETS:
Expenses.........      1.81%(1)     1.84%     1.89%      1.96%     1.93%     1.93%      1.92%     1.92%     1.88%      1.76%
 
Net investment
 income..........      4.40%(1)     4.45%     4.78%      5.24%     3.68%     3.44%      3.43%     4.34%     4.96%      4.93%
 
SUPPLEMENTAL DATA:
Net assets, end
 of period, in
 millions........      $263      $317      $234       $185      $190      $208       $218      $297      $413       $822
 
Portfolio
 turnover rate...        95%      182%      171%       138%      184%      221%       145%      133%       92%       167%
</TABLE>
 
- ---------------------
 
 *   Prior to July 28, 1997, the Fund issued one class of shares. All shares of
     the Fund held prior to that date have been designated Class B shares.
++   The per share amounts were computed using an average number of shares
     outstanding during the period.
 +   Does not reflect the deduction of sales charge. Calculated based on the net
     asset value as of the last business day of the period.
(1)  Reflects overall Fund ratios for investment income and non-class specific
     expenses.
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       63
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL HIGHLIGHTS, CONTINUED
 
<TABLE>
<CAPTION>
<S>                                                                     <C>                <C>
                                                                                            FOR THE PERIOD
                                                                          FOR THE YEAR      JULY 28, 1997*
                                                                             ENDED             THROUGH
                                                                         SEPTEMBER 30,      SEPTEMBER 30,
                                                                             1998++             1997++
- -----------------------------------------------------------------------------------------------------------
CLASS A SHARES
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................      $ 15.07            $ 14.31
                                                                             ------             ------
Net investment income.................................................         0.75               0.13
Net realized and unrealized gain (loss)...............................        (2.68)              0.78
                                                                             ------             ------
Total from investment operations......................................        (1.93)              0.91
                                                                             ------             ------
Less dividends from net investment income.............................        (0.69)             (0.15)
                                                                             ------             ------
Net asset value, end of period........................................      $ 12.45            $ 15.07
                                                                             ------             ------
                                                                             ------             ------
TOTAL INVESTMENT RETURN+..............................................       (13.38)%             6.40%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         1.05%(3)           1.15%(2)
Net investment income.................................................         5.16%(3)           5.03%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................         $963                $50
Portfolio turnover rate...............................................           95%               182%
</TABLE>
 
<TABLE>
<S>                                                                     <C>                <C>
CLASS C SHARES
PER SHARE OPERATING PERFORMANCE:
Net asset value, beginning of period..................................      $ 15.06            $ 14.31
                                                                             ------             ------
Net investment income.................................................         0.64               0.12
Net realized and unrealized gain (loss)...............................        (2.68)              0.77
                                                                             ------             ------
Total from investment operations......................................        (2.04)              0.89
                                                                             ------             ------
Less dividends from net investment income.............................        (0.59)             (0.14)
                                                                             ------             ------
Net asset value, end of period........................................      $ 12.43            $ 15.06
                                                                             ------             ------
                                                                             ------             ------
TOTAL INVESTMENT RETURN+..............................................       (14.07)%             6.26%(1)
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         1.81%(3)           1.92%(2)
Net investment income.................................................         4.40%(3)           4.52%(2)
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................       $2,390               $620
Portfolio turnover rate...............................................           95%               182%
</TABLE>
 
- ---------------------
 
 *   The date shares were first issued.
++   The per share amounts were computed using an average number of shares
     outstanding during the period.
 +   Does not reflect the deduction of sales charge. Calculated based on the net
     asset value as of the last business day of the period.
(1)  Not annualized.
(2)  Annualized.
(3)  Reflects overall Fund ratios for investment income and non-class specific
     expenses.
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       64
<PAGE>
MORGAN STANLEY DEAN WITTER CONVERTIBLE SECURITIES TRUST
FINANCIAL HIGHLIGHTS, CONTINUED
 
<TABLE>
<CAPTION>
<S>                                                                     <C>                <C>
                                                                                            FOR THE PERIOD
                                                                          FOR THE YEAR      JULY 28, 1997*
                                                                             ENDED             THROUGH
                                                                         SEPTEMBER 30,      SEPTEMBER 30,
                                                                             1998++             1997++
- -----------------------------------------------------------------------------------------------------------
 
CLASS D SHARES
 
PER SHARE OPERATING PERFORMANCE:
 
Net asset value, beginning of period..................................      $ 15.08            $ 14.31
                                                                             ------             ------
 
Net investment income.................................................         0.79               0.13
 
Net realized and unrealized gain (loss)...............................        (2.71)              0.80
                                                                             ------             ------
 
Total from investment operations......................................        (1.92)              0.93
                                                                             ------             ------
 
Less dividends from net investment income.............................        (0.72)             (0.16)
                                                                             ------             ------
 
Net asset value, end of period........................................      $ 12.44            $ 15.08
                                                                             ------             ------
                                                                             ------             ------
 
TOTAL INVESTMENT RETURN+..............................................       (13.19)%             6.42%(1)
 
RATIOS TO AVERAGE NET ASSETS:
Expenses..............................................................         0.81%(3)           0.89%(2)
 
Net investment income.................................................         5.40%(3)           4.94%(2)
 
SUPPLEMENTAL DATA:
Net assets, end of period, in thousands...............................       $1,696                $21
 
Portfolio turnover rate...............................................           95%               182%
</TABLE>
 
- ---------------------
 
 *   The date shares were first issued.
++   The per share amounts were computed using an average number of shares
     outstanding during the period.
 +   Calculated based on the net asset value as of the last business day of the
     period.
(1)  Not annualized.
(2)  Annualized.
(3)  Reflects overall Fund ratios for investment income and non-class specific
     expenses.
 
                       SEE NOTES TO FINANCIAL STATEMENTS
                                       65


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