DREYFUS INSTITUTIONAL MONEY MARKET FUND INC
N-30D, 2000-02-22
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Dreyfus

Institutional

Money Market Fund

ANNUAL REPORT December 31, 1999

(reg.tm)



The  views  expressed herein are current to the date of this report. These views
and  the  composition  of the fund's portfolio are subject to change at any time
based on market and other conditions.

   * Not FDIC-Insured
   * Not Bank-Guaranteed
   * May Lose Value

Year 2000 Issues (Unaudited)

The fund could be adversely affected if the computer systems used by Dreyfus and
the fund's other service providers do not properly process and calculate
date-related information from and after January 1, 2000. Dreyfus has taken steps
designed to avoid year 2000-related problems in its systems and to monitor the
readiness  of other service providers.  In addition, issuers of securities in
which the fund invests may be adversely affected by year 2000-related problems.
This could have an impact on the value of the fund's investments and its share
price.


                                 Contents

                                 THE FUND
- --------------------------------------------------

                             2   Letter from the President

                             3   Discussion of Fund Performance

                             6   Statement of Investments

                            11   Statement of Assets and Liabilities

                            12   Statement of Operations

                            13   Statement of Changes in Net Assets

                            14   Financial Highlights

                            16   Notes to Financial Statements

                            20   Report of Independent Auditors

                            21   Important Tax Information

                                 FOR MORE INFORMATION
- ---------------------------------------------------------------------------

                                 Back Cover

                                                                       The Fund

                                                          Dreyfus Institutional

                                                              Money Market Fund

LETTER FROM THE PRESIDENT

Dear Shareholder:

We  are  pleased  to  present this annual report for Dreyfus Institutional Money
Market  Fund, covering the 12-month period from January 1, 1999 through December
31,  1999.  Inside,  you' ll  find  valuable  information about how the fund was
managed  during  the  reporting  period,  including  a discussion with portfolio
manager Thomas S. Riordan.

When  the  reporting period began, investors were concerned that global economic
weakness  might  cause  a  slowdown in the U.S. economy. As it turned out, these
fears  were  unfounded.  In  fact,  it  became  apparent  early in the year that
international and domestic economies were growing faster than analysts expected,
giving   rise   to  concerns  that  long-dormant  inflationary  pressures  might
re-emerge. Consumers continued to spend heavily, unemployment levels reached new
lows  and  the  stock  market continued to climb. Because unsustainable economic
growth  may  trigger  unwanted inflationary pressures, the Federal Reserve Board
raised key short-term interest rates three times between June 30 and year-end in
an  attempt  to  forestall  an  acceleration  of inflation. In this environment,
yields on money market securities continued to rise.

We  appreciate  your  confidence over the past year, and we look forward to your
continued participation in Dreyfus Institutional Money Market Fund.

Sincerely,


Stephen E. Canter

President and Chief Investment Officer

The Dreyfus Corporation

January 14, 2000




DISCUSSION OF FUND PERFORMANCE

Thomas S. Riordan, Portfolio Manager

How did Dreyfus Institutional Money Market Fund  perform during the period?

For  the  12-month  period ended December 31, 1999, the fund produced a yield of
4.67%  for  its  Money  Market  Series  and  4.26% for its Government Securities
Series.  Taking  into account the effect of compounding, the effective yield was
4.77%  for  its  Money  Market  Series  and  4.34% for its Government Securities
Series.(1)

What factors influenced the fund's performance?

As  1999  began  the  United  States  economy was marked by fears of an economic
slowdown, largely as a result of the Asian financial crisis. The Federal Reserve
Board,  in  an attempt to cushion the economy from negative overseas events, had
sharply  lowered short-term interest rates. Global markets remained unsettled as
1999  began,  but  the  general atmosphere of crisis lifted. As fears waned, the
focus  of monetary policymakers shifted back to the domestic economy, and as the
economy  showed  no  signs  of  slowing,  the  Fed  began  to voice concern over
inflationary pressures.

The  performance  of  the  U.S.  economy  in  the first quarter of 1999 was much
stronger  than  expected.  But  while the Gross Domestic Product (GDP) grew at a
rate of 4.3%, inflation remained benign. Despite a tight labor market, there was
no  evidence  of  advancing  wage pressure. Many economic analysts believed that
advances  in  technology  might  make it possible for the economy to grow faster
than  previously  thought  possible  without igniting inflation. Notwithstanding
fears  that imbalances might eventually derail the nine-year expansion, the U.S.
economy  continued  to  grow  as  the Fed held steady on rates through the first
quarter of the year.

A surprisingly large jump in the Consumer Price Index in May pushed policymakers
closer to a rate hike.  Although the Fed did not
                                                             The   Fund


DISCUSSION OF FUND PERFORMANCE (CONTINUED)

immediately  raise interest rates, it did announce a significant shift, adopting
a  "bias"  towards  tightening  --  that is, raising short-term rates. With that
shift in bias came a resulting shift in market psychology, as participants began
to    anticipate    higher    rates.

That  highly  anticipated  move  came in June, when at its Open Market Committee
meeting,  the Fed raised short-term rates by 0.25 percentage points. At the same
time,  however,  it  announced  that it was shifting its bias back to neutral --
indicating  no intention of an immediate further rate increase. The market hoped
that this pre-emptive strike to head off the threat of inflation would signal an
end    to    Fed    tightening.

Such  hopes  would  be  short-lived as strong economic growth along with anxiety
over  rising wages and benefits renewed inflationary concern. At its August Open
Market  Committee  meeting,  the  Fed  raised  short-term  interest  rates by an
additional  0.25  percentage points, signaling its added resolve by also raising
the    discount    rate.

The  economy  continued to give mixed signals to the money market throughout the
third  quarter.  GDP growth accelerated back to a rapid 4.8%, but key indicators
of  employment costs, job creation and inflation were at lower levels than would
be  expected,  given such strong economic expansion. These mixed indications led
the Fed to hold off on further tightening until November, when continued fear of
inflationary  pressures  caused  it  to  increase  short-term  interest rates by
another 0.25 percentage points.

When the Fed took no action at its December meeting, many analysts considered it
to  be  an  attempt  to  quiet  markets  that were concerned about potential Y2K
disruption.  The  Fed also added significant reserves to the banking system over
year-end  to  quell  liquidity concerns, leading to temporary irregularities and
wide  fluctuations  in  short-term interest rates. Despite the temporary drop in
rates over year-end due to the added reserves, many believe, now that Y2K issues
have  been successfully navigated, that the issue of managing sustainable growth
should    take    center    stage    again.


What is the fund's current strategy?

In  response  to  a market environment marked by rising interest rates, the fund
took on a somewhat defensive strategy. We took two steps to position the fund in
the  current  market:  we  shortened  the  fund' s  average maturity and built a
liquidity cushion. Shorter maturities and a higher level of cash are designed to
enable the fund to take advantage of a possible further rise in interest rates.

January 14, 2000

(1)  EFFECTIVE YIELD IS BASED UPON DIVIDENDS DECLARED DAILY AND REINVESTED
MONTHLY. PAST PERFORMANCE IS NO GUARANTEE OF FUTURE RESULTS. YIELDS FLUCTUATE.
AN INVESTMENT IN THE FUND IS NOT INSURED OR GUARANTEED BY THE FDIC OR ANY OTHER
GOVERNMENT AGENCY. ALTHOUGH THE FUND SEEKS TO PRESERVE THE VALUE OF YOUR
INVESTMENT AT $1.00 PER SHARE, IT IS POSSIBLE TO LOSE MONEY BY INVESTING IN THE
FUND.

                                                             The Fund

STATEMENT OF INVESTMENTS

MONEY MARKET SERIES

December 31, 1999

<TABLE>
<CAPTION>


                                                                                              Principal
NEGOTIABLE BANK CERTIFICATES OF DEPOSITS--11.0%                                              Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Branch Bank & Trust Co.

   <S>                                                                                       <C>                      <C>
   5.04%, 1/10/2000                                                                          18,000,000               17,999,872

First National Bank of Maryland

   5.14%, 2/23/2000                                                                          26,000,000               25,998,727

Union Bank of California, N.A.

   5.98%, 8/1/2000                                                                           21,000,000               21,000,000

TOTAL NEGOTIABLE BANK CERTIFICATES OF DEPOSIT

   (cost $64,998,599)                                                                                                 64,998,599
- ------------------------------------------------------------------------------------------------------------------------------------

COMMERCIAL PAPER--45.4%
- ------------------------------------------------------------------------------------------------------------------------------------

Associates Corp. of North America

   4.00%, 1/3/2000                                                                           25,000,000               24,994,444

Associates First Capital Corp.

   4.00%, 1/3/2000                                                                           25,000,000               24,994,444

Atlantis One Funding Corp.

   5.92%-5.96%, 2/22/2000-3/24/2000                                                          25,000,000               24,766,567

BCI Funding Corp.

   6.05%, 5/2/2000                                                                           21,000,000               20,582,252

Commonwealth Bank of Australia

   6.00%, 4/6/2000                                                                           25,000,000               24,608,000

DaimlerChrysler North America Holding Corp.

   6.10%, 2/29/2000                                                                          25,000,000               24,755,806

Donaldson, Lufkin & Jenrette Securities Corp.

   6.22%, 3/3/2000                                                                           15,000,000               14,841,771

General Electric Capital  Corp.

   5.93%, 6/16/2000                                                                          20,000,000               19,468,383

General Electric Capital Services Inc.

   5.93%, 3/21/2000                                                                          10,000,000                9,870,889

Goldman Sachs Group Inc.

   6.07%, 2/29/2000                                                                          20,500,000               20,300,768

Heller Financial Inc.

   6.01%, 2/25/2000                                                                          15,000,000               14,864,333

Lehman Brothers Holdings Inc.

   6.20%, 3/8/2000                                                                           20,500,000               20,270,702

UBS Finance (DE) Inc.

   4.25%, 1/3/2000                                                                           25,000,000               24,994,097

TOTAL COMMERCIAL PAPER

   (cost $269,312,456)                                                                                               269,312,456


                                                                                              Principal
CORPORATE NOTES--19.9%                                                                       Amount ($)                 Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Bear, Stearns & Co. Inc.

   6.52%, 2/22/2000                                                                          20,000,000  (a)          20,001,551

Donaldson, Lufkin & Jenrette Securities Corp.

   5.96%, 5/26/2000                                                                           3,045,000                3,048,141

Ford Motor Credit Corp.

   5.70%, 5/5/2000                                                                           20,000,000  (a)          20,000,000

GTE Corp.

   6.22%, 6/12/2000                                                                          25,000,000  (a)          24,992,833

General Motors Acceptance Corp.

   5.96%-6.28%, 2/22/2000-8/7/2000                                                           19,800,000               19,824,368

Heller Financial Inc.

   6.17%, 4/13/2000                                                                          10,000,000  (a)          10,006,716

Paine Webber Group Inc.

   6.66%, 4/20/2000-9/15/2000                                                                20,000,000  (a)          20,000,000

TOTAL CORPORATE NOTES

   (cost $117,873,609)                                                                                               117,873,609
- ------------------------------------------------------------------------------------------------------------------------------------

SHORT-TERM BANK NOTES--6.7%
- ------------------------------------------------------------------------------------------------------------------------------------

First Tennessee Bank

   5.68%, 4/19/2000                                                                          20,000,000  (a)          19,997,665

Old Kent Bank & Trust Co.

   5.71%, 6/2/2000                                                                           20,000,000  (a)          19,995,163

TOTAL SHORT-TERM BANK NOTES

   (cost $39,992,828)                                                                                                 39,992,828
- ------------------------------------------------------------------------------------------------------------------------------------

U.S. GOVERNMENT AGENCIES--3.3%
- ------------------------------------------------------------------------------------------------------------------------------------

Federal Home Loan Banks

  5.82%, 5/5/2000-5/11/2000

   (cost $19,339,812)                                                                        19,740,000  (a)          19,339,812
- ------------------------------------------------------------------------------------------------------------------------------------

REPURCHASE AGREEMENTS--3.4%
- ------------------------------------------------------------------------------------------------------------------------------------

Barclays Capital Inc.

  dated 12/31/1999, 2.00% due 1/3/2000

  in the amount of $20,003,333

  (fully collateralized by U.S. Treasury Bills,

  due 7/31/2000, value $20,301,240)

   (cost $20,000,000)                                                                        20,000,000               20,000,000

                                                                                                     The Fund

STATEMENT OF INVESTMENTS

MONEY MARKET SERIES (CONTINUED)

                                                                                               Principal
TIME DEPOSITS--9.7%                                                                           Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Banc One N.A. (Grand Cayman)

   4.42%, 1/3/2000                                                                           25,000,000               25,000,000

Chase Manhattan Bank NA (London)

   4.00%, 1/3/2000                                                                           25,000,000               25,000,000

Republic National Bank of New York (London)

   4.50%, 1/3/2000                                                                            7,563,000                7,563,000

TOTAL TIME DEPOSITS

   (cost $57,563,000)                                                                                                 57,563,000
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL INVESTMENT

   (cost $589,080,304)                                                                            99.4%              589,080,304

CASH AND RECEIVABLES (NET)                                                                          .6%                3,365,359

NET ASSETS                                                                                       100.0%              592,445,663

(A) VARIABLE INTEREST RATE-SUBJECT TO PERIODIC CHANGE.

SEE NOTES TO FINANCIAL STATEMENTS.



STATEMENT OF INVESTMENTS

GOVERNMENT SECURITIES SERIES

December 31, 1999

                                                             Annualized
                                                              Yield on
                                                               Date of                       Principal
U.S. TREASURY BILLS--42.1%                                 Purchase (%)                     Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

1/13/2000

   (cost $34,939,442)                                              5.21                     35,000,000               34,939,442
- ------------------------------------------------------------------------------------------------------------------------------------

U.S. TREASURY NOTES--24.2%
- ------------------------------------------------------------------------------------------------------------------------------------

5.875%, 2/15/2000                                                  4.69                      8,000,000                8,010,066

5.50%, 3/31/2000                                                   4.66                      5,000,000                5,007,472

5.50%, 5/31/2000                                                   4.88                      4,000,000                4,006,474

5.875%, 6/30/2000                                                  5.23                      3,000,000                3,007,418

TOTAL U.S. TREASURY NOTES

   (cost $20,031,430)                                                                                                20,031,430
- ------------------------------------------------------------------------------------------------------------------------------------

REPURCHASE AGREEMENTS--33.3%
- ------------------------------------------------------------------------------------------------------------------------------------

Barclays Capital, Inc.

  dated 12/31/1999, due 1/3/2000

  in the amount of $3,567,446

  (fully collateralized by $3,605,000

  U.S. Treasury Notes 5.75%, due

   11/15/2000, value $3,622,603)                                   1.50                      3,567,000                 3,567,000

Bear, Stearns & Co. Inc.

  dated 12/31/1999, due 1/3/2000

  in the amount of $6,001,250

  (fully collateralized by $6,205,000

  U.S. Treasury Strips, due

   5/15/2000, value $6,076,557)                                    2.50                      6,000,000                 6,000,000

Donaldson, Lufkin & Jenrette Securities Corp.

  dated 12/31/1999, due 1/3/2000

  in the amount of $6,001,000

  (fully collateralized by $5,957,000

  U.S. Treasury Notes 5.375%, due

   7/31/2000, value $6,075,812)                                    2.00                      6,000,000                  6,000,000

Morgan Stanley Dean Witter & Co.

  dated 12/31/1999, due 1/3/2000

  in the amount of $6,001,235

  (fully collateralized by $6,145,000

  U.S. Treasury Notes 4.625%, due

   11/30/2000, value $6,090,310)                                   2.47                       6,000,000                  6,000,000

                                                                                                     The Fund

STATEMENT OF INVESTMENTS

GOVERNMENT SECURITIES SERIES (CONTINUED)

                                                             Annualized
                                                              Yield on
                                                               Date of                       Principal
REPURCHASE AGREEMENTS (CONTINUED)                          Purchase (%)                     Amount ($)                Value ($)
- ------------------------------------------------------------------------------------------------------------------------------------

Warburg Dillon Read, Inc.

  dated 12/31/1999, due 1/3/2000

  in the amount of $6,001,250

  (fully collateralized by $6,436,000

  U.S.Treasury Bills, due

   11/9/2000, value $6,120,636)                                    2.50                      6,000,000                6,000,000

TOTAL REPURCHASE AGREEMENTS

   (cost $27,567,000)                                                                                                27,567,000
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL INVESTMENTS

   (cost $82,537,872)                                             99.6%                                              82,537,872

CASH AND RECEIVABLES (NET)                                          .4%                                                 356,554

NET ASSETS                                                       100.0%                                              82,894,426

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>



STATEMENT OF ASSETS AND LIABILITIES

December 31, 1999
<TABLE>
<CAPTION>


                                                                                                      Money               Government
                                                                                                     Market               Securities
                                                                                                     Series                  Series
- ------------------------------------------------------------------------------------------------------------------------------------

ASSETS ($):
<S>                                                                                              <C>                     <C>
Investments in securities, at value (including repurchase
   agreements of $20,000,000 and $27,567,000
   for the Money Market Series and the Government
   Securities Series, respectively)--Note 2(b)                                                   589,080,304             82,537,872


Cash                                                                                                  69,795                131,583

Interest Receivable                                                                                3,603,554                268,845

Prepaid expenses                                                                                       7,843                  6,244

                                                                                                 592,761,496             82,944,544
- ------------------------------------------------------------------------------------------------------------------------------------

LIABILITIES ($):

Due to The Dreyfus Corporation and affiliates                                                        247,710                 14,187

Accrued expenses                                                                                      68,123                 35,931

                                                                                                     315,833                 50,118
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSETS ($)                                                                                   592,445,663             82,894,426
- ------------------------------------------------------------------------------------------------------------------------------------

COMPOSITION OF NET ASSETS ($):

Paid-in capital                                                                                  592,483,199             82,912,170

Accumulated net realized gain (loss) on investments                                                  (37,536)               (17,744)
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSETS ($)                                                                                   592,445,663             82,894,426
- ------------------------------------------------------------------------------------------------------------------------------------

SHARES OUTSTANDING

(unlimited number of $.001 par value shares of
   Beneficial Interest authorized)                                                               592,483,199             82,912,170
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSET VALUE, offering and redemption price per share ($)                                            1.00                   1.00

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>


                                                             The Fund

STATEMENT OF OPERATIONS

Year Ended December 31, 1999

                                                            Money     Government
                                                           Market     Securities
                                                           Series        Series
- --------------------------------------------------------------------------------

INVESTMENT INCOME ($):

INTEREST INCOME                                        29,482,363     3,919,354

EXPENSES--NOTE 2(C):

Management fee--Note 3(a)                               2,817,113       396,676

Shareholder servicing costs--Note 3(b)                    153,110        45,619

Custodian fees                                             70,439        33,865

Trustees' fees and expenses--Note 3(c)                     60,196         9,539

Registration fees                                          29,630        19,859

Professional fees                                          24,346        33,555

Prospectus and shareholders' reports                        6,942           786

Miscellaneous                                               3,150           528

TOTAL EXPENSES                                          3,164,926       540,427

INVESTMENT INCOME--NET                                 26,317,437     3,378,927

NET REALIZED GAIN (LOSS) ON INVESTMENTS--NOTE 2(B) ($):    (3,181)      (17,744)

NET INCREASE IN NET ASSETS RESULTING FROM OPERATIONS   26,314,256     3,361,183

SEE NOTES TO FINANCIAL STATEMENTS.


<TABLE>
<CAPTION>

STATEMENT OF CHANGES IN NET ASSETS

                                                              Money Market Series                    Government Securities Series
                                                  ----------------------------------------------------------------------------------

                                                            Year Ended December 31,                      Year Ended December 31,
                                                  ----------------------------------------------------------------------------------

                                                        1999                     1998                   1999                 1998
- ------------------------------------------------------------------------------------------------------------------------------------

OPERATIONS ($):

<S>                                               <C>                      <C>                     <C>                   <C>
Investment income--net                            26,317,437               27,102,277              3,378,927             3,992,120

Net realized gain (loss)
   on investments                                     (3,181)                 (27,774)               (17,744)               10,367

NET INCREASE (DECREASE)
   IN NET ASSETS RESULTING
   FROM OPERATIONS                                26,314,256               27,074,503              3,361,183             4,002,487
- ------------------------------------------------------------------------------------------------------------------------------------

DIVIDENDS TO SHAREHOLDERS
   FROM ($):

Investment income--net                            (26,317,437)             (27,102,277)            (3,378,927)           (3,992,120)

Net realized gain on
   investments                                           --                       --                     --                 (10,396)

TOTAL DIVIDENDS                                   (26,317,437)             (27,102,277)            (3,378,927)           (4,002,516)
- ------------------------------------------------------------------------------------------------------------------------------------

BENEFICIAL INTEREST
   TRANSACTIONS
   ($1.00 PER SHARE):

Net proceeds from
   shares sold                                 3,273,829,754            3,374,356,270            733,136,152           579,548,799

Dividends reinvested                               3,084,320                2,871,717              1,327,202             1,501,989

Cost of shares redeemed                       (3,224,747,435)          (3,368,354,264)          (720,166,100)         (620,130,251)

INCREASE (DECREASE) IN
   NET ASSETS FROM
   BENEFICIAL INTEREST
   TRANSACTIONS                                   52,166,639                8,873,723             14,297,254           (39,079,463)

TOTAL INCREASE (DECREASE)
   IN NET ASSETS                                  52,163,458                8,845,949             14,279,510           (39,079,492)
- ------------------------------------------------------------------------------------------------------------------------------------

NET ASSETS ($):

Beginning of Period                              540,282,205              531,436,256             68,614,916           107,694,408

END OF PERIOD                                    592,445,663              540,282,205             82,894,426            68,614,916

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>


                                                             The Fund

FINANCIAL HIGHLIGHTS Money Market Series

The  following tables describe the performance for the fiscal periods indicated.
Total  return  shows  how  much your investment in the fund would have increased
(or decreased) during each period, assuming you had reinvested all dividends and
distributions.  These  figures  have  been  derived  from  the  fund's financial
statements.

<TABLE>
<CAPTION>


                                                                                Year Ended December 31,
                                                                       --------------------------------------------

                                                                 1999        1998        1997        1996        1995
- ------------------------------------------------------------------------------------------------------------------------------------

PER SHARE DATA ($):

<S>                                                              <C>         <C>         <C>         <C>         <C>
Net asset value, beginning of period                             1.00        1.00        1.00        1.00        1.00

Investment Operations:

Investment income--net                                           .047        .050        .051        .049        .054

Distributions:

Dividends from investment income--net                           (.047)      (.050)      (.051)      (.049)      (.054)

Net asset value, end of period                                   1.00        1.00        1.00        1.00        1.00
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL RETURN (%)                                                 4.78        5.14        5.17        5.03        5.57
- ------------------------------------------------------------------------------------------------------------------------------------

RATIOS/SUPPLEMENTAL DATA (%):

Ratio of expenses to average net assets                           .56         .57         .57         .58         .62

Ratio of net investment income
   to average net assets                                         4.67        5.02        5.06        4.91        5.43
- ------------------------------------------------------------------------------------------------------------------------------------

Net Assets, end of period ($ x 1,000)                         592,446     540,282     531,436     483,156     401,032

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>



FINANCIAL HIGHLIGHTS Government Securities Series

<TABLE>
<CAPTION>


                                                                                Year Ended December 31,
                                                                       --------------------------------------------

                                                                 1999        1998        1997        1996        1995
- ------------------------------------------------------------------------------------------------------------------------------------
PER SHARE DATA ($):

<S>                                                              <C>         <C>         <C>         <C>         <C>
Net asset value, beginning of period                             1.00        1.00        1.00        1.00        1.00

Investment Operations:

Investment income--net                                           .043        .048        .048        .047        .052

Distributions:

Dividends from investment income--net                           (.043)      (.048)      (.048)      (.047)      (.052)

Net asset value, end of period                                   1.00        1.00        1.00        1.00        1.00
- ------------------------------------------------------------------------------------------------------------------------------------

TOTAL RETURN (%)                                                 4.34        4.87        4.95        4.84        5.36
- ------------------------------------------------------------------------------------------------------------------------------------

RATIOS/SUPPLEMENTAL DATA (%):

Ratio of expenses to average net assets                           .68         .68         .61         .63         .65

Ratio of net investment income
   to average net assets                                         4.26        4.76        4.84        4.74        5.23
- ------------------------------------------------------------------------------------------------------------------------------------

Net Assets, end of period ($ x 1,000)                          82,894      68,615     107,694     102,726     123,171

SEE NOTES TO FINANCIAL STATEMENTS.
</TABLE>


                                                             The Fund

NOTES TO FINANCIAL STATEMENTS

NOTE 1--General:

Dreyfus  Institutional  Money  Market  Fund (the "fund") is registered under the
Investment  Company  Act  of  1940,  as  amended  (the  "Act"), as a diversified
open-end  management investment company and operates as a series company issuing
two  classes  of Beneficial Interest: the Money Market Series and the Government
Securities  Series. The fund accounts separately for the assets, liabilities and
operations  of  each  series.  The  fund' s  investment  objective is to provide
investors  with  as  high  a  level  of current income as is consistent with the
preservation   of   capital  and  the  maintenance  of  liquidity.  The  Dreyfus
Corporation (the "Manager") serves as the fund's investment adviser. The Manager
is  a direct subsidiary of Mellon Bank, N.A., which is a wholly-owned subsidiary
of  Mellon  Financial  Corporation.  Premier  Mutual  Fund Services, Inc. is the
distributor  of the fund's shares, which are sold to the public without a  sales
charge.

It  is  the  fund's policy to maintain a continuous net asset value per share of
$1.00  for  each  series;  the  fund  has  adopted certain investment, portfolio
valuation and dividend and distribution policies to enable it to do so. There is
no assurance, however, that the fund will be able to maintain a stable net asset
value    per    share    of    $1.00    for    each    series.

The  funds'  financial  statements  are  prepared  in  accordance with generally
accepted accounting principles which may require the use of management estimates
and assumptions. Actual results could differ from those estimates.

NOTE 2--Significant Accounting Policies:

(a) Portfolio valuation: Investments in securities are valued at amortized cost,
which  has been determined by the fund's Board of Trustees to represent the fair
value of the fund's investments.

(b)  Securities  transactions and investment income: Securities transactions are
recorded  on  a  trade  date  basis.  Realized  gain  and  loss  from securities
transactions  are  recorded  on  the  identified  cost basis. Interest income is
recognized    on    the    accrual    basis.    Cost    of    investments

represents  amortized  cost. Under the terms of the custody agreement, the Money
Market  Series  receives  net  earnings credits based on available cash balances
left  on  deposit.  Under  the  terms  of  the custody agreement, the Government
Securities  Series received net earnings credits of $288 during the period ended
December  31,  1999  based  on available cash balances on deposit. Income earned
under    this    arrangement    is    included    in    interest.

The  fund  may  enter  into  repurchase  agreements with financial institutions,
deemed  to  be  creditworthy  by  the  fund' s  Manager, subject to the seller's
agreement  to repurchase and the fund's agreement to resell such securities at a
mutually   agreed   upon  price.  Securities  purchased  subject  to  repurchase
agreements are deposited with the fund's custodian and, pursuant to the terms of
the  repurchase  agreement,  must have an aggregate market value greater than or
equal  to  the repurchase price plus accrued interest at all times. If the value
of  the underlying securities falls below the value of the repurchase price plus
accrued  interest,  the  fund  will  require  the  seller  to deposit additional
collateral by the next business day. If the request for additional collateral is
not met, or the seller defaults on its repurchase obligation, the fund maintains
the  right  to  sell the underlying securities at market value and may claim any
resulting loss against the seller.

(c)  Expenses: Expenses directly attributable to each series are charged to that
series'  operations;  expenses which are applicable to both series are allocated
among them on a pro rata basis.

(d)  Dividends  to  shareholders:  It is the policy of the fund, with respect to
both  series,  to  declare dividends from investment income-net on each business
day;  such dividends are paid monthly. Dividends from net realized capital gain,
with  respect  to both series, are normally declared and paid annually, but each
series  may  make  distributions  on  a  more  frequent basis to comply with the
distribution  requirements of the Internal Revenue Code of 1986, as amended (the
" Code" ). However,  to  the  extent  that a net realized capital gain of either
series can be reduced by a capital loss carryover of that series, such gain will
not be distributed.

                                                             The Fund

NOTES TO FINANCIAL STATEMENTS (CONTINUED)

(e) Federal income taxes: It is the policy of each series to continue to qualify
as  a  regulated  investment  company,  if  such  qualification  is  in the best
interests  of  its  shareholders, by complying with the applicable provisions of
the  Code,  and to make distributions of taxable income sufficient to relieve it
from substantially all Federal income and excise taxes.

The  Money  Market  Series has an unused capital loss carryover of approximately
$37,500  available  for Federal income tax purposes to be applied against future
net securities profits, if any, realized subsequent to December 31, 1999. If not
applied,  $6,500  of  the  carryover  expires in fiscal 2002, $28,000 expires in
fiscal 2006 and $3,000 expires in fiscal 2007.

The  Government  Securities  Series  has  an  unused  capital  loss carryover of
approximately  $18,000  available  for Federal income tax purposes to be applied
against  future  net securities profits, if any, realized subsequent to December
31, 1999. If not applied, the carryover expires in fiscal 2007.

At  December 31, 1999, the cost of investments of each series for Federal income
tax  purposes  was  substantially  the  same as the cost for financial reporting
purposes (see the Statements of Investments).

NOTE 3--Management Fee and Other Transactions With Affiliates:

(a)  Pursuant to a management agreement with the Manager, the management fee for
each  series  is  computed  at  the annual rate of .50 of 1% of the value of the
average daily net assets of each series and is payable monthly.

(b)  Under the Shareholder Services Plan, each series reimburses Dreyfus Service
Corporation,  a  wholly-owned subsidiary of the Manager, an amount not to exceed
an  annual  rate  of  .25  of  1% of the value of each series' average daily net
assets  for  certain  allocated  expenses  of providing personal services and/or
maintaining  shareholder  accounts.  The  services provided may include personal
services  relating  to  shareholder  accounts,  such  as  answering  shareholder
inquiries  regarding  the  fund and providing reports and other information, and
services related to the mainte

nance  of  shareholder  accounts. During the period ended December 31, 1999, the
Money  Market  Series  and the Government Securities Series were charged $52,801
and $37,040, respectively, pursuant to the Shareholder Services Plan.

The  fund  compensates  Dreyfus Transfer, Inc., a wholly-owned subsidiary of the
Manager,   under  a  transfer  agency  agreement  for  providing  personnel  and
facilities  to  perform transfer agency services for the fund. During the period
ended  December  31, 1999, the Money Market Series and the Government Securities
Series  were  charged $92,446 and $3,363, respectively, pursuant to the transfer
agency agreement.

(c)  Each  trustee  who  is  not  an  "affiliated  person" as defined in the Act
received from the fund an annual fee of $4,500 and an attendance fee of $500 per
meeting.  The  Chairman  of  the  Board  received  an  additional  25%  of  such
compensation.

Each  non-affiliated trustee is a Board member of one or more funds comprising a
certain  group  of  funds  (" Fund Group") within the Dreyfus complex. Effective
January  1,  2000,  for  their  participation  as a trustee in a Fund Group, the
trustees receive an annual fee of $40,000 each, $6,000 for each meeting attended
in  person and $500 for each telephonic meeting in which they participate. These
fees  are allocated among the funds in the Fund Group. The Chairman of the Board
receives an additional 25% of such compensation.

                                                             The Fund

REPORT OF INDEPENDENT AUDITORS

Shareholders and Board of Trustees

Dreyfus Institutional Money Market Fund, Inc.

We  have audited the accompanying statement of assets and liabilities, including
the  statements  of  investments,  of  Dreyfus  Institutional Money Market, Inc.
(comprising, respectively, the Money Market Series and the Government Securities
Series) , as  of  December 31, 1999, and the related statement of operations for
the  year then ended, the statement of changes in net assets for each of the two
years  in  the period then ended, and financial highlights for each of the years
indicated  therein.  These financial statements and financial highlights are the
responsibility  of  the  Fund' s management. Our responsibility is to express an
opinion  on  these  financial  statements  and financial highlights based on our
audits.

We conducted our audits in accordance with auditing standards generally accepted
in the United States. Those standards require that we plan and perform the audit
to  obtain  reasonable  assurance  about  whether  the  financial statements and
financial  highlights  are  free  of  material  misstatement.  An audit includes
examining,  on  a test basis, evidence supporting the amounts and disclosures in
the  financial  statements  and  financial  highlights.  Our procedures included
confirmation  of securities owned as of December 31, 1999 by correspondence with
the  custodian  and  brokers.  An  audit  also includes assessing the accounting
principles  used  and  significant  estimates  made  by  management,  as well as
evaluating  the  overall  financial  statement presentation. We believe that our
audits provide a reasonable basis for our opinion.

In  our  opinion,  the financial statements and financial highlights referred to
above  present  fairly, in all material respects, the financial position of each
of  the  respective series constituting Dreyfus Institutional Money Market, Inc.
at  December  31, 1999, the results of their operations for the year then ended,
the  changes  in  their  net assets for each of the two years in the period then
ended,  and  the  financial  highlights  for  each  of  the  indicated years, in
conformity with accounting principles generally accepted in the United States.

                                                               Ernst & Young LLP


New York, New York

February 7, 2000



IMPORTANT TAX INFORMATION (Unaudited)

For  State  individual  income  tax  purposes,  the Government Securities Series
hereby designates 32.62% of the ordinary income dividends paid during its fiscal
year  ended  December  31,  1999  as attributable to interest income from direct
obligations  of  the  United  States.  Such  dividends are currently exempt from
taxation  for individual income tax purposes in most states, including New York,
California and the District of Columbia.

                                                             The Fund

                                                           For More Information

                        Dreyfus Institutional Money Market Fund

                        200 Park Avenue

                        New York, NY 10166

                        Manager

                        The Dreyfus Corporation

                        200 Park Avenue

                        New York, NY 10166

Custodian

The Bank of New York

100 Church Street

New York, NY 10286

Transfer Agent &

Dividend Disbursing Agent

Dreyfus Transfer, Inc.

P.O. Box 9671

Providence, RI 02940

Distributor

Premier Mutual Fund Services, Inc.

60 State Street

Boston, MA 02109

To obtain information:

BY TELEPHONE Call 1-800-645-6561

BY MAIL  Write to:  The Dreyfus Family of Funds 144 Glenn Curtiss Boulevard
Uniondale, NY 11556-0144

BY E-MAIL  Send your request to [email protected]

ON THE INTERNET  Information can be viewed online or downloaded from:

http://www.dreyfus.com

(c) 2000 Dreyfus Service Corporation                              179-195AR9912








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