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                                                                     EXHIBIT 8.1

              [FRIED, FRANK, HARRIS, SHRIVER & JACOBSON LETTERHEAD]


                               January 4, 2000


CoStar Group, Inc.
7475 Wisconsin Avenue
Bethesda, Maryland 20814

Dear Ladies and Gentlemen:

       We are acting as your counsel in connection with the proposed acquisition
by CoStar Group, Inc. ("CoStar") of COMPS.COM, Inc. ("Comps") pursuant to the
proposed merger (the "Merger") of Comps with and into Acq Sub, Inc. ("Sub"), a
wholly owned subsidiary of CoStar, whereupon Sub will be the surviving
corporation and the separate existence of Comps will cease. The Merger will be
consummated pursuant to the Agreement and Plan of Merger dated as of November 3,
1999 by and among CoStar, Sub, and Comps (the "Merger Agreement").

       CoStar has filed with the Securities and Exchange Commission under the
Securities Act of 1933, as amended (the "1933 Act"), a registration statement on
Form S-4 (the "Registration Statement"), with respect to the shares of CoStar
common stock to be issued to holders of shares of Comps common stock in
connection with the Merger. In addition, CoStar has prepared, and we have
reviewed, a Joint Proxy Statement/Prospectus (the "Joint Proxy Statement") which
is contained in and made a part of the Registration Statement, and the
Appendices thereto, including the Merger Agreement. In rendering the opinion set
forth below, we have relied upon the facts and representations stated in the
Joint Proxy Statement and upon such other documents as we have deemed
appropriate, including the representations of CoStar and Comps referred to in
the Joint Proxy Statement and set forth in certain officer's certificates from
CoStar and Comps.

       We have assumed that (i) all parties to the Merger Agreement, and to any
other documents reviewed by us, have acted, and will act, in accordance with the
terms of the Merger Agreement and such other documents, (ii) all facts,
information, statements and representations qualified by the knowledge, belief,
expectation and/or intention of CoStar,


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                    FRIED, FRANK, HARRIS, SHRIVER & JACOBSON


CoStar Group, Inc.
January 4, 2000
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Comps or Sub will be complete and accurate as of the effective time as though
not so qualified, (iii) the merger will be consummated at the effective time
pursuant to the terms and conditions set forth in the Merger Agreement without
the waiver or modification of any such terms and conditions, and (iv) the Merger
is authorized by and will be effected pursuant to applicable state law.

       Based upon and subject to the foregoing, and to the qualifications,
limitations, representations and assumptions contained in the portion of the
Joint Proxy Statement captioned "Material Federal Income Tax Consequences," the
portion of the Joint Proxy Statement captioned "Material Federal Income Tax
Consequences" is accurate in all material respects. No opinion is expressed on
any matters other than those specifically referred to herein.

       This opinion is furnished to you for your use in connection with the
Registration Statement and may not be used for any other purpose without our
prior express written consent. We hereby consent to the filing of this opinion
as an annex to the Registration Statement and to the references to us contained
in the Registration Statement. In giving such consent, we do not thereby admit
that we are in the category of persons whose consent is required under Section 7
of the 1933 Act.


                                       Yours very truly,

                                       /s/

                                       FRIED, FRANK, HARRIS, SHRIVER & JACOBSON