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                                                                    EXHIBIT 23.1

               CONSENT OF ERNST & YOUNG LLP, INDEPENDENT AUDITORS


     We consent to the reference to our firm under the caption "Experts" in
Amendment No. 1 to the Registration Statement (Form S-4 No. 333-92579) and
related prospectus of CoStar Group, Inc. for the registration of 2,242,000
shares of its common stock, and to the incorporation by reference therein of our
report dated February 12, 1999, with respect to the consolidated financial
statements of CoStar Group, Inc. included in its Annual Report (Form 10-K) for
the year ended December 31, 1998, filed with the Securities and Exchange
Commission.


                                          /s/ Ernst & Young LLP

McLean, Virginia

January 3, 2000