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                                                                    EXHIBIT 23.3

               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 5, 1999, with respect to the financial statements of
Jamison Research, Inc. included in CoStar Group, Inc.'s 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