CoStar Group, Inc.

________________________________

                                                                                                                                                                                                                                                                                  2 Bethesda Metro Center

                                                                                                                  10th Floor

                                                                                                                  Bethesda, MD 20814-5388

                                                                                                                  ________________________________

                                                                                                                  301.215.8300  tel

                                                                                                                  800.204.5960  toll free

                                                                                                                  301.718.2444  fax

                                                                                                                  800.613.1301  toll  free fax

                                                                                                                  ________________________________

                                                                                                                  www.costar.com

                                                                                                                  NASDAQ:  CSGP

July 14, 2010

VIA EDGAR AND FACSIMILE

Kathleen Collins

Accounting Branch Chief

Securities and Exchange Commission

Division of Corporation Finance

One Station Place

100 F Street, N.E.

Washington, D.C.  20549-4561

Re: CoStar Group, Inc.

Form 10-K for Fiscal Year Ended December 31, 2009

Filed February 26, 2010

File No 000-24531

Dear Ms. Collins:

We received your comment letter dated July 1, 2010 concerning the CoStar Group, Inc. filing listed above.  As discussed yesterday with Mr. Ryan Housel, SEC

Division of Corporation Finance Staff Attorney, we respectfully advise the Staff that we currently expect to respond no later than August 2, 2010.

Please feel free to contact the undersigned at (301) 664-9132 or Jonathan Coleman, our General Counsel, at (301) 280-3859 with any questions you may have.

Sincerely,

/s/ Brian J. Radecki      

Brian J. Radecki

Chief Financial Officer

cc:           Megan Akst, Staff Accountant, Division of Corporation Finance

Ryan Housel, Staff Attorney, Division of Corporation Finance

David Orlic, Staff Attorney, Division of Corporation Finance

Jonathan Coleman, CoStar Group, Inc.

Stephen Glover, Gibson, Dunn & Crutcher LLP