UNITED STATES
SECURITIES AND EXCHANGE COMMISSION
Washington, D.C. 20549


SCHEDULE 13G

Under the Securities Exchange Act of 1934


Amendment No.:      *


Name of Issuer: CoStar Group, Inc. 


Title of Class of Securities: Common Stock


CUSIP Number: 22160N10-9


Date of Event Which Requires Filing of this Statement: 10/31/2005


Check the appropriate box to designate the rule pursuant to which this Schedule 
is filed.

[X] Rule 13d-1(b)
[ ] Rule 13d-1(c)
[ ] Rule 13d-1(d)

* The remainder of this cover page shall be filled out for a reporting
person's initial filing on this form with respect to the subject class
of securities, and for any subsequent amendment containing information which 
would alter the disclosures provided in a prior cover page.

The information required in the remainder of this cover page shall not be deemed
to be "filed" for the purpose of Section 18 of the Securities
Exchange Act of 1934 ("Act") or otherwise subject to the liabilities of
that section of the Act but shall be subject to all other provisions of
the Act (however, see the Notes).




CUSIP No.: 22160N10-9					

1.  NAME OF REPORTING PERSON
    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
    Janus Capital Management LLC
    EIN #75-3019302

2.  CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP
    a.   ___
    b.   ___
	
3.  SEC USE ONLY

4.  CITIZENSHIP OR PLACE OF ORGANIZATION
         Delaware

NUMBER OF SHARES BENEFICIALLY OWNED BY EACH REPORTING PERSON WITH:

    5.   SOLE VOTING POWER
         2,010,804**

    6.   SHARED VOTING POWER
         -0-  
    
    7.   SOLE DISPOSITIVE POWER
         2,010,804**

    8.   SHARED DISPOSITIVE POWER
         -0-

    9.   AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
         2,010,804**

    10.  CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES
         Not applicable
         
11. PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW (9)
    10.9%**

12. TYPE OF REPORTING PERSON
    IA, HC

**  See Item 4 of this filing




Item 1.

    (a). Name of Issuer: CoStar Group, Inc. ("CoStar")
    
    (b). Address of Issuer's Principal Executive Offices:

         2 Bethesda Metro Center, 10th Floor
         Bethesda, MD  20814

Item 2.

    (a).-(c). Name, Principal Business Address, and Citizenship of Persons
              Filing:

         (1)  Janus Capital Management LLC ("Janus Capital")
              151 Detroit Street
              Denver, Colorado  80206
              Citizenship:  Delaware        
         
    (d). Title of Class of Securities:  Common Stock 

    (e). CUSIP Number:  22160N10-9

Item 3. 

This statement is filed pursuant to Rule 13d-1 (b) or 13d-2(b) and the person
filing, Janus Capital, is an investment adviser in accordance with Section 
240.13d-1(b)(ii)(E) as well as a parent holding company/control person in 
accordance with Section 240.13d-1(b)(ii)(G).  See Item 4 for additional 
information.

Item 4.  Ownership

The information in items 1 and 5 through 11 on the cover page(s) on Schedule 13G
is hereby incorporated by reference.

Janus Capital has an indirect 77.5% ownership stake in Enhanced Investment 
Technologies LLC ("INTECH") and an indirect 30% ownership stake in Perkins, 
Wolf, McDonnell and Company, LLC ("Perkins Wolf").  Due to the above ownership 
structure, holdings for Janus Capital, Perkins Wolf and INTECH are aggregated 
for purposes of this filing. Janus Capital, Perkins Wolf and INTECH are 
registered investment advisers, each furnishing investment advice to various 
investment companies registered under Section 8 of the Investment Company Act of
1940 and to individual and institutional clients (collectively referred to 
herein as "Managed Portfolios").

As a result of its role as investment adviser or sub-adviser to the Managed 
Portfolios, Janus Capital may be deemed to be the beneficial owner of 2,010,804
shares or 10.9% of the shares outstanding of CoStar Common Stock held by such
Managed Portfolios.  However, Janus Capital does not have the right to receive
any dividends from, or the proceeds from the sale of, the securities held in the
Managed Portfolios and disclaims any ownership associated with such rights.



Item 5.  Ownership of Five Percent or Less of a Class
         
Not applicable.
         
Item 6.  Ownership of More than Five Percent on Behalf of Another Person
         
The Managed Portfolios, set forth in Item 4 above, have the right to receive all
dividends from, and the proceeds from the sale of, the securities held in their 
respective accounts.
         
The interest of any one such person does not exceed 5% of the class of 
securities.
         
These shares were acquired in the ordinary course of business, and not with the 
purpose of changing or influencing control of the Issuer.
         
Item 7.  Identification and Classification of the Subsidiary Which
Acquired the Security Being Reported on by the Parent Holding Company

Not applicable.
         
Item 8.  Identification and Classification of Members of the Group

Not applicable.
         
Item 9.  Notice of Dissolution of Group
         
Not applicable.
         
Item 10. Certification

By signing below I certify that, to the best of my knowledge and belief, the 
securities referred to above were acquired in the ordinary course of business 
and were not acquired for the purpose of and do not have the effect of changing
or influencing the control of the issuer of such securities and were not 
acquired in connection with or as a participant in any transaction having such 
purposes or effect.



				
SIGNATURES                                            

After reasonable inquiry and to the best of my knowledge and belief, I certify 
that the information set forth in this statement is true, complete and correct.

JANUS CAPITAL MANAGEMENT LLC

By  /s/  Bonnie M. Howe	                       11/10/2005    
  Bonnie M. Howe,                              Date
    Vice President and Assistant General Counsel