<PAGE>
 
                                 UNITED STATES
                      SECURITIES AND EXCHANGE COMMISSION
                            WASHINGTON, D.C. 20549


                                 SCHEDULE 13G



                   Under the Securities Exchange Act of 1934

                             (Amendment No. ___)*


                       COMPUTER TASK GROUP, INCORPORATED
--------------------------------------------------------------------------------
                               (Name of Issuer)


                                 Common Stock
--------------------------------------------------------------------------------
                        (Title of Class of Securities)


                                   205477102
--------------------------------------------------------------------------------
                                (CUSIP Number)

*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).


Schedule is filed pursuant to Rule 13d-1(b).

                              Page 1 of 7 pages 

<PAGE>
 
--------------------                                          -----------------
CUSIP No.  205477102                   13G                    Page 2 of 7 Pages
--------------------                                          -----------------

 
------------------------------------------------------------------------------
      NAME OF REPORTING PERSON
 1    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
                                                                                
      Wanger Asset Management, L.P. 36-3820584
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* 
 2                                                              (a) [_]
                                                                (b) [_]
      Not Applicable
------------------------------------------------------------------------------
      SEC USE ONLY
 3
 
     
------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4                                 
      Delaware
      
------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5                                                       
     NUMBER OF            None
                           
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6       
                          1,279,500
     OWNED BY
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7       
    REPORTING             None
 
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8       
                          1,279,500
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9                           
      1,279,500
------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
10    
                                                                        [_]
      Not Applicable
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11           
      6.13%                                                                  

------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON* 
12           
      IA

------------------------------------------------------------------------------
                    * SEE INSTRUCTIONS BEFORE FILLING OUT!

<PAGE>
 
--------------------                                          -----------------
CUSIP No.  205477102                   13G                    Page 3 of 7 Pages
--------------------                                          -----------------

 
------------------------------------------------------------------------------
      NAME OF REPORTING PERSON
 1    S.S. OR I.R.S. IDENTIFICATION NO. OF ABOVE PERSON
                                                                                
      Wanger Asset Management, Ltd.
------------------------------------------------------------------------------
      CHECK THE APPROPRIATE BOX IF A MEMBER OF A GROUP* 
 2                                                              (a) [_]
                                                                (b) [_]
      Not Applicable
------------------------------------------------------------------------------
      SEC USE ONLY
 3
 
     
------------------------------------------------------------------------------
      CITIZENSHIP OR PLACE OF ORGANIZATION
 4                                 
      Delaware
      
------------------------------------------------------------------------------
                          SOLE VOTING POWER
                     5                                                       
     NUMBER OF            None
                           
      SHARES       -----------------------------------------------------------
                          SHARED VOTING POWER
   BENEFICIALLY      6       
                          1,279,500
     OWNED BY
                   -----------------------------------------------------------
       EACH               SOLE DISPOSITIVE POWER
                     7       
    REPORTING             None
 
      PERSON       -----------------------------------------------------------
                          SHARED DISPOSITIVE POWER
       WITH          8       
                          1,279,500
------------------------------------------------------------------------------
      AGGREGATE AMOUNT BENEFICIALLY OWNED BY EACH REPORTING PERSON
 9                           
      1,279,500
------------------------------------------------------------------------------
      CHECK BOX IF THE AGGREGATE AMOUNT IN ROW (9) EXCLUDES CERTAIN SHARES*
10    
                                                                        [_]
      Not Applicable
------------------------------------------------------------------------------
      PERCENT OF CLASS REPRESENTED BY AMOUNT IN ROW 9
11           
      6.13%                                                                  

------------------------------------------------------------------------------
      TYPE OF REPORTING PERSON* 
12           
      CO

------------------------------------------------------------------------------
                    * SEE INSTRUCTIONS BEFORE FILLING OUT!

<PAGE>
 
Item 1(a)  Name of Issuer:  

           COMPUTER TASK GROUP, INCORPORATED

      (b)  Address of Issuer's Principal Executive Offices:

           800 Delaware Avenue
           Buffalo, NY 14209

Item 2(a): Name of Person Filing:  

           Wanger Asset Management, L.P. ("WAM")
           Wanger Asset Management Ltd., the general partner of WAM ("WAM LTD.")

      (b)  Address of Principal Business Office:

           WAM and WAM LTD. are each located at:

           227 West Monroe Street, Suite 3000
           Chicago, Illinois 60606

      (c)  Citizenship:  

           WAM is a Delaware limited partnership; and WAM LTD. is a Delaware
           corporation.

      (d)  Title of Class of Securities:  

           Common Stock.

      (e)  CUSIP Number:  

           205477102

Item 3:    Type of Person:

               (d)  WAM is an Investment Adviser registered under section 203 of
                    the Investment Advisers Act of 1940; and WAM LTD. is the
                    General Partner of the Investment Adviser.

                               Page 4 of 7 Pages

<PAGE>
 
Item 4:    Ownership:

           As of December 31, 1999:

           (a)   Amount owned "beneficially" within the meaning of
                 rule 13d-3:

                 1,279,500

           (b)   Percent of class:

                 6.13%

           (c)   Number of shares to which such person has:

                 (i)    sole power to vote or to direct the vote:

                        none
                    
                 (ii)   shared power to vote or to direct the vote:

                        1,279,500

                 (iii)  sole power to dispose or to direct the disposition of:

                        none

                 (iv)   shared power to dispose or to direct the disposition of:
          
                        1,279,500

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 shares reported herein have been acquired on behalf of
           discretionary clients of WAM. Persons other than WAM and WAM LTD. are
           entitled to receive all dividends from, and proceeds from the sale
           of, those shares.

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.

                               Page 5 of 7 Pages

<PAGE>
 
Item 10:   Certification:

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

                               Page 6 of 7 Pages

<PAGE>
 
                                   Signature
                                   ---------

     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.


Date: February 11, 2000

                         The undersigned corporation, on the date above written,
                         agrees and consents to the joint filing on its behalf
                         of this Schedule 13G in connection with its beneficial
                         ownership of the security reported herein.

                                   WANGER ASSET MANAGEMENT, LTD.
                                      for itself and as general partner of
                                      WANGER ASSET MANAGEMENT, L.P.


                                   By:  /s/ Bruce H. Lauer
                                        --------------------------
                                            Bruce H. Lauer
                                            Vice President

                               Page 7 of 7 Pages