SECURITIES AND EXCHANGE COMMISSION

                              Washington, DC 20549

                  Under the Securities and Exchange Act of 1934



                               (Amendment No. 4 )


                                   AMBAC INC.
                                (Name of Issuer)

                                     Common
                         (Title of Class of Securities )


                                    023139108
                                 (CUSIP NUMBER)

                         A cross appears below if a fee
                        is being paid with this statement






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1)      Name of Reporting
        SS or IRS Identification      Pioneering Management
        Nos. of Above Persons         Corporation

2)      Check the Appropriate Box     (a)
        of A Member of  Group
        (See Instructions)            (b)  X

3)      SEC Use Only

4)      Citizenship of Place of
        Organization

        Number of                     (5)  Sole Voting Power      3508900
        Shares
        Beneficially Owned            (6)  Shared Voting Power    0
        by Each Reporting
        Person With                   (7)  Sole Dispositive       271200
                                            Power

                                      (8)  Shared Dispositive     3237700
                                             Power

9)      Aggregate Amount Bene-        3508900
        ficially Owned by Each
        Reporting Person

10)     Check if the aggregate
        Amount in Row  (9) Ex-
        clude Certain Shares (See
        Instructions)

11)     Percent of Class Represented
        By Amount in Row 9.           10.02%

12)     Type of Reporting
        Person (See Instructions)     IA





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Item 1(a)         Name of Issuer.

                  AMBAC INC.

Item 1(b)         Address of User's Principal Executive Office's

                  Mr.  Frank  J.  Bivona
                  Chief Financial Officer
                  AMBAC INC.
                  One State Street Plaza
                  New York, New York 10004

Item 2(a)         Name of Person Filing.

                  Pioneering Management Corporation

Item 2(b)         Address of Principal Business Office:

                  60 State Street, Boston, MA 02109

Item 2(c)         Citizenship:

                  State Of Delaware - Pioneering Management Corporation.

Item 2(d)         Title of Class of Securities.

                  Common Stock

Item 2(e)         CUSIP Number.

                  023139108

Item 3            The person filing this statement pursuant to Rule 13-1(b)
                  or 13d-2 is:

                  (a)      Investment Adviser registered under Section
                           203 of the Investment Advisers Act of 1940.




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Item 4.  Ownership

(a)       Amount Beneficially Owned                                     3508900

(b)       Percent of Class                                              10.02%


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

         (i)      sole power to vote or to direct the vote             3508900

         (ii)     shared power to vote or to direct vote               0

         (iii)    sole power to dispose or to direct disposition of    271200

         (iv)     shared power to dispose or to direct disposition     3237700

Item  5. Ownership of Five Percent or Less of a Class.

         Inapplicable.


Item 6.  Ownership of More than Five Percent on Behalf of Another Person.

         Inapplicable.

Item 7.  Identification and Classification of the Subsidiary Which Acquired
         the Security Being Reported On By the Parent Holding Company.

         Inapplicable.

Item 8.  Identification and Classification of Members of the Group.

         Inapplicable.

Item 9.  Notice of Dissolution of the Group.

         Inapplicable.



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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  transactions  having
such purposes or effect.

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


                  August  9,1996
                  Date




         /s/ William H. Keough
                  Signature


         William H. Keough, Senior Vice President
         Chief Financial Officer and Treasure
         Type Name and Title