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
<PAGE> 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
<PAGE> 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.
<PAGE> 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.
<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 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