Exhibit 32.1

CERTIFICATION
PURSUANT TO 18 U.S.C. SECTION 1350,
AS ADOPTED BY SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

     Each of the undersigned officers of Quanta Services, Inc. (the “Company”) certify, pursuant to 18 U.S.C. § 1350, as adopted by Section 906 of the Sarbanes-Oxley Act of 2002, to such officer’s knowledge that:

     (1) the accompanying Form 10-K report for the period ending December 31, 2005 as filed with the U.S. Securities and Exchange Commission (the “Report”) fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934, as amended; and

     (2) the information contained in the Report fairly presents, in all material respects, the financial condition and results of operations of the Company.

Dated: March 1, 2006

/s/ JOHN R. COLSON  

John R. Colson, 

Chief Executive Officer 

Dated: March 1, 2006

/s/ JAMES H. HADDOX  

James H. Haddox, 

Chief Financial Officer