Arrowhead Pharmaceuticals (ARWR) 10-K 2006 FY Annual report

Exhibit 32.2

CERTIFICATION OF CHIEF FINANCIAL OFFICER

PURSUANT TO RULE 13a-14(b) OR RULE 15d-14(b)

OF THE SECURITIES EXCHANGE ACT OF 1934

AND 18 U.S.C. SECTION 1350

I, Joseph T. Kingsley, President and Chief Financial Officer of Arrowhead Research Corporation (the “Company”), certify, pursuant to Rule 13(a)-14(b) or Rule 15(d)-14(b) of the Securities Exchange Act of 1934 and 18 U.S.C. Section 1350, that (i) the Annual Report on Form 10-K of the Company for the fiscal year ended September 30, 2006 fully complies with the requirements of Section 13(a) or 15(d) of the Securities Exchange Act of 1934, and (ii) the information contained in such Annual Report on Form 10-K fairly presents in all material respects the financial condition and results of operations of the Company.

Date: December 14, 2006

/s/    JOSEPH T. KINGSLEY        

Joseph T. Kingsley

Interim President and Chief Financial Officer

(Principal Accounting Officer)

A signed original of these written statements required by 18 U.S.C. Section 1350 has been provided to Arrowhead Research Corporation and will be retained by Arrowhead Research Corporation and furnished to the Securities and Exchange Commission or its staff upon request.