EXHIBIT 5.1
                                February 23, 2004

Arrowhead Research Corporation
150 S. Los Robles, Suite 480
Pasadena, California 91101

     Re:     Registration Statement on Form S-3
             ----------------------------------

Ladies and Gentlemen:

     The  undersigned  has examined the Registration Statement on Form S-3 to be
filed  by Arrowhead Research Corporation (the "Company") with the Securities and
Exchange  Commission  on  or  about  February  __,  2004  (the  "Registration
Statement"), in connection with the registration for resale under the Securities
Act  of  1933,  as  amended (the "Securities Act"), of Common Stock and Warrants
previously issued to the Selling Security Holders identified therein, along with
additional Common Stock issuable upon exercise of the Warrants.  This opinion is
furnished  in  accordance  with the requirements of Item 16 of Form S-3 and Item
601(b)(5)(i)  of  Regulation  S-B.

     For  purposes  hereof, the undersigned has examined such matters of law and
such  documents,  corporate records and other instruments as the undersigned has
deemed  necessary,  assuming the genuineness of all signatures, the authenticity
of  all  documents  submitted  as  originals, the conformity to originals of all
documents  submitted  as  certified,  photostatic  or  conformed copies, and the
authenticity  of  the  originals of all such documents. The undersigned has also
assumed  the due execution and delivery of all documents where due execution and
delivery  are prerequisites to the effectiveness thereof. With your consent, the
undersigned has relied upon certificates of public officials and certificates of
officers  of the Company for the accuracy of material, factual matters contained
therein  which  were  not  independently  established.

     Based  upon  the  foregoing,  and relying thereon, it is the opinion of the
undersigned  that  the  shares  of  Common Stock and Warrants were, and that the
shares  of  Common  Stock,  when  issued and sold in the manner described in the
Registration  Statement, will be, legally and validly issued, and are or will be
fully  paid  and  non-assessable.

     No  opinion  is  expressed  herein as to the applicability or effect of any
laws,  orders  or  judgments  of  any  state  or jurisdiction other than federal
securities  laws and the general corporate laws of the State of Delaware and the
State  of  California.  Furthermore,  this opinion is based solely upon existing
laws,  rules  and  regulations,  and the undersigned undertakes no obligation to
advise  you  of  any  changes  that  may  be  brought  to  the  attention of the
undersigned  after  the  date  hereof.

     The  undersigned  consents  to the filing of this opinion letter as Exhibit
5.1  to  the  Registration  Statement.

                                                Very truly yours,


                                                /s/  James M. Phillips, Jr.
                                                ----------------------------
                                                James M. Phillips, Jr.


                                      II-6