Arrowhead Research Corporation Scheduled to be Listed on Russell Microcap Index

PASADENA, California – (BUSINESS WIRE) – June 21, 2006 – Arrowhead Research (NASDAQ:ARWR) announced today that it is scheduled to join the Russell Microcap(TM) Index when the Russell Investment Group reconstitutes its family of U.S. indexes on June 30th, 2006, according to a preliminary membership list posted on Friday, June 16th, 2006, on www.russell.com.

"We are very pleased with the scheduled addition of Arrowhead to the Russell Microcap Index," said R. Bruce Stewart, Chairman and CEO of Arrowhead. "We believe the selection of Arrowhead represents recognition by Russell of the significant progress of our operating companies over the last two years. Inclusion in this index will increase our visibility with investors and institutions that rely on the Russell indexes as part of their investment strategy."

Russell, a global leader in multi-manager investment services, provides investment products and services in 44 countries. Russell manages more than $167 billion in assets and advises clients worldwide representing $2.4 trillion. Russell indexes are widely used by investment managers and institutional investors for index funds and as benchmarks for both passive and active investment strategies. An industry-leading $3.8 trillion in assets currently are evaluated and benchmarked against these Russell indexes.

About Arrowhead Research Corporation

Arrowhead Research Corporation (www.arrowheadresearch.com) is a publicly-traded nanotechnology company commercializing new technologies in the areas of life sciences, electronics, and energy. Arrowhead is building value for shareholders through the progress of majority owned subsidiaries founded on nanotechnologies originally developed at universities. The company works closely with universities to source early stage deals and to generate rights to intellectual property covering promising new nanotechnologies. Currently, Arrowhead has four subsidiaries commercializing nanotech products and applications, including anti-cancer drugs, RNAi therapeutics, carbon-based electronics and compound semiconductor materials.

Safe Harbor Statement under the Private Securities Litigation Reform Act of 1995:

This news release contains forward-looking statements within the meaning of the "safe harbor" provisions of the Private Securities Litigation Reform Act of 1995. These statements are based upon our current expectations and speak only as of the date hereof. Our actual results may differ materially and adversely from those expressed in any forward-looking statements as a result of various factors and uncertainties, including the recent economic slowdown affecting technology companies, the future success of our scientific studies, our ability to successfully develop products, rapid technological change in our markets, changes in demand for our future products, legislative, regulatory and competitive developments and general economic conditions. Our Annual Report on Form 10-K and 10-K/A, recent and forthcoming Quarterly Reports on Form 10-Q and 10-Q/A, recent Current Reports on Forms 8-K and 8-K/A, our Registration Statement on Form S-3, and other SEC filings discuss some of the important risk factors that may affect our business, results of operations and financial condition. We undertake no obligation to revise or update publicly any forward-looking statements for any reason.

Contacts

Virginia Dadey; Vice President, Investor Relations
Telephone: 212.541.3707
Email: vdadey@arrowres.com