The Hessian fly occurs in the western United States in parts of California, Idaho, Oregon, and Washington. Although wheat is a major host, the fly also infests barley, rye, triticale, and various grass species. Of major concern in the northwestern U.S. is Hessian fly injury to wheat and barley. The most reliable and economical control of the Hessian fly on wheat has been achieved by growing resistant wheat varieties, when available. Cultural practices that reduce the availability of acceptable host plants at critical stages in the fly life cycle complement the use of resistant varieties in many areas. The use of wheat varieties with specific resistance genes results in development of fly races (biotypes) that can survive on and injure wheat plants carrying these genes. Therefore, it is important to monitor Hessian fly population to determine evolution of virulent biotypes and subsequent loss of effectiveness of resistant varieties. The objectives of our research were to; 1) determine the biotype composition of Hessian fly populations from Idaho and Washington, 2) test the response of representative populations to wheat varieties and lines that carry presently deployed or new genes for resistance, and 3) screen barley accessions from the USDA/ARS collection at Aberdeen, Idaho for Hessian fly resistance. This information will help us select the most effective genes to incorporate into future varieties adapted to the northwestern U.S. Hessian fly populations were sampled from Pullman, WA in 1995, Dusty and Walla Walla, WA in 1997, and Garfield, WA and Lewiston, ID in 1998. Following biotype testing, a population from Dusty, WA and the Lewiston, ID population were tested against three resistant wheat selections from the Purdue/USDA wheat breeding program, and check varieties (four with presently deployed resistance genes and a susceptible variety). The wheat selections include INW9811, a soft winter wheat variety released in 1998 by Purdue/USDA that carries H13 resistance, Purdue line 9547B1-12 that carries resistance from Portugal 2536, and Purdue line CI17960-1-1-2-4-2-10 that carries resistance from CI17960. The resistance genes in the two Purdue lines are unnamed, but thought to be different than presently identified genes based on genetic studies conducted to date. Results of biotype tests demonstrated that the Washington Hessian fly populations, with the exception of Garfield, still contained a relatively high frequency of biotype GP (47-75%), a biotype that cannot survive on wheat plants that carry any of the tested resistance genes. The Garfield, WA and Lewiston, ID populations contained 25% GP. Biotypes E, F, G, and O were the most common biotypes found in the six populations. These biotypes are virulent to resistance genes H3 (biotype E, G), H6 (biotype F, G, O), and H5 (biotype O). The reduced effectiveness of these genes in contrast to that of H7H8 was demonstrated in the tests with wheat selections and controls. Although the highest level of virulence was found to H3 and H6, the frequency of biotype 'GP' in these populations would indicate that all presently deployed genes would provide moderate to high levels of resistance. IN9811 and CI17960-1-1-2-4-10 were highly resistant to the Dusty, WA and Lewiston, ID populations. The level of resistance in 9547B1-12 was somewhat lower to both populations (81 to 92%). The effectiveness of H13 resistance was very encouraging since this is carried in a soft winter wheat background with excellent agronomic traits and could be incorporated into wheat varieties adapted to the Pacific Northwest. Hessian fly resistance was demonstrated in some barley accessions evaluated in greenhouse test. Results on barley screening tests will be discussed.