The Russian wheat aphid, Diuraphis noxia (Kurdjumov) is one of the most destructive insect pests of wheat, Triticum aestivum L., and Barley, Hordeum vulgare L., in the agricultural industry. The use of D. noxia resistant cultivars came into existence due to difficulties in managing the D. noxia pests with other insect pest management strategies like insecticides or biological controls. Several D. noxia (Dn) genes from wheat have been used to develop cultivars resistant to D. noxia. The recently originated virulent (Biotype 2) found in the United States is virulent to all the Dn genes except for the Dn7 gene from rye and Dnx from wheat. There is a significant need to identify and characterize the unique sources of D. noxia resistance to both biotypes 1 and 2. This has been accomplished by constructing gut cDNA libraries from both the biotypes fed on Dn4 (susceptible) wheat plants at different time points. An overall of 4000 clones from biotypes 1 and 2 were sequenced using an ABI 3700 DNA analyzer and the analysis is in progress. Virtual subtraction will be done to identify subset of genes expressed in each of the cDNA libraries, thus enabling us to study the sequences of similarity and homology in both up and down regulated genes. Northern blots will be done to confirm the expression of the genes of interest. A detailed understanding of the molecular basis of both the virulence and avirulence biotypes will complement in finding newer mechanisms and strategies for plant resistance.