Hessian fly, Mayetiola destructor (Say), is one of the major pests of wheat, which cause enormous economic loss. Several lines of evidence suggest that Hessian fly larvae inject saliva into host plants during feeding and cause severe stunting and plant death at seedling stage. To identify components injected into host plants by the Hessian fly, we adopted an EST approach to identify and characterize cDNAs and genes that encode proteins with a secretion signal peptide. BLAST nucleotide similarity searches indicated that two of the identified clones code for protease inhibitors. Northern blot data shows that the identified clones are expressed only at first instar stage, which is critical for the larvae to establish infestation. Southern analysis shows that these genes are present in multiple copies in the genome. Full-length cDNAs were expressed in E. coli system. Purified proteins exhibited inhibitory activity towards commercial serine proteases. Studies are underway to further characterize individual inhibitors biochemically.