Technical Abstract: Over 10,000 Expressed Sequence Tags (ESTs) were generated and assembled into 2,007 clusters (contigs and singletons). The most striking feature of the Hessian fly larval transcriptome is the existence of a large number of transcripts coding for so-called small secretory proteins (SSP) with amino acids less than 250. Eleven of the 30 largest clusters were SSP transcripts with the largest cluster containing 11.3% of total ESTs. Micorarray and qPCR analyses of representative SSP transcripts revealed that most of them were predominantly present in the gut tissue and the transcript levels of many SSP were affected by plant types on which larvae feed. Transcripts coding for diverse digestive enzymes, detoxification and metabolic proteins were also identified. The putative digestive enzymes included serine proteinases (trypsins and chymotrypsins), cysteine proteases, aspartic protease, endo-oligopeptidase, aminopeptidases, carboxypeptidases, and a-amylases. Putative detoxification proteins included cytochrome P450s, glutathione S-transferases, peroxidases, ferritins, catalase, peroxiredoxins, and others. This study represents the first global analysis of gut transcripts from a gall midge. The identification of a large number of SSP transcripts in the Hessian fly larval gut provides a foundation for future study on the functions of these genes. The impact of plant types on the expression of SSP genes suggests that the SSP genes may play critical roles in Hessian fly – plant interactions and are potentially useful targets for pest management. The discovery of transcripts encoding various kinds of digestive enzymes and detoxification proteins also provides important sources for future studies on these groups of genes.