Characterization of the enzymes involved in the chitin biosynthetic pathway in mosquitoes is critical due to the importance of chitin in the formation of the peritrophic matrix [PM] and its potential impact on vector competence. Chitin is the homopolymer of the amino sugar N-acetyl-D glucosamine [GlcNAc]. The final step of incorporation of GlcNAc into the chitin polymer is catalyzed by the enzyme chitin synthase [CS]. CS is a membrane bound enzyme, but the mechanism of its action in the biosynthesis of the PM is not understood. We have isolated and sequenced a CS-encoding cDNA clone from the mosquito Aedes aegypti, compared its sequence with CS from other organisms and studied its RNA expression. The cDNA is 3.5 kb in length with an open reading frame of 2.6 kb that encodes a protein of 865 amino acids with a predicted molecular mass of 99.5 kDa. The putative translation product shares 90% similarity to two CS proteins from Caenorhabditis elegans and 50% similarity to Saccharomyces cerevisiae in the catalytic domain of CS enzymes. Data suggest that CS is a single copy gene. RT-PCR analysis shows CS message in whole non-blood-fed females, whole blood-fed females, non-blood-fed midguts and in midguts dissected at different time points post-blood-feeding. In situ hybridization studies of midgut samples revealed that CS mRNA increases following a bloodmeal and is localized to the periphery of the epithelial cells facing the midgut lumen.