Mosquitoes play an important role as vectors for a variety of pathogens. Insecticide use for control means results in selection of tolerant/resistant mosquitoes. An esterase cDNA was isolated from Culex nigripalpus Theobald (Diptera: Culicidae), and its spatial expression was characterized. GenBank tblastx and VectorBase searches with the translation product of the longest cDNA fragment (397 bp, CN Temsha est-1) of the Cx. nigripalpus esterase revealed similarity (95%) to the estalpha2 esterase gene product of Culex pipiens quinquefasciatus Say, suggesting it is a member of the esterase family. Transcripts encoding Cx. nigripalpus esterase were detected in midgut tissue, thoraces, and abdomens of female Cx. nigripalpus, findings that may support possible roles in feeding, digestion, or reproduction. Gene expression studies to determine the role of Cx. nigripalpus esterase in the mosquito are being conducted.