Mosquitoes in the Culex pipiens complex are the primary enzootic vectors for transmission of West Nile(WN) and St. Louis encephalitis(SLE) viruses in the United States. This vector species is therefore of important public health concern. Within this complex there is distinction between sibling species found in the north, Culex pipiens(L.1758) and those found in the south, Culex quinquefasciatus(Say,1823). Hybridization can result along the geographic range in which these two species overlap. Tennessee falls within this hybridization zone. In 2006 the Vector-Borne Diseases Branch of the Tennessee Department of Health, in conjunction with Memphis/Shelby County Vector-Control, collected Culex species mosquitoes. The aim was to characterize the prevalence of hybrid mosquitoes to determine if genetic differences in Culex populations may help explain, in part, differences seen in intensity of WN transmission in Memphis(high-intensity) vs. Nashville(low-intensity). Mosquitoes were collected using gravid traps from early June to late October, 2006. Eight ecologically distinct sites in Memphis/Shelby County and four ecologically distinct sites in Nashville/Davidson County were chosen. Mosquitoes were identified using primers designed for nucleotide sequence differences in the acetylcholinesterase gene(Ace.2) from a modified version of published protocols(Smith,2004 and Aspen,2003). Nucleic acid extractions from individual mosquitoes were used. We report preliminary results from our study. The majority of mosquitoes collected from sites in Shelby County in late-summer were identified as Culex quinquefasciatus. Mosquitoes collected in late-summer from sites in Davidson County were identified as either Culex pipiens or hybrids. The genetic composition of these mosquito vectors in conjunction with epidemiological data may provide insight into patterns of West Nile transmission seen along hybrid zones.