Mosquitoes have been the ideal transmitters of a wide variety of disease agents due to their repeated blood feedings over the life span. Culex quinquefasciatus (Say) is an important disease vector in the Southeastern USA. Mechanisms of resistance in mosquitoes have attracted attention, since they elucidate pathways of resistance development and help those designing novel strategies to prevent or minimize the spread and evolution of resistance. A mosquito strain of Cx. quinquefasciatus, HAmCq, from Huntsville, Alabama, has been established and further selected with permethrin in the laboratory. HAmCq parental and selected mosquito strains exhibit high levels of resistance to pyrethroids. The results of our current study indicate that high levels of pyrethroid resistance in the HAmCq mosquitoes are conferred by multiple mechanisms, including P450 monooxygenase-, hydrolase- and/or GST-mediated detoxification. Incomplete suppression of pyrethroid resistance by PBO, DEF and/or DEM suggests that one or more additional mechanisms are involved in overall resistance of HAmCq. The L-to-F kdr mutation of the sodium channel, the target site of pyrethroid insecticides, plays a very important role in pyrethroid resistance. Our study reveals multiple genotype isoforms of the sodium channel gene presented in Cx. quinquefasciatus. These results strongly suggest the role of posttranscriptional regulation in the connection of the sodium channel genotype and its mutation-mediated resistance phenotype in Culex mosquitoes.