The para-type sodium channel in insects is the primary target of pyrethroid and DDT insecticides. However, modifications in the target protein structure such as point mutations or substitutions, resulting from single nucleotide polymorphisms (SNP), cause insensitivity of the insect's nervous system to pyrethroids and DDT and, in turn, result in insecticide resistance. Among these mutations, substitution of leucine to phenylalanine (L to F) in the 6th segment of domain II (IIS6) has been clearly associated with pyrethroid and DDT resistance in many insect species, including mosquitoes. Here, multiple copies of the sodium channel gene were identified in the mosquito Culex quinquefasciatus by Southern blot analysis and polymerase chain reaction (PCR) analysis. Two genomic DNA fragments of the mosquito sodium channel gene (509 and 181 bp) were detected by a single PCR primer pair. Sequence analysis indicated the lack of an intron sequence in the 181 bp sodium channel fragment. Single nucleotide polymorphism (SNP) analysis revealed a strong correlation among the frequencies of L-to-F allelic (T) expression at the RNA level, the frequencies and resistance allele (T) at the L-to-F site of the 509 bp genomic DNA fragment, which did include an intron sequence, and the levels of insecticide resistance. Taking together, this study, for the first time, not only revealed multiple copies of the sodium channel gene presented in the Culex mosquito genome but also suggested that the one with the intro sequence may be a functional copy of the sodium channel gene in the Culex mosquitoes. (C) 2011 Elsevier Inc. All rights reserved.