The voltage-gated sodium channel mediates the rapid rising phase of action potentials in almost all excitable cells and is a molecular target of a variety of neurotoxins including pyrethroid insecticides. Most studies have focused on the expression of sodium channel genes in the adult stage, information on other developmental stages, however, is limited. In this study, we characterized the para sodium channel orthologous gene (BmNa(v)) of the silkworm Bombyx mori, a model insect of Lepidopteran species. The BmNa(v) gene covers a 31 kb genome region and contains 36 exons. The longest ORF contained 6258 bp and encoded 2085 amino acid residues, which shares 74%, and 77% overall amino acid sequence identities with the sodium channel proteins from Drosophila melanogaster and Blattella germanica, respectively. Using high-throughput Solexa sequence technology we conducted sequence analysis of BmNa(v) cDNAs from embryos, larvae, pupae and adults of the silkworm, identified alternative splicing sites and determined the frequencies of these splicing events in four developmental stages. Three optional exons, two sets of mutually exclusive exons, and one internal spliced exon were identified. One optional exon is unique to BmNa(v), while the others are conserved in other insect sodium channel genes. Interestingly, the expression of the mutually exclusive exons is developmentally regulated.