Abstract: A cyhalothrin-resistant strain(BaoD-R) was established by selecting 40 generations of cotton bollworm, Helicoverpa armigera Hübner collected from cotton field of Baoding county, Hebei province on Aug. 1992 by cyhalothrin during 19921997. Resistance ratio to the insecticide has been 117-fold in BaiD-R strain compared with the homologous control population (BaoD-S) without any insecticide treatment since the collection based on LD 50 values. Coding sequence of 300bp DNA fragment of domain III-IV linker of cotton bollworm -para- homologous sodium channel gene was amplified using RT-PCR. The results showed that there are 4 nucleotide differences of the DNA fragment between BaoD-R resistant and BaoD-S susceptible strains, but no any difference in their deduced amino acid sequences. Alignment of the deduced amino acid sequence of the amplified DNA fragment with those of the para gene of -D. melanogaster, hscp(para- homologous) gene of Heliothis virescens, and CSMA (para homologous) gene of Blattella germanica showed that there are 88.9%, 98.1%, and 93.5% amino acid identity in Helicoverpa armigera compared with the other 3 species, respectively.