Transgenic cotton expressing Bacillus thuringiensis endotoxins (Bt cotton) has been widely adopted to control major cotton insects. Resistance development in target insects to Bt cotton is a main threat to Bt cotton technology which is a potential strategy for long-term cotton insect management program. Field surveys showed an increasing trend of population density of the cotton bollworm, Helicoverpa armigera (Hübner), since 2001 in Qiu County of Hebei Province in China, where Bt cotton has been planted commercially since 1998. This region was therefore selected in 2006 and 2007 for estimating frequency of gene alleles conferring resistance to Bt cotton in field population of the bollworm using the F₁ screening technique (i.e. single-pair mating field-collected male to laboratory resistant female). F₁ offspring from each single-pair were screened for resistance alleles based on larval growth rate and developmental speed on Bt cotton leaf for 5 days. Results indicated that a high level of resistance allele frequency was detected in H. armigera.