Abstract: Development of Bt-susceptible and resistant Helicoverpa armigera and their reciprocal crosses feeding on Bt cotton leaves for 5 d were compared，and individuals ≥0.6 mg/larva and surviving beyond the mid-2nd-instar on Bt cotton leaves for 5 d were considered as the standard for homozygous resistant individuals. In 2006，the frequency of allele conferring resistance to Bt cotton in Qiuxian County，Hebei Province，was estimated by using the F₁ screening method. The results showed that 24 out of 127 field-collected male moths were detected to carry resistance alleles and the resistance allele frequency was estimated as 0.094 （95%CI： 0.044–0.154）. This is the first report that resistance allele frequency increases to such a high level in fields in China. Long-term adoption of Bt sprays，dominant planting of uni-toxin-producing Bt cotton，and lack of conventional cotton refuge system may accelerate the resistance evolution in this region，and it is necessary to establish and implement effective resistance management strategy as soon as possible. Factors influenced resistance evolution and the resistance risk in fields were discussed as well.

Key words: Cotton bollworm (Helicoverpa armigera), transgenic Bt cotton, resistance monitoring, F1 screening method