The influence of Bacillus thuringiensis Berliner (Bt) cotton on population dynamics of cotton aphid, Aphis gossypii Glover, was investigated during 1999?2000 in northern China. The field experiments were conducted in plots of Bt cotton and conventional cotton that received no insecticide applications, and in plots of conventional cotton in which pyrethroid and organophosphate insecticides were used regularly for control of Helicoverpa armigera. The results indicate that resistance of cotton aphids to majority of insecticides used for control of H. armigera, and lower densities of predators in late June and early July caused by insecticide use, causes population densities of cotton aphids to become significantly higher in plots of insecticide-treated conventional cotton than in Bt cotton plots. These results suggest that Bt cotton planting not only played an important role in the control of H. armigera, but also efficiently prevented cotton aphid resurgence in response to insecticide use.