The cotton bollworm, Helicoverpa armigera, is one of the most important insect pests in cotton growing regions of China. Transgenic cotton that expresses a gene derived from the bacterium Bacillus thuringiensis (Bt) has been deployed for combating cotton bollworm since 1997. Natural refugees derived from the mixed planting system consisting of cotton, corn, soybean, vegetables, peanut and others on single-family farms of a small scale were used for delaying the evolution of resistance to Bt cotton. Susceptibility of H. armigera field populations to the Bt insecticidal protein Cry1Ac was monitored from 1997 to 2006. The results indicate that the field populations are still susceptible to Cry1Ac, and monitoring indication no apparent shifts in susceptibility in field populations of this important pest.