1. Changes in the frequency of Cry1Ac resistance genes and shifts in tolerance of cotton bollworm, Helicoverpa armigera, to the Cry1Ac toxin were assessed using bioassays of F1 and F2 offspring of isofemale lines from Anci County of Hebei Province (a multiple-crop system including corn, soybean, peanut, and Bt cotton) and Xiajin County of Shandong Province (an intensive Bt cotton planting area) in Northern China during 2002?2005.2. A conservative analysis of the overall results indicated that there was a small increase in the frequency of major, non-recessive resistance genes over time.3. The relative average development ratings [RADR ? growth rate of a line on a Bt diet in proportion to the growth rate on a non-Bt (NBT) diet] of the bollworm larvae in F1 tests increased significantly from year to year, indicating a gradual trend towards higher tolerance to Cry1Ac in the field populations.4. There were also significant positive correlations between RADR of the lines in the F1 generation and the RADR of their F2 offspring, indicating that the tolerance was genetically based.5. Quantitative genetic simulation analysis showed that resistance of H. armigera to Bt cotton in Xiajin could evolve to a high level in 11?15 years if no effective resistance management measures are carried out.