Transgenic crops that produce toxins from Bacillus thuringiensis Berliner (Bt) to control insects are grown widely, but rapid evolution of resistance by pests could nullify the benefits of Bt. Cotton bollworm, Helicoverpa ermigere Hübner, is an economically important lepidopteran pest that has experienced intense selection for resistance to Bt-Cry1Ac cotton in China since 1997. Since 2002, changes in the frequency of alleles for resistance and the quantitative shifts in tolerance to Cry1Ac were monitored for 7 years using bioassays of F₁ and F₂ offspring of isofemale lines of larvae of H. armigera from the field in Xiajin County of Shandong Province (an intensive Bt cotton planting area) and Anci County of Hebei Province (a multiple-crop system including maize, Zee mays L.; soybean, Glycine max L.; peanut, Arechis hypogaea L.; and Bt cotton, Gossypium hirsutum L.) in Northern China. Here, we summarized the monitoring data from Xiajin and Anci regions during 2002–2008. The monitoring results during this period indicated that the frequency of alleles for resistance has remained low in both regions of China. The results suggested that natural refugia may be part of an effective management strategy for delaying the evolution of resistance in H. armigera to Bt cotton in Northern China.