Susceptibility of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae) field populations to the Cry1Ac toxin of Bacillus thuringiensis Berliner (Bt) were monitored from 1997 to 2004 in China. During 2001–2004, 53 strains from the Bt cotton planting region were sampled. The range of concentration producing 50% inhibition of larval development to third instar (IC₅₀) values among different populations in 2001, 2002, 2003, and 2004 was 0.014–0.046, 0.010–0.062, 0.005–0.062, and 0.005–0.035 μg/ml, respectively. Diagnostic concentration studies (IC₉₉) showed that the percentage of individuals reaching third instar ranged from 0 to 9.09%, with only four of the 53 tested populations showing values above 0%. Considering these data, it was determined that the susceptibility to Cry1Ac of the field populations sampled was not different from the baseline in 1997, and no movement toward resistance among H. armigera populations was apparent.