Sensitivities of Helicoverpa armigera (Hübner) field populations to Bacillus thuringiensis (Bt) insecticidal protein Cry1Ac were monitored during 1998–2000 in China. A total of 41 strains was sampled, and most of them were collected from Bt cotton planting regions. The range of IC₅₀ values (concentration producing 50% inhibition of larval development to third instar) among different populations in 1998, 1999, and 2000 were 0.020–0.105 μg/ml, 0.016–0.099 μg/ml, and 0.016–0.080 μg/ml, respectively. Diagnostic concentration studies (IC₉₉) showed that the percentage of individuals reaching third instar ranged from 0 to 4.35%, with only eight of the 41 tested populations showing values above 0%. Also interesting was a trend over successive years in which fewer populations contained individuals that survived the diagnostic concentration by reaching third instar. Considering these data, it was determined that the field populations sampled during the 3-yr study were susceptible to Cry1Ac protein, and that movement toward resistance among H. armigera populations was not apparent.