Susceptibility to the Cry1Ac protein from Bacillus thuringiensis was determined for 17 populations of Helicoverpa armigera (Hübner) (Lepidoptera: Noctuidae), collected from vegetable growing areas in India. Neonate larvae from the field-collected populations were exposed to artificial diet incorporated with Cry1Ac protein at different concentrations, and mortality and growth inhibition were evaluated after 7d. There was 10-fold interpopulation variation in the insect susceptibility to the Cry1Ac protein indicated by mortality, whereas there was 20-fold variation in growth inhibition. The lethal concentration (LC₅₀) ranged from 0.01 to 0.11 µg of Cry1Ac / ml of diet and the effective concentration (EC₅₀) ranged from 0.0001 to 0.002 µg of Cry1Ac / ml of diet. The baseline data was used to estimate two candidate diagnostic concentrations, LC₉₉ and EC₉₉, that cause 99% mortality and growth inhibition, respectively. Studies are in progress to validate the candidate diagnostic concentrations using field-collected populations from India.