Baseline susceptibility of the Okra shoot and fruit borer, Earias vittella (Fabricius) (Lepidoptera: Noctuidae), for the Cry 1Ac protein was determined from twenty populations collected from okra growing regions in India. The Cry 1Ac diet incorporation bioassays were carried out with the neonate larvae of the field-collected populations, and mortality and growth inhibition were evaluated after 7 d. There was 5-fold interpopulation variation in the insect susceptibility to the Cry1Ac protein indicated by mortality, whereas there was 10-fold variation in growth inhibition. The baseline data was used to estimate two candidate diagnostic concentrations, LC₉₉ (1.65 ppm) and EC₉₉ (0.50 ppm), that cause 99% mortality and growth inhibition, respectively. Studies are in progress to validate the candidate diagnostic concentrations using field-collected populations from India.