The baseline toxicity of Cry1A i-endotoxins (derived from Bacillus thruringiensis (Kurstaki)) on field populations of the cotton bollworm, Helicoverpa armigera (Hub.), was determined through log dose probit analysis. All the three Cry1A proteins were found to be toxic to the bollworm larvae. Cry1Ac was found to be the most toxic followed by Cry1Aa andCry1Ab. LC50 values ranged from 0.07 to 0.99 wg/ml(14- fold) for Cry1Aa, 0.69 to 9.94 wg/ml (14-fold) for Cry1Ab and 0.01 to 0.67 wg/ml of diet (67-fold) for Cry1Ac. The LC50 values deduced from the cumulative log dose probit response of the data pooled from all assays, were 0.62 wg/ml for Cry1Aa, 4.43 wg/ml for Cry1Ab and 0.100 wg/ml of diet for Cry1Ac. The respective LC50 values represent the baseline susceptibility indices for resistance monitoring through the conventional log dose probit assays. The LC99 values derived from the cumulative data were 515 wg/ml for Cry1Aa, 13385 wg/ml for Cry1Ab and 75 wg/ml of diet for Cry1Ac. These values represent the diagnostic doses for routine monitoring of resistance to the respective toxins through discriminating dose assays after the introduction of Bt transgenic crops.