Studies were carried out to determine the baseline susceptibility of the American bollworm, Helicoverpa armigera to Bacillus thuringiensis var. kurstaki (B.t.k.) HD-1 and HD-73 strains for the populations collected from different places in India. The populations from Delhi (field), Raichur (F1 and F2) and Bangalore were least susceptible to the toxicity of B.t.k. HD-1. On the contrary, the Hyderabad and Madurai populations were most susceptible. B.t.k. HD-1 caused neonate mortality of 37.4% at 10 ppm and 68.6% mortality at 100 ppm after 96 h of treatment. Mehna and Nagpur populations were least susceptible to B.t.k. HD-73 whereas Guntur, Bapatla, Hyderabad, Madurai and Vijayawada populations were most susceptible. HD-73 caused mortality of 62.3% at 100 ppm and 91.7% at 500 ppm after 96 h of treatment. Temporal variations of insect mortality showed that LC50 (96 h) of B.t.k. HD-1 increased from 9.47 ppm in October 1998 to 51.04 ppm in December 1998 for the insect population on pigeonpea in Delhi. The baseline susceptibility studies show that there is a possibility of tolerance (resistance) in some populations. These studies are discussed in relation to their importance vis-à-vis growing use of B.t.k. and future cultivation of B.t. transgenic crops in the country.