The diet incorporation technique has been used to detennine the variability in the baseline susceptible response to Bt in a number of field populations of both Helicoverpa armigera & Helicoverpa punctigera. The commercial Bacillus thuringiensis subspecies kurstaki formulation DiPel 2X (32,000 IU/mg) was incorporated into the standard soyflour/ wheat germ/ agar artificial diet. Newly moulted early third instar larvae were confined on the treated diet in 24 well tissue culture trays for up to 8 days at 25 degrees C. The criterion for assessing mo1tality was the same as for conventional insecticides: no co-ordinated movement when prodded. LCSOs are expressed in mg DiPel 2X per ml of diet.