Pink bollworm larvae from eggs of the APHIS laboratory strain were exposed to concentrations of a commercially available formulation of Bt or to concentrations of lyophilized transgenic plant parts incorporated into a standard laboratory diet. Dose response curves for the Bt formulation showed that as little as 21 IU/ml (0.1 mg/ml) of diet produced high larval mortality and significantly slowed larval development. Doses that exceeded 0.4 mg/ml of lyophilized plant material from the T81 strain had significant effects on both numbers of pupae and on larval development. Different strains of transgenic plants had different effects on the mortality of the larvae developing in the formulated diets. Strains T624 and T249 had no significant effect on larval development than control leaf material at any dose up to 6 gm of leaf material per 1.51 of diet.