Laboratory experiments were conducted to evaluate the behavior of bollworm, Helicoverpa zea (Boddie), and tobacco budworm, Heliothis virescens (F.), larvae on meridic diet with different concentrations of the Cry1Ac and Cry2Ab proteins from Bacillus thuringiensis subsp. kurstaki Berliner. The proteins used in these experiments are the ones in commercially available Bollgard and Bollgard II cotton. Both bollworms and tobacco budworms selectively fed on nontreated diet compared with diet treated with Cry1Ac. In addition, bollworms exhibited a concentration response with Cry1Ac. In general, bollworms selected diet with low concentrations of Cry1Ac compared with diet with higher concentrations of Cry1Ac. For Cry2Ab, the avoidance was not as prominent as that observed for Cry1Ac. Based on results from no-choice assays, the Cry1Ac and Cry2Ab concentrations used in choice assays represented a wide range of biological activity on both species. The lower concentrations provided low levels of mortality, whereas the higher concentrations provided high levels of mortality. Also, the developmental times of larvae were longer at higher concentrations of both proteins. These data provide important information about the behavioral response of key cotton pests to the B. thuringiensis proteins found in commercially available transgenic cotton. This information will be important to develop accurate scouting and management procedures for Bollgard and Bollgard II cotton.