Transgenic cotton plants (Bt cotton) expressing an insecticidal protein, delta-endotoxin, have been developed to reduce crop injury from certain Lepidoptera. Little is known about the feeding preference, growth, and survival of these insects in mixed stands of Bt and non-Bt cotton. A feeding preference study of tobacco budworm, Heliothis virescens (F.), and bollworm, Helicoverpa zea (Boddie), and an age specific (instar specific) mortality study of bollworm were conducted in 1992 at the Texas A&M Research and Extension Center, Corpus Christi, TX. The objectives were to: (1) determine if third instar budworm/bollworm showed a feeding preference for flower buds of non-Bt cotton compared to transgenic Bt cotton; and (2) to determine percent survival, length of immature stage, and larval and pupal weight of each bollworm instar on flower buds of Bt and non-Bt cotton. Each instar was reared on diet until it reached the age (=instar) to be bioassayed. At each instar a cohort of larvae were fed on flower buds of Bt or non-Bt cotton. In the preference study, significantly more bollworms were found on non-Bt flower buds (100%) than Bt flower buds (30%) at 2 hours after infestation while differences were not observed in the number of bollworm larvae on Bt and non-Bt flower buds at 20-49 hours after infestation in a no-choice study. In the age specific mortality study, the first, second, third, or fourth instar larvae did not survive to the pupal stage when fed on Bt flower buds. A higher percentage of fifth instar larvae fed on non-Bt flower buds (81.3%) pupated compared with Bt flower buds (51.3%). Higher percentages of fifth instar larvae fed on non-Bt flower buds (48.7%) survived to the adult stage compared with larvae fed on Bt flower buds (29.3%). This study showed that Bt cotton influenced tobacco budworm and bollworm feeding preference and also increased mortality of bollworm larvae in all instars.