A series of field studies evaluated the performance of transgenic cotton expressing the Vip3A insecticidal proteins against heliothine larvae in Louisiana. Larval infestations and fruiting form injury were estimated by sampling 25 structures weekly through out the production season. To confirm efficacy levels, selected transgenic and conventional (non-Bt) plants were artificially infested with heliothine larvae. Plants were visually inspected after 3 d for damage to the fruiting structure at the site of infestation and for the presence of larvae. Thereafter, whole plants were inspected every 2 d for damage to flower buds (squares), flowers, and fruit (bolls) until no larvae were detected. In both studies, heliothine larvae damaged more fruiting forms on the conventional non-Bt plants than on the Vip3A cotton plants.