Field studies were conducted to measure the direct insecticidal activity of transgenic cotton expressing delta-endotoxin of Bacillus thuringiensis (BT cotton) against Heliothis virescens (F) the tobacco budworm, as well as the combined effects of BT cotton and natural mortality on survival of tobacco budworm. The direct effect of BT cotton was studied by measuring the survival of caged tobacco budworm larvae of various ages on various plant structures for various lengths of exposure. Survival of H. virescens was not affected by the plant tissue on which the larvae were caged. Larvae which were exposed to BT cotton for only 24 h had higher initial survival than larvae exposed for 48, 72, and 96 h. Four-day-old larvae survived at significantly higher rates than 1- and 2-day-old larvae. The combined effect of BT cotton and natural enemies on tobacco budworm was studied in subplots, each composed of two adjacent 1-meter row of BT and CK cotton, contained within 5-acre main plots of BT and CK cotton. Measurements of the number of surviving larvae, fruit damage and yield was made in each meter of the subplots. The experiment was repeated at the pinhead square, first bloom, and peak bloom stages of plant development. At all phenological stages, the BT-meter subplots suffered less fruit damage and had lower larval survival at 2-days post-infestation than the CK-meter subplots; however, there were no differences in yield between BT and CK-meter subplots within a main plot, except for infestation at pinhead square.