The behavior of third-instar Heliothis virescens (F.) was observed on six genetically-modified (transgenic) Coker 312 cotton lines (Bt) expressing the ë-endotoxins produced by altered genes from Bacillus thuringiensis var. kurstaki. On the transgenic Bt lines the percentage of time larvae spent feeding was reduced to 18% and resting increased to 60% compared to 50% and 33%, respectively, for the nontransgenic control, Coker 312. Larvae on some of the transgenic lines spun-down (i.e., dropped off the plant on a silken thread) and abandoned plants more frequently than larvae on the control. Reduced feeding and increased plant abandonment on some transgenic lines may be due, in part, to midgut paralysis and/or repellency of the ë-endotoxin. Two cage experiments (glasshouse and field) were conducted to evaluate H. virescens survival and plant damage on the Bt lines. Larval survival was less than 2% for all Bt lines. Some Bt lines had no larvae surviving the 9 or 10-day-confinement. Percent damaged flower buds and bolls on all the Bt lines were less than Coker 312. Some Bt lines had no damaged flower buds or bolls compared to 64% and 74% damaged, respectively, on the control. Another glasshouse experiment was conducted where each plant was inoculated with 100 eggs of Helicoverpa zea (Boddie). The number of larvae surviving for 14 days on two of the Bt lines was lower than a nonexpressing control. Percent undamaged flower buds and bolls were 98 and 94%, respectively, on one Bt line, compared to 46 and 45%, respectively, for the control. Results with these genetically-modified cottons indicate that they offer excellent potential for suppression of H. virescens and H. zea damage to cotton, and for reduction of insect control costs in production agriculture.