Behavior of third-instar Heliothis virescens (F.) was observed on six genetically modified cotton lines (transgenic) expressing the δ-endotoxins produced by altered genes from Bacillus thuringiensis Berliner var. kurstaki (strains HD1 or HD73). On transgenic lines the percentage of time larvae spent feeding was reduced to about 18% and resting increased to about 60% compared with 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 ‘Coker 312’. Reduced feeding and increased plant abandonment on some transgenic lines may be due, in part, to midgut paralysis and/or repellency of the δ5-endotoxin. Two cage experiments (one greenhouse and one field), were conducted to evaluate H. virescens survival and plant damage on the transgenic lines. Larval survival was Helicoverpa zea (Boddie). Percentage of damaged flower buds and bolls was 2 and 6%, respectively, on a transgenic line expressing the HD73 endotoxin, compared with 54 and 55%, respectively, for the control. These data indicate that transgenic cottons offer excellent potential for suppression of H. virescens and H. zea damage and for reduction of insect control costs in production agriculture.