Behavior of third-instarHeliothis virescens(F.) was observed on six genetically modified cotton lines (transgenic) expressing the δ-endotoxins produced by altered genes fromBacillus thuringiensisBerliner 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<2% for all transgenic lines. Some transgenic lines had no larvae surviving the 10-d confinement. Percentage of damaged flower buds and bolls on all the transgenic lines were less than on ‘Coker 312’. Some transgenic lines had no damaged flower buds or bolls compared with 64 and 74% damaged, respectively, for ‘Coker 312’. Another greenhouse experiment was conducted where transgenic lines and nonexpressing ‘Coker 312’ controls were inoculated with 100 eggs ofHelicoverpa 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 ofH. virescensandH. zeadamage and for reduction of insect control costs in production agriculture.