A laboratory bioassay was used to evaluate the effect of Dimilin in combination with transgenic cotton (Bollgard^TM), which incorporates the genes to manufacture the Bacillus thuringiensis (Bt) endotoxin, on several lepidopteran species. Bollgard was highly effective in minimizing leaf feeding and causing mortality of third instar larvae of tobacco budworm (Heliothis virescens), cotton bollworm (Heliocoverpa zea) and soybean looper (Pseudoplusia includens), but was less efficacious for beet armyworm (Spodoptera exigua) and fall armyworm (Spodoptera frugiperda). For beet armyworm Dimilin (diflubenzuron) in combination with Bt cotton was more effective in reducing larval leaf feeding and increasing larval mortality than either Dimilin or Bt cotton alone. Instances of synergistic responses to Dimilin and Bt cotton for both larval feeding and mortality were found for this species. Although Dimilin alone was highly effective on fall armyworm, the advantage of Dimilin on Bt cotton over Dimilin on non-Bt cotton was not evident for this species.