Our dataset on susceptibility of Helicoverpa zea and Heliothis virescens to insecticidal proteins expressed in Bt cotton is large, spans nearly two decades and may be suitable for exploration of a number of insect resistance management questions. Both species exhibit wide variability among field populations in response to toxins incorporated into meridic diet, and both species can be selected for elevated responses to toxin challenge in diet assays. Shifts in measured susceptibility over time and geographic location may be important indicators of changing levels of field resistance, but more work is needed to associate variability observed in diet assays with variability in survival of insects on toxin expressing plants in the field.