To determine the effect of insecticide pressure on a series of generations of a field-collected strain (FCS) of the tobacco budworm, Heliothis virescens (F.), permethrin was topically applied to mid-to-late third instar larvae in the first and sixth through thirteenth generations from the field and LD-50 and LD-95 values were recorded 48 and 96 h post-treatment. Larval numbers were increased over generations 2-5. The LD-50 values after 48 h for generations 6-8 were significantly (the 95% confidence interval overlaps) lower than that for generation 1, but values for generations 1 and 9-12 did not differ. Also, the LD-50 values for generation 13 dropped significantly, indicating that resistance was not enhanced by selection. Confidence intervals for LD-50 values after 96 h and LD-95 values after 48 and 96 h indicated wide variation in response. Resistance was observed after treating progeny of a single pair in generation 11 and of 20 mass-mated pairs in generation 12, but in the next two generations, the LD-50 values did not differ significantly from the LD-50 for the first generation from the field although even these values significantly exceeded those for a laboratory strain treated simultaneously.