A laboratory strain of the tobacco budworm, Heliothis virescens, originally collected from North Carolina was selected topically with technical spinosad (spinosyns A and D) for successive generations, producing high levels of resistance. The toxicity ratio for spinosad based on differences in the LC₅₀ between the parental and selected strains in a larval dip bioassay (48 h after treatment) was 245-fold. Insects were also resistant when fed on spinosad-treated artificial diet for 48 h. The resistance ratio increased drastically depending on when mortality was assessed. At 1 day after the treatment the resistance ratio was 150-fold while at 4 days the value increased to 2670-fold. When the selected budworms were placed on spinosad treated cotton for 48 h, the resistance ratio was only 91.1-fold. The selected strain was also resistant by injection. The parental strain six day LD₅₀ was 0.059 g/larva while 10 g/larva produced only 30% mortality in the selected strain. Preliminary studies indicated no differences in the rate of penetration across the cuticle or metabolism of 2 -O-[¹⁴C]-methyl spinosyn A between susceptible and selected larvae suggesting that these mechanisms are not contributing to the observed differences in spinosad susceptibility.