An investigation into the toxicity and resistance mechanisms of the first laboratory strain of tobacco budworm that is resistant to spinosad determined that the toxicity ration for spinosad based on differences in the LC(50) between the parental and selected strains in a larval dip bioassay was 245 fold. Insects were also resistant when fed spinosad treated diets for 48 hours. The resistance ration increased from 150 fold one day after treatment to 2,670 fold four days after treatment. When selected budworms were placed on spinosad treated cotton for 48 hours, the resistance ration was only 91.1 fold. The selected strain was also resistant by injection. The parental strain six day LD(50) was 0.059 microgram per larva. Ten micrograms per larva produced only 30 percent mortality in the selected strain. Neither the rate of penetration across the cuticle or the metabolism of 2'-O-[(14)C]-methyl spinosyn A accounted for differences in spinosad susceptibility. 16 refs.