In 1996 and 1997 field collections of North Carolina tobacco budworm larvae, Heliothis virescens (F.), were established as a laboratory (parental) strain. Larvae from this colony were repeatedly selected for spinosad resistance by topical application of technical spinosad (a mixture of spinosyns A and D) over 13 generations. Topical LD50s 18 days after treatment were 0.11 µg of active ingredient per larva for the parental (susceptible) strain and 73.55 µg per larva for generation (G) 19 of the selected strain. When the resistant and the susceptible strains were crossed, and the F1(R x S) budworms then backcrossed to the resistant strain, spinosad resistance was found to be the result of a partially recessive, single gene. The F1 budworms were estimated to be 4- to 5-fold resistant compared to the susceptible strain. The stability of resistance in the absence of immigration and exposure to spinosad for 5 generations in a cohort of the resistant strain was examined. The LD50 decreased by 1.4-fold compared to the resistant budworms. Comparing the developmental biology of the susceptible (parental) and resistant strains, resistant males developed a little slower as larvae and emerged as adults later than susceptible males and had slightly smaller 1 day old pupal wet weights. When susceptible (S) and resistant (R) moths of both sexes were allowed to mate freely in a mixed population (80%R X 20%S), the majority of the offspring (84.6%) were susceptible to spinosad. This indicates reduced reproductive competitiveness for the resistant strain.