The sunflower moth,Homoeosoma electellum(Hulst), a major pest of commercial sunflower in North America, is susceptible toBacillus thuringiensisBerliner. Besides being used as a biological insecticide, theB. thuringiensisendotoxin gene may be used to create transgenic sunflower (Helianthus annuusL.). Acute and chronic effects and resistance to the pathogen were evaluated in two populations of the sunflower moth. A susceptible sunflower moth population and a laboratory-selected,B. thuringiensis-resistant population did not differ in mortality, developmental periods, pupal weight, female/male ratios, or fecundity whenB. thuringiensiswas not present. However, when larvae were reared on diets treated withB. thuringiensis, the susceptible population had a significantly greater mortality, especially of females, than did the resistant population. Sublethal effects on the susceptible population included reduced pupal weights and lowered fecundity in surviving females. In the resistant population, females were not more likely to die than males, and egg deposition was normal when larvae were treated withB. thuringiensis. Under present cultivation conditions, management ofB. thuringiensisuse in sunflower is not needed to prevent the development of aB. thuringiensis-resistant sunflower moth.