Results from conventional resistance monitoring and F₂ screens have found no evidence of major resistance alleles to Cry1Ac in the bollworm, Helicoverpa zea (Boddie), in US cotton, Gossypium hirsutum L., fields. Furthermore, sentinel field plot results show no changes in field performance of Cry1Ac cotton despite 14 years of commercial use. Although insecticide-resistant management practices such as refugia, insecticide oversprays, and pyramided traits in Bt cotton have likely contributed to the lack of observed resistance development and changes in field efficacy, fitness costs associated with the development of Cry1Ac resistance in bollworm probably also have contributed to this outcome. This paper discusses various fitness costs associated with Cry1Ac resistance in bollworm and concludes with a discussion of how exclusive use of diet-based laboratory bioassays may underestimate their role in delaying development of resistance to Bt.