We evaluated effects of entomopathogenic nematodes on the fitness cost of resistance to Bt toxin Cry1Ac in the pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae). P. gossypiella is a major pest of cotton in the United States. In the southwestern United States, it is currently controlled with transgenic cotton that produces Cry1Ac. Although resistance to Cry1Ac has not increased in field populations of P. gossypiella, laboratory selection has produced several resistant strains, indicating that field-evolved resistance remains a threat. In conjunction with refuges of non-Bt cotton, fitness costs can help to delay resistance. We hypothesized that the fitness cost of Bt-resistance can be increased by exposure to entomopathogenic nematodes. To test this hypothesis, we compared the fitness cost of Bt resistance across several concentrations of nematodes for two hybrid populations of P. gossypiella, both of which contain Bt-resistant and Bt-susceptible individuals. We tested both Arizona-native and commercially available entomopathogenic nematodes. Our data address the extent to which incorporating entomopathogenic nematodes into an integrated pest management strategy might slow or prevent the evolution of resistance to Bt toxins.