The widespread use of crops engineered to produce toxins from the bacterium Bacillus thuringiensis (Bt) imposes strong selection on insect populations to evolve resistance. In conjunction with a high dose/refuge strategy, fitness costs of Bt resistance can slow or prevent the evolution of resistance. The pink bollworm Pectinophora gossypiella is a major pest of cotton in the southwestern United States currently controlled by transgenic cotton that produces the Bt toxin Cry1Ac. We tested whether two species of entomopathogenic nematodes, Steinernema riobrave and Heterorhabditis bacteriophora, increase the fitness cost of resistance to Bt cotton in pink bollworm, and whether this effect is altered when larval diet included the cotton phytochemical gossypol. Our assessment of fitness costs was combined with measurements of nematode reproduction, as an indictor of the potential persistence of these nematodes in agroecosystems. Our results cast light on the potential for tritrophic interactions to enhance resistance management programs by slowing or reversing the spread of Bt-resistance alleles.