Technical Abstract: Evolution of resistance by pests can reduce the efficacy of transgenic crops that produce insecticidal toxins from the bacterium Bacillus thuringiensis (Bt). However, fitness costs can slow the evolution of resistance. We tested whether four species of entomopathogenic nematodes (Steinernematidae and Heterorhabditidae) could magnify fitness costs of resistance to Bt toxin Cry1Ac in pink bollworm, Pectinophora gossypiella (Lepidoptera: Gelechiidae). In laboratory experiments, the magnitude and dominance of fitness costs were significantly increased by the nematode Steinernema riobrave, but not by Steinernema carpocapsae, Steinernema sp. (ML18), or Heterorhabditis sonorensis. Results of computer simulations incorporating nematode-imposed fitness costs suggest that in a landscape where non-Bt crop refuges account for 20% of all fields, treating 8% of refuge fields with S. riobrave could substantially increase the delay in pink bollworm resistance to Bt cotton. We conclude that entomopathogenic nematodes could bolster management of insect resistance to Bt crops, but the success of this approach is likely to depend on careful selection of an appropriate nematode species.