The western corn rootworm (Diabrotica virgifera virgifera LeConte) is serious pest of corn, particularly in the Midwestern United States. One strategy used to manage western corn rootworm is the planting of transgenic corn that produces one or more Cry toxins derived from Bacillus thuringiensis (Bt). Refuges of non-Bt corn function to delay the development of resistance to these toxins. The greatest delay in resistance development is expected when the inheritance of resistance is recessive and there are associated fitness costs. We characterized the inheritance of resistance of two strains of western corn rootworm with field-derived resistance to the Bt toxin Cry3Bb1 (Elma and Monona) and tested for fitness costs of resistance. The inheritance of resistance to Cry3Bb1 was investigated by crossing resistant and susceptible western corn rootworm and evaluating the survival of their progeny on Bt using diet-based assays and experiments with whole plants and seedling mats. For both strains, resistance was found to be non-recessive. In a greenhouse experiment in which larvae were reared on whole corn plants in field soil, no fitness costs of resistance were detected for Monona. In a laboratory experiment with Elma, in which larvae experienced intraspecific and interspecific competition for food, a fitness cost of delayed larval development was identified, however, no other fitness costs were found. These findings underscore the need for rootworm resistance management beyond the refuge strategy. Multiple tactics such as crop rotation, the planting of non-Bt corn with soil-applied insecticide, and planting pyramids of multiple Bt toxins that target western corn rootworm must be used to maintain the effectiveness of available Bt toxins.