Central to most theories that explain the diversity of life is the concept that organisms face trade-offs. Theoretical work has shown that the precise shape of a trade-off relationship affects evolutionary predictions. One common trade-off is that between competitive ability and resistance to predators, parasitoids, pathogens or herbivores. We used a microbial experimental system to elucidate the shape of the relationship between parasitoid resistance and competitive ability. For each of 86 bacteriophage-resistant isolates of the bacterium Escherichia coli B, we measured the degree of resistance to bacteriophage T2 (a viral parasitoid) and relative competitive ability in both the resource environment in which strains were isolated and in two alternate environments. We observed that environmental change can alter trade-off shape, and that different physiological mechanisms can lead to different trade-off shapes and different sensitivities to environmental change. These results highlight the important interaction between environment and trade-off shape in affecting ecological and evolutionary dynamics.