The evolution of ecological trade-offs is an important component of ecological specialization and adaptive radiation. However, the pattern that would show that evolutionary trade-offs have occurred between traits among species has not been clearly defined. In this paper, we propose a phylogeny-based definition of an evolutionary trade-off, and apply it to an analysis of the evolution of trade-offs in locomotor performance in emydid turtles. We quantified aquatic and terrestrial speed and endurance for up to 16 species, including aquatic, semi-terrestrial and terrestrial emydids. Emydid phylogeny was reconstructed from morphological characters and nuclear and mitochondrial DNA sequences. Surprisingly, we find that there have been no trade-offs in aquatic and terrestrial speed among species. Instead, specialization to aquatic and terrestrial habitats seems to have involved trade-offs in speed and endurance. Given that trade-offs between speed and endurance may be widespread, they may underlie specialization to different habitats in many other groups.