IL-15 receptor α (IL-15Rα)is a component ofthe heterotrimeric plasma membrane receptor for the pleiotropic cytokine IL-15. However, IL-15Rα is not merely an IL-15 receptor subunit, as mice lacking either IL-15 or IL-15Rα have unique phenotypes.IL-15 and IL-15Rα have been implicated in muscle phenotypes, but a role in muscle physiology has not been defined. Here, we have shown that loss ofIL-15Rα induces a functional oxidative shift in fast muscles, substantially increasing fatigue resistance and exercise capacity.