The transcriptional coactivator peroxisome proliferator-activated receptor gamma coactivator 1alpha (PGC-1alpha) is a key integrator of neuromuscular activity in skeletal muscle. Ectopic expression of PGC-1alpha in muscle results in increased mitochondrial number and function as well as an increase in oxidative, fatigue-resistant muscle fibers. Whole body PGC-1alpha knock-out mice have a very complex phenotype but do not have a marked skeletal muscle phenotype. We thus analyzed skeletal muscle-specific PGC-1alpha knock-out mice to identify a specific role for PGC-1alpha in skeletal muscle function. These mice exhibit a shift from oxidative type I and IIa toward type IIx and IIb muscle fibers. Moreover, skeletal muscle-specific PGC-1alpha knock-out animals have reduced endurance capacity and exhibit fiber damage and elevated markers of inflammation following treadmill running. Our data demonstrate a critical role for PGC-1alpha in maintenance of normal fiber type composition and of muscle fiber integrity following exertion.