Local hyperthermia has been widely used as physical therapy for a number of diseases such as inflammatory osteoarticular disorders, tendinitis, and muscle injury. Local hyperthermia is clinically applied to improve blood and lymphatic flow to decrease swelling of tissues (e.g., skeletal muscle). As for muscle repair following injury, the mechanisms underlying the beneficial effects of hyperthermia-induced muscle repair are unknown. In this study, we investigated the direct effects of continuous heat stress on the differentiation of cultured mammalian myoblasts. Compared with control cultures grown at 37 degrees C, incubation at 39 degrees C (continuous mild heat stress; CMHS) enhanced myotube diameter, whereas myotubes were poorly formed at 41 degrees C by primary human skeletal muscle culture cells, human skeletal muscle myoblasts (HSMMs), and C2C12 mouse myoblasts. In HSMMs and C2C12 cells exposed to CMHS, mRNA and protein levels of myosin heavy chain (MyHC) type I were increased compared with the control cultures. The mRNA level of MyHC IIx was unaltered in HSMMs and decreased in C2C12 cells, compared with cells that were not exposed to heat stress. These results indicated a fast-to-slow fiber-type shift in myoblasts. We also examined upstream signals that might be responsible for the fast-to-slow shift of fiber types. CMHS enhanced the mRNA and protein levels of peroxisome proliferator-activated receptor-gamma coactivator (PGC)-1alpha in HSMMS and C2C12 cells but not the activities of MAPKs (ERK1/2 and p38 MAPK) in HSMMs and C2C12 cells. These data suggest that CMHS induces a fast-to-slow fiber-type shift of mammalian myoblasts through PGC-1alpha.