We identified and characterized the function of CD45+/collagen I+ fibrocytes in acutely injured skeletal muscle of wild-type (WT) and Ccr2−/− mice, and in quadriceps and diaphragm muscles of mdx 5cv mice, a mouse model for Duchenne muscular dystrophy. Fibrocytes were not detected in peripheral blood of WT mice after acute muscle injury or mdx 5cv mice. Fibrocytes were detected in acutely injured muscles and in mdx 5cv quadriceps and diaphragm muscles. These cells expressed F4/80 and CCR2, and they were mostly Ly6C lo. They expressed a low level of collagens but a high level of profibrotic growth factors as compared with im fibroblasts. Fibrocyte expression of collagens and profibrotic growth factors was not increased in Ccr2−/− mice as compared with WT controls. Fibrocyte expression of both proinflammatory and profibrotic cytokines was significantly higher in mdx 5cv diaphragm than in mdx 5cv quadriceps. In cocultures, fibrocytes from the mdx 5cv diaphragm stimulated a higher level of fibroblast expression of extracellular matrix genes than did those from the mdx 5cv quadriceps. Our findings suggest that im fibrocytes most likely originate from infiltrating monocytes/macrophages and differentiate within injured muscles. They likely contribute to the normal muscle injury repair by producing growth factors. They do not appear to contribute to the persistent muscle fibrosis associated with poor injury repair in Ccr2−/− mice. However, they likely contribute to the persistent inflammation and progressive fibrosis in the mdx 5cv diaphragm.