The observation that CD45⁺ stem cells injected into the circulation participate in muscle regeneration raised the question of whether CD45⁺ stem cells resident in muscle play a physiological role during regeneration. We found that CD45⁺ cells cultured from uninjured muscle were uniformly nonmyogenic. However, CD45⁺ cells purified from regenerating muscle readily gave rise to determined myoblasts. The number of CD45⁺ cells in muscle rapidly expanded following injury, and a high proportion entered the cell cycle. Investigation of candidate pathways involved in embryonic myogenesis revealed that Wnt signaling was sufficient to induce the myogenic specification of muscle-derived CD45⁺ stem cells. Moreover, injection of the Wnt antagonists sFRP2/3 into regenerating muscle markedly reduced CD45⁺ stem cell proliferation and myogenic specification. Our data therefore suggest that mobilization of resident CD45⁺ stem cells is an important factor in regeneration after injury and highlight the Wnt pathway as a potential therapeutic target for degenerative neuromuscular disease.