A stem cell population isolated from murine skeletal muscle has recently been shown to differentiate into hematopoietic cells after transplantation in vivo. In the present study, we tested the hypothesis that this cell population would also, under appropriate culture conditions, differentiate into skeletal muscle cells in vitro. Lower-extremity skeletal muscle tissue isolated from 3- to 4-wk-old mice was dissected free from bone and vessels, enzymatically digested, and flow cytometrically sorted to yield CD45⁻Sca-1 c-Kit⁻ (S ) cells. These cells were further sorted into CD34 and CD34⁻ fractions and examined for skeletal, cardiac, and hematopoietic lineage–specific messenger RNA (mRNA) transcripts immediately after isolation and after a 10- to 14-d culture period. Freshly isolated S CD34 cells lacked expression of skeletal-, cardiac-, or hematopoietic-specific mRNA transcripts, whereas S CD34⁻ cells expressed c-met, a marker for skeletal muscle satellite cells. During 10–14 d in culture, both S CD34 and S CD34⁻ cell populations underwent a period of attachment followed by elongation and, ultimately, fusion to create large multinucleated contractile myotubes expressing skeletal muscle lineage mRNA transcripts but not hematopoietic or cardiac lineage transcripts. We conclude that murine skeletal muscle possesses two populations of progenitor cells that can be directly isolated. One population expressing the phenotype S CD34⁻ may contain satellite cells, whereas the S CD34 population is devoid of satellite cell markers. Both populations possess the ability to differentiate into skeletal muscle cells in vitro.