We examined whether freshly isolated (native) human muscle satellite cells (HMSC), as well as their proliferating clonal progenies, were heterogeneous. We studied the expression of the cytoskeletal proteins, desmin (DSM), alpha-sarcomeric and alpha-smooth muscle actins (alpha-SR actin, alpha-SM actin), three markers that may be expressed prior to the fusion process. We found that native HMSC constituted a homogeneous population of cells expressing desmin and giving rise to similar clones in vitro. The clonal progeny of HMSC was heterogeneous, including several subpopulations of myoblasts with different cytoskeletal phenotypes, commitment states and fusion abilities. A major subpopulation that expressed both alpha-sarcomeric actin and desmin during the proliferative stage corresponded to a "predifferentiated" population of myoblasts, committed to fusion. Another subpopulation, expressing exclusively desmin, and phenotypically similar to native HMSC, failed to fuse under fusion-promoting conditions and could represent a new generation of HMSC born in culture.