Intramuscular implanted myoblasts can fuse with existing myofibers. Here we report that implanted primary myoblasts marked with retroviral transgenes can also persist as muscle precursor cells. These cells can be recovered as viable myoblasts from muscles of recipent mice even months after myoblast implantation, and they can fully resume expression of the transgenes in culture. Upon re-implantation into muscles, they again not only fuse with existing myofibers, but also survive as muscle precursor cells in the tissue. These reserve myogenic cells should be able to contribute to host myofibers in muscle regeneration when the recombinant myofibers are damaged, providing an additional mechanism to maintain a persistent expression of transgenes delivered by myoblast-mediated gene transfer.