Evidence is presented which shows that myoblasts, isolated at different stages during chick and quail limb bud development, will form, in culture, myotubes which can be distinguished with a combination of morphological as well as biochemical criteria. Hind limb bud myoblasts isolated from 5-day-old embryos form very short myotubes which synthesize a myosin, the light chains of which are predominantly LC1F and LC2S. Myoblasts isolated from the limb buds of 7-8-day-old embryos form large myotubes which synthesize a myosin the light chains of which are predominantly LC1F, LC2S and LC2F. Myoblasts isolated from the thigh muscle of embryos older than 10 days form large myotubes which synthesize a myosin the light chains of which are predominantly LC1F and LC2F. These results have been confirmed by hybridization of the cellular mRNA with a molecular probe specific for LC2F. These results lead us to suggest the existence of at least two classes of myoblasts which appear at different times during limb bud development. The first class, or 'early' myoblasts, is present in the limb buds of 5-day-old embryos, whereas the second class, or 'late' myoblasts, is present in the muscles of embryos older than 8 days. This result, however, is also compatible with the hypothesis that all muscle cells are the same at all times during development, and that the different phenotypes simply reflect differences in the environmental conditions.