We have examined the expression of MyoD1, a potential determination factor of myogenic cells, in permissive and inducible C2 myoblasts. These two types of myoblasts exhibit distinct requirements to undergo terminal differentiation. Unlike permissive cells, inducible cells fail to differentiate in the presence of growth medium plus fetal calf serum and require insulin to undergo terminal differentiation. We show that while expression of MyoD1 is constitutive in permissive cells, no trace of MyoD1 transcripts is found in inducible cells at the myoblast stage. In these cells, however, expression of MyoD1 accompanies differentiation. This indicates that MyoD1 may not be required for the maintenance of the myoblast phenotype, and could act as an effector of terminal differentiation in already determined muscle cells. Our results provide new evidence that permissive and inducible cells represent two distinct stages of the progression of determined muscle cells toward terminal differentiation.