The levels and cellular distribution of PV were analyzed at the molecular level in normal and mutant Xenopus laevis embryos. In developing tadpoles, PV protein expression was restricted to fast-twitch myotomal cells, as shown by immunofluorescence experiments with myosin isoform-specific monoclonal antibodies. Both PV expressing and non-expressing myotomal cells can be cultured in vitro without neurons, and non-expressing cells have been shown to lack hybridizable levels of PV mRNA. In a paralyzed mutant, unresponsive, the level of PV protein was normal, but its cellular distribution was not. Thus, while neural input may not be necessary for PV expression in embryonic cells, one gene sequence appears needed for proper cellular distribution of PV in tadpole tail muscles.