Fluorescence microscopy of chicken cervical somites revealed that muscle-specific proteins began to appear at stage 11 (Hamburger and Hamilton numbering), and the onset of the expression of all the proteins examined in the present study had occurred by stage 17. Muscle proteins were classified into six groups according to the stage of their appearance. Since all these proteins were expressed before emergence of nerve fibers in myotomes, switching-on of their synthesis does not seem to require neuronal influence. However, since isoproteins other than adult muscle types disappeared and diversification of muscle fiber types occurred coordinately with the clustering of acetylcholine receptors in cervical muscles, switching-off of the synthesis of the nonadult isoforms might have been accelerated by the formation of functional neuromuscular junctions. The absence of nebulin and C-protein in early stages seems to indicate that these proteins are not required for the initial assembly of myofilaments and/or myofibrils. Further, this absence might be considered to facilitate exchangeabilities of proteins in nascent myofibrils, thereby changing the isoforms to adult types.