Polypeptides obtained by limited tryptic digestion of several myosins have been analyzed by a two-dimensional electrophoresis technique. The different maps thus obtained exhibit some common and distinct features specific of the myosins studied. Myosins from rabbit, fast and slow muscle and cardiac tissue, as well as beef cardiac myosin, have been compared. The polypeptides obtained vary in molecular weight from 120 000 60 15 000. The light chains LC1 and LC2, have disappeared. A peptide which comigrates with fast type LC3 is however found. The fast myosin hydrolyzate is very different from that obtained by the hydrolysis of slow and cardiac myosin. Numerous peptides are common to cardiac and slow myosins. However a few peptides are specific of the two myosin types. In the hydrolyzate from fetal calf myosin, some of the typical slow, fast and cardiac peptides can be found. However several apparently unique fetal peptides are also present. The comparison of the fetal calf tissue myosin hydrolyzate and that of [35S] methionine labeled myosin from myotubes in cultures shows qualitatively a very great homology. Thus the protein synthesized by cultured cells seems to be very similar to or the same as that of the embryonic tissue.