Two major embryonic myosin heavy chains are expressed in embryonic chick breast muscle until the first week after hatching. Of these, one is already detected in the 8-day-old embryo. The other appears on day 12. Two putative slow embryonic isoforms represent minor components transiently expressed between days 8 and 12. A neonatal heavy chain is expressed at low concentrations on day 8 and increases with development. It is the only isoform two weeks after hatching, and is ultimately replaced by the fast myosin heavy chain in the adult muscle.