The contractile proteins of adult skeletal and cardiac muscle tissue are very similar. Some of these proteins are identical in amino acid sequence or differ only slightly. Various structural studies have shown that the heavy and light chains of myosin are also homologous in skeletal and cardiac muscles. In developing skeletal muscles, certain myosin subunits are present which are not found in the corresponding adult muscle. Whether fetal cardiac muscle also contains myosin subunits homologous to these early forms is not known. We now report that ventricular myosin of fetal rats has a light chain polypeptide corresponding to the skeletal muscle embryonic light chain. This result provides further evidence that a form of myosin light chain, not detectable in adult skeletal or ventricular myosin, is characteristic of the early stages of striated muscle development.