We have isolated a full length complementary DNA clone (pCTnCl) from a 19-day embryonic chicken heart library corresponding to cardiac troponin C (TnC). Sequence analysis demonstrated varying homologies with TnC complementary DNA clones isolated from developing chick skeletal muscle. Using pCTnCl as a hybridization probe, we have determined that cardiac TnC is constitutively expressed in both atria and ventricles of the developing and adult heart. These data along with previous immunochemical studies of TnC expression demonstrate that the slow skeletal and cardiac muscle Isofonn is the only TnC expressed in the heart. In contrast, expression of slow skeletal and cardiac muscle TnC is developmentally regulated in skeletal muscles of the chicken. The tightly controlled expression of slow skeletal and cardiac muscle TnC in the varying myocyte types of the heart suggests a physiologically significant role of this regulatory protein.