The ultrastructure of the developing fetal lamb myocardium was studied in a series of animals spanning 29 days of gestation to term, and compared with newborn and adult animals. All major ultrastructural features which characterise the adult myocyte were found in early fetal life, although with considerably different degrees of development of specific features. Notably, myofibrils at 29 days of gestation are sparse and show little organisation. With advancing gestation there is an increasing number of myofibrils and the development of well defined striations. Thus, at term, the fetal tissue is not substantially different from the adult myofibril in the appearance of sarcomere structure. The observation of contractile tissue paucity and disorder in early fetal lamb myocardium is difficult to reconcile with available physiological data, which show an extraordinary pumping performance of the heart in vivo, and requires further investigation.