In vertebrate cardiomyocytes, DNA synthesis and mitosis occur in the presence of myofibrils. The myocardium of the adult vertebrates has no cambial elements. We have studied the ultrastructural features and replicative abilities of the heart cells of Ascidia obliqua, a representative of the tunicates, which are considered to be closely related to the immediate precursors of vertebrates. The ascidian heart is composed of a pericardium and a myocardium. The pericardium consists of mononucleated cells with well-developed Golgi complexes and expanded channels of rough endoplasmic reticulum. The myocardium consists of mono- and binucleated myoepithelial cells, whose polarity is recognizable in basement lamina underlying the basal surface; centrioles, cilia, and a well-developed Golgi complex located at the apical side; and cross-striated myofibers, one per cell, running along the basal membrane. Four hours after injection of tritiated thymidine (3 H Tdr), labeled nuclei were found both in pericardial and myocardial cells. Electron microscope autoradiographs show that the 3 H Tdr-labeled nuclei in the myocardium belong to myofiber-containing cells. We failed to observe mitosis; however, the occurrence of centrioles and the high number of binucleated myocytes testify to the ability of these cells to undergo karyokinesis. Large numbers of 3 H Tdr-labeled and prophasic nuclei have been observed in the raphe region, the site of transition of the pericardium into myocardium. The morphological features, such as loss of labyrinth junctions and the acquisition of gap junctions, that distinguish the cells of the raphe region from ordinary pericardial cells give evidence of their premyocytic nature. The resemblance of ascidian myocardium to the vertebrate embryonic one and the presence of cambial zone in the ascidian heart are discussed.