The fiber architecture of the ventricular myocardium has been studied in elasmobranch (Isurus oxyrhinchus, Galeorhinus galeus, Prionace glauca) and teleost (Xiphias gladius, Thunnus thynnus, Thunnus alalunga) fish species with hearts displaying mixed types of ventricular musculature (compact and trabecular). In all cases, the compact myocardium is organized in layers of fiber bundles with an orderly arrangement within the ventricular walls. The number of these layers appears to be dependent on the relative thickness of the compact myocardium. Differences in the pattern of myocardial fiber arrangement were observed among the different fish species. In elasmobranchs the compact myocardium at the level of the atrioventricular orifice is continuous with the trabeculated myocardium. Furthermore, in elasmobranchs the trabeculated myocardium displays a precise arrangement in arcuate trabeculae running from the auriculoventricular to the conoventricular orifices. In teleosts, the compact myocardium is independent of the trabeculated myocardium and a large number of fibers insert into the bulboventricular fibrous ring. The trabeculated myocardium in these species displays an anarchic arrangement except at the level of the bulboventricular orifice, where the fibers tend to be aligned longitudinally, also being inserted into the fibrous ring. Minor differences, consisting mainly of the presence of extra bundles of fibers, were also observed among different individuals of the same species. The possible relationship between myocardial fiber architecture and ventricular shape is discussed.