The zebrafish has recently emerged as an excellent model for studies of heart development and regeneration. The physiology of the zebrafish heart has been suggested to resemble that of the human heart in many aspects, whereas, in contrast to mammals, the zebrafish has a remarkable ability to regenerate after heart injury. Thus, zebrafish have been proposed as a cost-effective model for genetic and pharmacological screens of factors affecting heart function and repair. However, realizing the full potential of the zebrafish heart as a model will require a better understanding of the electrophysiology of the adult zebrafish myocardium. Here, we characterize action potentials (APs) from intact adult atria and ventricles and find that the overall shape of zebrafish APs is similar to that of humans. We show that zebrafish, like most mammals, display functional acetylcholine-activated K⁺ channels in the atrium, but not in the ventricle. Furthermore, the zebrafish AP upstroke is dominated by Na⁺ channels, L-type Ca²⁺ channels contribute to the plateau phase and I_Kr channels are involved in repolarization. However, despite these similarities between zebrafish and mammalian electrophysiology, we also identified important differences. In particular, zebrafish display a robust T-type Ca²⁺ current in both atrial and ventricular cardiomyocytes. Interestingly, in most mammals T-type Ca²⁺ channels are only expressed in the developing heart or under pathophysiological conditions, indicating that adult zebrafish cardiomyocytes display a more immature phenotype.