To characterize the catecholaminergic systems in the zebrafish medulla, immunocytological studies were performed by using antibodies directed against tyrosine hydroxylase and dopamine-beta-hydroxylase. Catecholaminergic neurons could be categorized into three populations based on location, dendritic morphology, axonal projection pattern, and targets: an interfascicular group, a vagal area group, and an area postrema group. All groups contained both dopaminergic and noradrenergic neurons. Interfascicular neurons formed a loose longitudinal column of approximately 16-20 multipolar neurons on each side of the medulla, whose rostrocaudal extension coincided roughly with the vagal lobe. These neurons were relatively large and had dendrites that arborized throughout the reticular formation and in the vagal lobe. They also contributed axonal processes to the longitudinal catecholamine bundle. Neurons associated with structures in the vagal area were mostly dopaminergic. Some cells had a short, thin apical process that arborized into a dense plexus near the ventricular surface, and all cells had a basal dendrite that divided into two main branches: one extended caudally to terminate in the commissural nucleus of Cajal and among the postobecular catecholaminergic cell group; the other extended laterally and joined the longitudinal catecholamine bundle. The caudal extent of this cell group reached the medullospinal junction. The area postrema cell group consisted of densely packed, bipolar neurons. One process of these neurons contacted the ventricular surface in the area postrema, and one terminated in the commissural nucleus of Cajal. Collaterals from the latter innervated the superficial laminae of the vagal lobe and joined the longitudinal catecholamine bundle. The longitudinal catecholamine bundle ascended through the medulla ventral to the secondary gustatory tract. Whether some fibers extended more rostrally is not known. The majority of the terminal fields of medullary catecholaminergic neurons appeared to be restricted to the medulla and were strongly associated with sensory systems. With the exception of some cells in the vagal area, catecholamine-containing neurons in the zebrafish medulla were not obviously homologous to those in the mammalian brainstem.