In the diencephalons of the adult zebrafish brain, all catecholamine-containing neurons are dopaminergic. The organization and projection pattern of these neurons are studied using tyrosine hydroxylase immunocytochemistry. By their locations, 3 neuronal complexes and 17 cell groups are identified on the bases of their morphology, staining intensity, and projection pattern: 1) the preoptic complex (5 groups); 2) the posterior tuberal complex (4 groups); and 3) the hypothalamic complex (5 groups). In addition, three other groups can be distinguished: one group in the ventral thalamus; one in the pretectal area, and one found in the postoptic commissure and above the pituitary stalk in a few brains. Two dopaminergic pathways are defined: 1) the preoptico-hypophyseal tract runs in close association with the lateral forebrain bundle along the base of the brain between the preoptic area and the pituitary stalk, and neurons of the preoptic complex are major contributors to this pathway; additional fibers come from the large periventricular organ-associated neurons of the posterior tuberal; 2) the endohypothalamic tract links neurons of the hypothalamic complex and consists mainly of processes from hypothalamic neurons. Axons from neurons of the suprachiasmatic, periventricular organ-associated, and posterior tuberal nuclei also join this pathways after entering the hypothalamus. Several groups of neurons contact the cerebrospinal fluid. These appear to be primarily local neurons because none have processes that join the two major pathways. The preoptic area, dorsal thalamus, tuberal and hypothalamic areas, optic tectum, and pituitary are the major targets of diencephalic dopaminergic neurons. The dorsal telencephalon does not receive input from these cells. The large periventricular organ-accompanying neurons have descending projections beyond the diencephalon and isthmus. Some cells of this group terminate in the crista cerebellaris. A few axons also exit the medulla via a branch of the octavolateralis nerve. J. Comp. Neurol. 460:13–37, 2003. © 2003 Wiley-Liss, Inc.