Little has been written about the cells here termed cerebellar melanoneurons. This paper describes and illustrates their cytologic features and topographic relationships. In the human brain these large pigmented neurons are scattered in a narrow layer near the lateral wall, dorsal angle and roof of the fourth ventricle. They form an inconspicuous part (group A4) of the system of catecholamine, neuromelanin-containing cells well known in the brain stem. Rostrally, a few of them provide a tenuous continuity with the locus ceruleus but topographically the two nuclei are independent. With ordinary stains the cerebellar cells can be seen as early as the 26th week of gestation (the earliest period examined). Brown neuromelanin granules do not appear until two and a half years of age but argentaffin granules, foreshadowing the production of pigment, are found in increasing numbers in the fetal and postnatal period. Homologues of the human cerebellar cells are reported in two species of monkey, Macaca nemestrina and Lagothrix sp. Neuromelanin, not previously observed in non-human cerebellar cells, occurs in M. mulatta and M. nemestrina. The proximity of the cerebellar melanoneurons to the ventricle raises the possibility that they are related to functions of the ependyma, or that they influence, or are affected by, constituents of the cerebrospinal fluid. The pathologic changes they undergo in Parkinson's disease and other disorders are to be described elsewhere.