Male Cebus monkeys are all dichromats, but about two thirds of the females are trichromats. M and P retinal ganglion cells were studied in the male Cebus monkey to investigate the relationship of their morphology to retinal eccentricity. Retinal ganglion cells were retrogradely labeled after optic nerve deposits of biocytin to reveal their entire dendritic tree. Cebus M and P ganglion cell morphology revealed by biocytin retrograde filling is similar to that described for macaque and human M and P ganglion cells obtained by in vitro intracellular injection of HRP and neurobiotin. We measured 264 and 441 M and P ganglion cells, respectively. M ganglion cells have larger dendritic field and cell body size than P ganglion cells at any comparable temporal or nasal eccentricity. Dendritic trees of both M and P ganglion cells are smaller in the nasal than in the temporal region at eccentricities greater than 5 mm and 2 mm for M and P ganglion cells, respectively. The depth of terminal dendrites allows identification of both inner and outer subclasses of M and P ganglion cells. The difference in dendritic tree size between inner and outer cells is small or absent. Comparison between Cebus and Macaca shows that M and P ganglion cells have similar sizes in the central retinal region. The results support the view that M and P pathways are similarly organized in diurnal dichromat and trichromat primates.