M and P retinal ganglion cell morphology revealed by biocytin retrograde labelling was compared in two closely related New-World monkeys, Cebus and Aotus, to investigate whether nocturnal and diurnal species of primates have similar cell classes. Monkey and cat ganglion cells from regions of matching cell class densities were also compared. Cat alpha, cat beta, Aotus M, and Cebus M cells were similar in many aspects, but Cebus M cells had higher branching density. Cebus and Aotus P cells formed a distinct group and represent a primate specialization common to diurnal and nocturnal simians.