In this study, we have examined the genesis of neurons of the retinal ganglion cell layer of the opossum Didelphis marsupialis by [3H]-thymidine autoradiography. Our results suggest that most neurons surviving to adulthood are generated in postnatal life from day 1 to day 23. Cells are generated according to a coarse gradient from the retinal geometric center to the periphery. Regional analysis of soma size distributions in different cohorts suggest that this gradient is actually formed by two partially-overlapping, concentric waves of cell proliferation. Most medium and large ganglion cells are formed during the early wave, whereas most displaced amacrine cells and small ganglion cells are formed during the late wave. Our results confirm the appropriateness of the opossum as a model for studies of development of the mammalian visual system.