Cell differentiation in the retina of the mouse during the postnatal period was studied by autoradiography. Animals were injected with 3H-thymidine at ages extending from the day of birth through postnatal day 11. Six weeks later the distribution of labeled nuclei in the cells of the mature neural retina was analyzed to determine when these cells completed their final mitosis prior to differentiating. Central and peripheral zones were analyzed separately. Cell division ceases by 5-6 days in the center of the retina and by 11 days in the periphery. Among cells produced postnatally, 73% differentiate as rods, 20% as bipolar cells, 6% as Müller cells, and 1% as amacrine and ganglion cells. At all stages of embryogenesis, the differentiation of at least three and as many as six distinct types of specialized cells is initiated simultaneously within the ventricular cell population.