Histogenesis of the photoreceptor layer in the retina of goldfish was studied morphometrically and with thymidine radioautography at the light microscopic level. At hatching, the photoreceptor layer is poorly differentiated. The cells in the outer nuclear layer that are postmitotic' at hatching differentiate into cones. Two to 3 days after hatching, rod nuclei appear in the outer nuclear layer , and rods begin to differentiate. Rods and rod nuclei accumulate across the retina during larval and juvenile life. It is proposed that rods are generated in the outer nuclear layer by dividing progenitor cells that may be analogous to subventricular cells. Rod progenitors appear to derive from clusters of undifferentiated cells in the inner nuclear layer, which are called neurogenic clusters. Some of these cells apparently migrate across the outer plexiform layer from the inner to the outer nuclear layer and then divide to produce rods. The ontogenetic sequence of photoreceptor development in the larval retina is recapitulated continuously in the circumferential growth zone at the margin of the juvenile and adult retina. New rods continue to be inserted, but at a slower rate, throughout the adult retina; this may explain how the density of rods is maintained constant despite the expansion of the retinal surface that accompanies growth of the eye.