Observations were made on the pre- and postnatal development of the photoreceptors in the central region of the retina in the diurnal rhesus monkey. Histologic findings on the level of development of the retina at birth revealed a macula with the characteristic adult foveal depression. Observations on the ultrastructural development of the receptor inner and outer segments revealed that the photoreceptors were well differentiated even at birth. An evaluation of the prenatal morphogenesis of the photoreceptors indicated that the initial differentiation may take place as early as 100–125 days after conception. At 100 days after conception, the pigment epithelium consisted of a single layer of cuboidal cells with cytoplasmic content of smooth endoplasmic reticulum, but was only sparsely populated with mitochondria and rough endoplasmic reticulum. The photoreceptor inner segments contained vesiculation in the supranuclear region, while the incipient outer segments consisted of outbudding ciliary processes. By 125 days postconception, the pigment epithelium contained such cytoplasmic specializations as increases in rough endoplasmic reticulum and mitochondria, as well as greater organelle complexity. In the photoreceptor outer segments there were arrays of closed bimembranous discs, with vesiculation at the apical tip in the inner segments. A comparison of the pre- and postnatal ultrastructural changes in the choroid and the interstitial zone comprising the photoreceptor outer segments and the cell processes of the pigment epithelium suggested that the changes in these areas represent a critical aspect in the maturation of the retina.