The retinal photoreceptor cells and pigment epithelium of the lamprey, Petromyzon marinus have been examined with the electron microscope. The pigment epithelial cells in this avascular retina are characterized by surface specializations in the form of apical microvilli and basal infoldings. These cells contain myriad myeloid bodies, some phagosomes and residual bodies, as well as sparse, apically positioned melanin granules. Two types of photoreceptor cells are described in the retina of this animal. The long receptors have conical outer segments, slender elongated myoid regions and their nuclei are positioned in the outer portion of the outer nuclear layer near the external limiting membrane. The pyramid-shaped synaptic terminals of these cells are found deep in the outer plexiform layer, are less electron dense than those of the short receptors and their synaptic vesicles appear to be less densely packed than in the long receptor terminals. The short receptors have slender, elongated outer segments, their nuclei are positioned deeper in the outer nuclear layer, while the spherical synaptic terminals are located in the scleral region of the outer plexiform layer. The outer segments of both cell types however, show similar cone-like characteristics at the ultrastructural level, being composed of membrane-bound stacks of double-membrane discs, some of which are in continuity with the extra-cellular space. The outer segment of both types of receptors is joined to the inner segment through a slender ciliary stalk and is surrounded by calycal processes. Autoradiographic analysis shows that protein renewal in the photoreceptor outer segments of both long and short receptors takes place in a similar diffuse manner, giving rise are to the suggestion that both photoreceptor types in the retina of Petromyzon marinus cones.