An electron microscopic analysis of photoreceptor degeneration in a congenitally blind strain of chickens is presented. The mutation was named rd, meaning 'retinal degeneration'. Although the chicks were behaviorally and electrophysiologically blind at the time of hatching, their retinas appeared morphologically comparable to normal chicks at this stage. Both groups had well-developed photoreceptor cells, although outer segments were typically disoriented or misaligned. In the normal, and to some degree in the rd, retina, outer segments became organized within the first week posthatching. In the rd retina at that time, however, more outer segments were disorganized and disoriented. Disc-like membranes were also seen in some inner segments. Many photoreceptors had distended inner segment tips containing a granular cytoplasm. Membraneous debris was present in the subretinal space. Over the next 2-3 weeks there was a reduction in number of inner segments, outer segments and photoreceptor nuclei of both rods and cones. Photoreceptor cell bodies in the outer nuclear layer were replaced by Mueller cell processes. By the end of the second month, a larger cone:rod ratio was apparent, and a large proportion of the remaining cones were double cones. Intact outer segments were rarely seen at that time. Few and sporadic cone cells, identified by a pale-staining oil droplet, were the predominant surviving photoreceptors by 6 months of age. At the later stages examined, the pigment epithelium (PE) appeared to be undergoing degenerative changes. A general thinning of cells and hypopigmentation of PE cells was apparent, although hyperpigmented, hypertrophied PE cells were also present which bulged into the subretinal space. Pigmented cells of unknown origin were also noted in the subretinal space at the later time points.