Kittens and adult cats fed a semipurified diet containing casein developed a retinal degeneration that initially involved photoreceptor outer segments in the area centralis. By electron microscopy, cone and rod outer segment lamellar discs could be seen to become vesiculated, frayed, disoriented and twisted. Shortening and subsequent disappearance of outer segments was followed by loss of photoreceptor nuclei, primarily in the area centralis but also in the midperipheral retina. The electroretinogram (ERG) indicated progressive reduction in cone and rod amplitudes and a delay in the temporal aspects of the cone response. When dietary casein was replaced by egg albumin in the diets of cats with minimal to moderately advanced degeneration, the degeneration was reversed; rod ERG function and structure returned essentially to normal, whereas some abnormalities of cone outer segment structure and a delay in the temporal aspects of the cone ERG persisted. The data provide strong evidence that dietary casein is a factor in this retinopathy and suggest that an alteration in protein metabolism of the photoreceptor may result from the dietary protein inadequacy.