Progressive rod-cone degeneration (prcd) is a recessively inherited visual cell disease. Neither the genetic abnormality nor the corresponding biochemical defect have yet been identified. Unique abnormalities of visual cell structure, function and renewal, however, characterize the disease phenotype and act as a marker for the prcd gene. The disease was first described in miniature poodle dogs (MP) but broadly similar retinal degenerations have been recognized, clinically, in other breeds. Crossbreeding experiments with prcd-affected MP and retinal degenerate English (ECS) and American (ACS) cocker spaniels now demonstrate that all the progeny are affected with a retinal degeneration indistinguishable from prcd in the MP. This indicates that the gene mutation in each breed is at the same (prcd) locus. In purebred prcd-affected ECS (prcd-ECS), however, the disease phenotype consistently differs from that in prcd-MP in its rate of progression and in the topographical distribution of disease within the retina. Ultrastructural variation in disease expression are also recognizable between the two phenotypes. These differences in disease phenotype may be ascribable to different genetic backgrounds in the two breeds, reflecting the effect of modifying genes, or may indicate separate, allelic, mutations at the same locus.