Photoreceptor degeneration is a common cause of inherited blindness worldwide. We have identified a blind zebrafish mutant with rapid degeneration of cone photoreceptors caused by a mutation in the cone phosphodiesterase c ( pde6c) gene, a key regulatory component in cone phototransduction. Some rods also degenerate, primarily in areas with a low density of rods. Rod photoreceptors in areas of the retina that always have a high density of rods are protected from degeneration. Our findings demonstrate that, analogous to what happens to rod photoreceptors in the rd1 mouse model, loss of cone phosphodiesterase leads to rapid degeneration of cone photoreceptors. Furthermore, we propose that cell density plays a key role in determining whether rod photoreceptors degenerate as a secondary consequence to cone degeneration. Our zebrafish mutant serves as a model for developing therapeutic treatments for photoreceptor degeneration in humans.