We have found four mutations in the human gene encoding the β–subunit of rod cGMP phosphodiesterase (PDE β) that cosegregate with autosomal recessive retinitis pigmentosa, a degenerative disease of photoreceptors. In one family two affected siblings both carry allelic nonsense mutations at codons 298 and 531. Affected individuals have abnormal rod and cone electroretinograms. PDE β is the second member of the phototransduction cascade besides rhodopsin that is absent or altered as a cause of retinitis pigmentosa, suggesting that other members of this pathway may be defective in other forms of this disease.