MICE homozygous for the rd mutation display hereditary retinal degeneration and the classic rd lines serve as a model for human retinitis pigmentosa^1,2. In affected animals the retinal rod photo-receptor cells begin degenerating at about postnatal day 8, and by four weeks no photoreceptors are left³. Degeneration is preceded by accumulation of cyclic GMP in the retina⁴ and is correlated with deficient activity of the rod photoreceptor cGMP-phospho-diesterase⁵. We have recently isolated a candidate complementary DNA for the rd gene⁶ from a mouse retinal library and completed the characterization of cDNAs encoding all subunits of bovine photoreceptor phosphodiesterase⁷. The candidate cDNA shows strong homo logy with a cDNA encoding the bovine phospho-diesterase β subunit. Here we present evidence that the candidate cDNA is the murine homologue of bovine phosphodiesterase β cDNA. We conclude that the mouse rd locus encodes the rod photoreceptor cGMP-phosphodiesterase β subunit.