We have characterized overlapping cDNA clones encoding cGMP phosphodiesterase (PDE) α- and β-subunits of mouse retinal rod photoreceptors. The open reading frames predict an α-subunit of 100 kDa (856 residues), and a β-subunit of 99 kDa (853 residues). Sequence analysis of two of twelve β-subunit clones predicts the presence in the retina of an additional PDE, termed β, which is generated by alternative splicing of the β-subunit gene, β differs from β only at the C-terminus being 55 residues shorter and lacking the Caax motif found at the C-termini of both the α- and β-subunits. A 300 residue segment thought to contain the active site is present in the C-terminal half of α, β and β. © 1991.