Rods and cones of the two vertebrate lateral eyes hyperpolarize when illuminated, a response generated by a cyclic GMP cascade leading to cGMP hydrolysis and consequently the closure of cGMP-gated, non-selective cation channels that are open in darkness(1-4). Lizards and other lower vertebrates also have a parietal (third) eye(5), which contains ciliary photoreceptors that under dark-adapted conditions depolarize to light instead(6). Depolarizing Light responses are characteristic of most invertebrate rhabdomeric photoreceptors, and are thought to involve a phosphoinositide signalling pathway (see, for example, refs 7-9). Surprisingly, we have found in excised membrane patches a cGMP-gated channel that is selectively present at high density on the outer segment (the presumptive light-sensitive part) of the parietal eye photoreceptor. Like the light-activated channel of the cell, it is non-selective among cations. Inositol trisphosphate (InsP(3)) had no effect on the same membrane patches. These findings suggest that the photoreceptors of the parietal eye, like rods and cones, use a cGMP cascade and not an InsP(3)-mediated pathway for phototransduction, but in this case light increases cGMP. A unifying principle of evolutionary significance emerges : that phototransductions in various ciliary photoreceptors, whether hyperpolarizing or depolarizing, uniformly use a cGMP cascade and a cGMP-gated channel to generate the Light response, although there are rich variations in the details.