Patients with congenital stationary night blindness enjoy normal daytime vision, which is mediated by cone photoreceptors, but are blind when ambient light is so dim that a normal individual would utilize only rod photoreceptors to see without colour discrimination. The disease is genetically heterogeneous. One form of dominantly inherited congenital night blindness is eponymously named "Nougaret' because pedigree analysis reveals that the disease originated in Jean Nougaret (1637-1719), a butcher who lived in Vendémian in southern France. Here we report that his affected descendants carry a missense mutation in the gene encoding the alpha subunit of rod transducin the G-protein that couples rhodopsin to cGMP-phosphodiesterase in the phototransduction cascade. Based on these results, rod transducin joins rhodopsin and the beta subunit of rod cGMP-phosphodiesterase to become the third component of the rod phototransduction cascade where a defect is implicated as a cause of stationary night blindness. Interestingly, the amino acid residue in transducin affected by the Nougaret mutation is in the position homologous to that affected by the oncogenic mutation originally reported in p21ras, a distant relative in the G-protein superfamily.