Affected members of a family with autosomal dominant retinitis pigmentosa were found to have a 3 base pair deletion at codon 118 or 119 of the retinal degeneration slow gene. This mutation causes the loss of a highly conserved cysteine residue in the predicted third transmembrane domain of peripherin-rds, a photo-receptor specific structural glycoprotein localised to both rod and cone outer segment disc membranes. Four of these individuals underwent detailed clinical, psychophysical, and electroretinographic testing in order to characterise their photoreceptor dysfunction. Nyctalopia was reported early in the second decade by all patients. Global rod and cone dysfunction was recorded by the third decade with severe reduction of both photopic and scotopic function by age 30 years. This retinal degeneration slow gene mutation may lead to the primary loss of both rod and cone photo-receptor function.