The distribution of retinal rod and cone photoreceptors shows considerable variation across mammalian species. We were interested to see how these differences relate to different adaptive pressures in closely related species. The Octodontidae are endemic rodents from South America and show a marked diversity at the species level in their lifestyle (diurnal vs nocturnal) and habitat use. We have studied the population densities and distributions of the rods and the different spectral cone types in the retinae of three Octodon species: Octodon degus, crepuscular-diurnal;O.lunatus and O. bridgesi, both nocturnal. Animals were killed by a lethal intraperitoneal dose of ketamine and xylazine. In isolated formalin-fixed retinae, rods were labelled with the rod opsin-specific mouse antibody rho4D2. The middle-to-longwave-sensitive (L-)cones were labelled with the L-opsin-specific rabbit antiserum JH492, and the short-wave-sensitive (S-)cones with the S-opsin-specific rabbit antiserum JH455 or goat antiserum sc-14363. In all three species, rods are the most abundant photoreceptors. All three species possess L-cones and S-cones (cone dichromacy). The cone proportion among the photoreceptors is about 30% in the diurnal O. degus and only about 2% in the nocturnal O.bridgesi and O.lunatus. In all three species, the S-cones form a minority of 5% or less of the cones, with the exception of ventral O.bridgesi retina where they attain up to 17%. The range of rod densities across the retina is 200,000-320,000/mm² in O.bridgesi and 200,000-430,000/mm² in O.lunatus. The cones show a centro-peripheral density gradient with central maxima: 7000-2700 L-cones/mm² and 700-40 S-cones/mm² in O.bridgesi, 8500 L-cones/mm² and 525-20 S-cones/mm² in O.lunatus. Double-labelling for L-cone and S-cone opsin was performed by incubating the tissue in a mixture of antisera JH 492 and sc-14363. In O.degus and O.bridgesi, each and every cone exclusively expresses either the L- or the S-opsin. In O.lunatus, a marked proportion of the S-cones co-express the L-opsin (dual pigment cones). In conclusion, our observation of highly different cone proportions in diurnal vs nocturnal species from the same genus Octodon suggest that their photoreceptor arrangements were shaped by species-specific visual demands, rather than representing phylogenetic relationships. The functional role of dual pigment cones is still enigmatic.