Immunocytochemistry revealed in the retina of the Mongolian gerbil three immunologically distinct photoreceptor cell types. Rods comprising about 87% of the total receptor population were selectively recognized by an antirhodopsin serum (AO). The most abundant cone type (11-13% of photoreceptors) was labeled by the monoclonal antibody COS-1, specific in mammals to the middle-to-long-wave sensitive cone visual pigments. A minor cone population (2.5-5% of the cones) reacted with the monoclonal antibody OS-2, shown earlier to bind to the blue cones in mammalian species. Color substitution experiments revealed on the ERG level a color discrimination capability which must be attributed to the cooperative activity of green-sensitive (COS-1 positive) and blue-sensitive (OS-2 positive) cones. We conclude that the Mongolian gerbil has a well developed cone system, and that it may possess dichromatic green-blue color vision.