Squirrel monkeys (Saimiri sciureus) have a striking color-vision polymorphism; each animal has one of six different types of color vision. These arise from individual variation in the presence of three different middle- to long-wavelength cone pigments. The distribution of cone phenotypes was established for a large sample of squirrel monkeys, including several families, through analysis of a retinal gross potential. The results indicate that the inheritance of color vision in the squirrel monkey can be explained by assuming that the three middle- to long-wavelength cone pigments are specified by three alleles at a single locus on the X chromosome. This arrangement is discretely different from that found in Old World monkeys and humans.