It was earlier proposed that the polymorphism of color vision observed in some neotropical monkeys could be accounted for by assuming that these animals have only a single photopigment gene locus on the X-chromosome. Three kinds of evidence have been added to existing data sets in an effort to evaluate the adequacy of the single locus model: (1) photopigment complements of squirrel monkeys (Saimiri sciureus) have been determined using electroretinogram flicker photometry; (2) photopigment pedigrees have been established for several families of squirrel monkey; (3) X-chromosome pigment genes obtained from six dichromatic monkeys (three squirrel monkeys; three tamarins--Saguinus fuscicollis) have been examined to search for sequence polymorphisms at those gene loci believed crucial for spectral tuning. All of these results are in accord with the idea that some species of platyrrhine primate have only a single type of photopigment gene on the X-chromosome.