Microspectrophotometric measurements of visual pigments have been obtained for a large sample of New World monkeys of the species Callithrix jacchus jacchus. These animals exhibit a polymorphism of visual pigments. The rods (Lmax 499 nm) and the short-wave receptors (Lmax 423 nm) appear to be common to all animals but individuals differ in the number and spectral position of pigments in the green-yellow spectral region. The latter pigments cluster near 545, 559 and 567 nm. Male monkeys draw one pigment from this set and female monkeys may draw one or two. The results are generally consistent with a genetic theory that postulates in Callithrix three possible alleles for a single locus in the X-chromosome. It appears that polymorphisms of cone pigments may be widespread among neotropical primates.