The olivaceous background coloration of the platyfish, Xiphophorus maculatus, results from the presence of micromelanophores interspersed among small xanthophores in the epidermis of the scales and from the arrangement of the micromelanophores in a reticular pattern in the dermis of the flank. Two autosomal loci, St and R, with two alleles each have been identified which control the presence or absence of these pigment cells. St - R - fish have the wild-type coloration. The St - r r genotype, gray lack the xanthophores, but the micromelanophore pattern is indistinguishable from that of St fish. Fish of genotype st st R are yellow all over. They are virtually free of micromelanophores in the scales and dermis, but the number of xanthophores is increased manyfold in the scales. The dermis and the perineural lining of the brain are densely and uniformly populated by xanthophores. By contrast, xanthophores are absent from these two tissues in the St fish. The double recessive fish, st st r r, have a whitish appearance, ghost, and lack micromelanophores and the background xanthophores throughout the integument. In st st R - and st st r r fish, melanophores are also absent from the perivascular lining of the blood vessels, the gill arches and the wall of the swim bladder. Neither the St nor the R locus have any detectable effect on the retinal pigment layer or on the melanophore pigmentation of the connective tissue sheath of the spinal cord. Dopa positive cells could be demonstrated in the scales and in the dermis of st st R - and in the scales of st st r r fish. The dermis of the double recessive was Dopa negative. The St locus has at best a minor effect on the expression of the macromelanophore patterns which are controlled by sex-linked genes. Similarly, the R locus has no effect on the expression of the sex-linked xanthophore and xantho-erythrophore patterns. The existence of several main lineages of pigment cells can be discerned in X. maculatus and the action of St and R loci are restricted to only one of these lines. The St locus of X. maculatus may be homologous to the B locus of Poecilia reticulata and the R locus of X. maculatus may be homologous to the R locus of P. reticulata and the Gr locus of Poeciliopsis viriosa. No gene homologous to the G locus of P. reticulata, which also effects the background melanophores and xanthophores, has been identified in X. maculatus.