The sex-determining mechanism of swordtails (Xiphophorus, Poeciliidae, Pisces) is poorly understood, because many laboratory strains of these species are characterized by biased sex ratios in either the male or female direction. Because of these ratios sex determination is said to be controlled by numerous male and female factors scattered over many chromosomes. However, direct experimental evidence for the polygenic theory is absent. Three strains of X. alvarezi were derived from natural populations in Guatemala, two of which (Dolores, Candelaria) always exhibit a ratio of 1:1, whereas the third one (San Ramón) shows a preponderance of males. Males of X. alvarezi, like those of other species of swordtails, develop a caudal appendage or sword at maturity. Chromatophores with carotenoid pigment (orange) are present in the sword of Dolores males and pigment cells containing sepiapterin (green) are present in the swords of males and the other two stocks. F1 and backcross hybrids were produced between the Dolores and Candelaria stocks. The sex ratios of all hybrid pedigrees was always in statistical agreement with unity. The pattern of inheritance of the two pigmentary traits demonstrated sex-linkage with female heterogamety (WY female female-YY male male).