We describe the life-history pattern of Phalloceros caudiomaculatus on the basis of wild-caught and preserved fish from four different South American locations and laboratory data on livestock from a fifth location. This species is matrotrophic; the estimated dry weight of the young at birth is up to five times the weight of eggs at fertilization. In addition, these fish lack superfetation, meaning that there is only one brood of developing young at a time. This observation suggests that matrotrophy and superfetation can evolve independently of one another. The size distribution of mature males was bimodal in two of three samples and significantly skewed in the third. Such size distributions have indicated the presence of a genetic polymorphism for age and size at maturity in other species of Poeciliidae.