Viviparous fishes of the family Poeciliidae have been model systems in the study of life histories because of their remarkable variation in life-history traits both within and among species, and because of the diversity of selective environments that they occupy. However, the life histories of several poeciliids remain unknown. In this study, we describe the life history of Gambusia vittata, a poorly known poeciliid endemic to eastern Mexico. We analyze variation in number of embryos per reproductive female, size of individual embryos, size at maturity, reproductive effort, the amount of maternal transfer of nutrients to developing embryos, and the relationship between female size and life-history traits. We collected individuals from three sites across 3 months. We found that brood size varied significantly across sites and that brood size, embryo mass, and reproductive allotment varied across months within sites. Brood size, embryo mass, and reproductive allotment all increased with female size. We found that this species exhibits moderate matrotrophy, meaning that females transfer small amounts of nutrients to embryos during development. We compare our results to other poeciliid species and provide tentative explanations for the patterns of intraspecific variation that we observed in the life history of G. vittata.