We performed a common garden experiment to assess the existence of genetic differences on growth and body size between two populations of Poecilia vivipara inhabiting extremes of an environmental gradient caused by water salinity in lagoons of Northern Rio de Janeiro State, Brazil: the Campelo lagoon (freshwater) and Açu lagoon (brackish/saltwater). The two populations show extreme differences in average phenotypes for body size, shape and life history (freshwater populations with smaller body size, lower fecundity and larger reproductive allotment). Pregnant females were brought to the lab and the offspring from both groups were kept in a common recirculating system with freshwater. Standard length and survival were measured weekly over a period of 200 days and growth models were fitted and selected with information criteria. The offspring originally from the brackish water lagoon presented larger asymptotic length, higher maximum growth rate but lower survival than the offspring originally from the freshwater lagoon. Potential confounding variables such as density differences due to mortality and maternal effects (offspring size) were included as covariates in comparisons of growth rates between groups.The results are consistent with phenotypic differences among populations having some genetic basis, and with the existence of a trade-off between growth and maintenance due to the high growth/low survival observed in the group that changed from salt to freshwater. Comparisons of captive and natural populations suggest that the influence of environmental factors, such as salinity, food availability, fish density and predation should also be considered relevant to explain phenotypic variation in this system.