Poecilia vivipara (Bloch & Schneider. 1801) is a species that lives in the Rodrigo de Freitas lagoon, a coastal lagoon in the city of Rio de Janeiro. The objective of this study was to increase the knowledge about reproductive aspects of P. vivipara population including: the female size when it begins reproduction, the number of offsprings per gestation, embrionary development stages and the number of pregnant females. The results show that P. vivipara reproduces throughout the whole year with its first maturation occurring at 29 mm. The proportion found of females to males was 3:1. Embryos showed four developmental stages until birth with sexual differentiation occurring after 15mm.