Poeciliids provide a model system for comparative studies of life-history variation because of the relatively large number of species and the diversity of environments they occupy. Brachyrhaphis parismina is a narrow-bodied poeciliid that occupies rivers and streams in the eastern lowlands of Costa Rica. Detailed life-history information on species in the narrow-bodied clade of the genus Brachyrhaphis is lacking compared to the many studies on the round-bodied species of the genus. We test for variation in life history among five populations of Brachyrhaphis parismina, and for patterns of allometry associated with the reproductive value hypothesis among individuals. Life-history traits exhibited little variation among populations in contrast to variation observed among populations of round-bodied congeners. Furthermore, within locations females exhibit isometric patterns of reproductive allocation with female body size, thus life history varies little over a female's lifetime. This pattern is inconsistent with that expected from the reproductive value hypothesis. Variation among and within populations in this species may be constrained by the high flow environment they occupy.