Females from two closely related reproductive complexes of Poeciliopsis reared under common conditions differed in weight of offspring at birth. The two fish strains from a more variable, upstream environment had significantly larger offspring than did three downstream fish strains. There was no consistent pattern of within-clutch variation for the two reproductive complexes. We interpret these data by examining the biotic and abiotic environmental factors in the natural habitats of these two reproductive complexes that may affect offspring size at birth, and relate these observations to current adaptive explanations for differences in offspring size. Larger offspring were significantly less likely to be cannibalized by adult Poeciliopsis. Cannibalism, combined with the possibility of size-selective predation by insect predators in the upstream habitat, might lead to an advantage of producing larger offspring in these streams. No trends between these two reproductive complexes were found in within-clutch variation in offspring size consistent with predictions of "bet-hedging" life-history models.