Given a trade-off between offspring size and number and an advantage to large size in competition, theory predicts that the offspring size that maximizes maternal fitness will vary with the level of competition that offspring experience. Where the strength of competition varies, selection should favor females that can adjust their offspring size to match the offspring's expected competitive environment. We looked for such phenotypically plastic maternal effects in the least killifish, Heterandria formosa, a livebearing, matrotrophic species. Long-term field observations on this species have revealed that some populations experience relatively constant, low densities, whereas other populations experience more variable, higher densities. We compared sizes of offspring born to females exposed during brood development to either low or high experimental densities, keeping the per capita food ration constant. We examined plastic responses to density for females from one population that experiences high and variable densities and another that experiences low and less-variable densities. We found that, as predicted, female H. formosa produced larger offspring at the higher density. Unexpectedly, we found similar patterns of plasticity in response to density for females from both populations, suggesting that this response is evolutionarily conserved in this species.