Differences in maturation time, body size and clutch size among populations of the freshwater copepod Mesocyclops edax inhabiting lakes of different seasonalities (Lake Thonotosassa, Florida, and Douglas Lake, Michigan) were maintained through two generations under common laboratory conditions. In most cases, Florida individuals matured more rapidly and had larger body sizes and clutch sizes than Michigan individuals over a range of temperature from 15 to 30 deg. C. The greatly reduced fecundity and longer maturation time of the Michigan population relative to the Florida population at 15 deg. C may reflect adaptations to different temperature regimes encountered by these geographically distant populations. Genotype-environment interactions were observed for body size, but not for maturation time or clutch size. Some local variation was evident in both the Michigan and Florida locales, indicating that forces operating on a local scale may result in substantial variation that is superimposed upon any broad scale, geographic patterns. Selection for short maturation time and large clutch size is hypothesized to have occurred in the Thonotosassa population, which is exposed to heavy fish predation and high food levels. The small bodies and clutches in the Douglas population, which experiences chronically low food levels, may reflect selection to minimize energy demands where food is scarce.