The mechanistic basis of differences in adult body size between two closely related freshwater amphipod species (Hyalellaspp.) are investigated. The study populations, Duck Lake and George Pond, are representative of two sibling species found in southeast Michigan, U.S.A that differ in adult body size and life history. An analysis of relative growth rates across size classes indicated that growth rates were similar in small size classes, but divergent at larger size classes, with higher growth rates inHyalellafrom George Pond. Because the timing of the divergence coincides with the onset of reproduction in the Duck Lake population, which reaches sexual maturity at a smaller body size than individuals in the George Pond population, tradeoffs in allocation of resources between growth and reproduction probably contribute substantially to population differences in adult body size. Resource consumption rates were similar between populations in small and intermediate size classes, but large Duck Lake adults had lower consumption rates than George Pond individuals of similar size. Thus, differences in resource consumption rate may also contribute to differences in adult growth rate and body size.