We are interested in the impact of predators as a selective force, an evolutionary constraint, and as a modifier of interactions in their prey communities. Using measurements of size, population number, energy content of crabs, and midden content of octopuses, we show evidence of size-selection of crabs by intertidal octopuses (Enteroctopus dofleini) in Prince William Sound Alaska. Carapaces of live crabs on average were smaller than those of crab remains found at octopus middens. Energetic results show no difference in calories per unit mass among crab species. Therefore, size alone is a good predictor of energy per crab and so rate-maximizing foragers should select larger crabs. With more than six years of data from sites with high and low octopus densities, we show that this size selectivity impacts the composition of intertidal crab communities. At sites with greater octopus abundance, crabs commonly found in middens had smaller size distributions and smaller relative abundances than crabs rarely found in middens. Although species richness was similar, indexes of diversity show that sites with greater octopus abundance were more even, indicating the importance of octopuses in mediating negative interactions between crab species.