Zooplankton communities are typically comprised of smaller-bodied species when size-selective fish predators are abundant, but become dominated by large-bodied species when fish predators are scarce. Superiority by larger-bodied grazers over smaller-bodied species in competition for algal resources has been proposed to be the mechanism responsible for this observed pattern. To investigate this mechanism, we performed a laboratory experiment with two freshwater zooplankton species, the larger-bodied Daphnia pulicaria and smaller-bodied D. mendotae, obtained from Square Lake (Washington County, Minnesota). The Daphnia species were grown in monoculture and in combination over a 24-day period to assess the outcome of competition between the two species and their effect on algae cell densities. We hypothesized that the larger-bodied D. pulicaria species would outcompete D. mendotae, and that D. pulicaria would exert greater control on algae levels than would D. mendotae. Results of the experiment strongly supported these hypotheses, and were consistent with findings of a recently completed field study of Square Lake that discovered that terminating the program of stocking rainbow trout (a zooplanktivorous predator) in the lake resulted in D. pulicaria replacing D. mendotae as the dominant Daphnia species and in the reduction of algae levels in the lake's surface waters.