The feasibility of using the zooplankton speciesDaphnia pulexas a biological control agent of nuisance algal blooms was investigated experimentally in two Illinois lakes. Two questions were posed: (1) Can grazing byDaphnia pulexbuffer the effects of increased nutrient loading to an initially low-nutrient lake? and (2) CanD. pulexsignificantly reduce algal biomass when introduced into a lake that already exhibits excessive phytoplankton growth? These questions were answered by introducingD. pulexindividuals into large enclosures (1,000 liters) suspended in the lakes, which do not naturally containD. pulex. The first question was addressed in Dynamite Lake, an oligo-mesotrophic quarry lake that does not normally exhibit algal blooms. Four treatments were employed within the enclosures: (1) a control, (2)D. pulexadded, (3) nutrients (nitrogen and phosphorus) added, and (4)D. pulexand nutrients added. Nutrients were not added until theD. pulexpopulations became established and were added at weekly intervals after initial addition. In an experiment performed in 1982D. pulexdisplayed the ability to substantially buffer the effects of increased nutrient loading. Although addition of nutrients increased phytoplankton biomass in all enclosures to which they were added, by the end of the experiment enclosures withoutD. pulexhad phytoplankton densities (measured as chlorophyll a concentration) greater than those withD. pulex. The second question was addressed in Larimore Pond, a highly eutrophic farm pond with a dense summer surface bloom of phytoplankton. One experiment in the summer of 1982 was performed with a control and withD. pulexadded. The results were quite striking: by the end of the experiment (roughly 6 weeks) enclosures withoutD. pulexexhibited phytoplankton densities an order of magnitude greater than those withD. pulex. These results demonstrate thatD. pulexcan effectively control phytoplankton biomass even in lakes in whichD. pulexis not a natural inhabitant. Therefore, management strategies designed to facilitate introduction and survival ofD. pulexor other large grazers should be encouraged as a viable within-lake technique for mitigating the symptoms of eutrophication.