The application of 936 metric tons of alum to eutrophic Medical Lake, WA in 1977 interrupted the lake's internal phosphorus cycle, lowering its trophic status. Observations of the zooplankton community were made before, during, and for three years following the restoration. A decline in percent composition of rotifers with a corresponding increase in crustaceans has occurred both in response to the altered trophic status and as a result of coexploitive interactions within the community. The Rotifera numerically comprised approximately 90 percent of the zooplankton standing crop prior to the restoration but only 43.5 percent of the standing crop in 1980. The Crustacea (i.e., Cladocera and Eucopepoda) collectively comprised 9.9 percent of the total standing crop in 1977 increasing to 56.5 percent in 1980. Indices of species diversity also indicated a trend toward a more diverse zooplankton community with less annual variability.