Aluminum sulfate was applied to eutrophic Eau Galle Lake to control internal phosphorus recycling from anoxic sediments for a period of five years, and to provide increased user benefits at a reasonable cost. Treatment dose (11.3 g Al/m2) was based on mass balance estimates of summer Internal phosphorus loading rates; no attempt was made to curtail external loading rates. Phosphorus concentrations in bottom waters, internal phosphorus loading rate, and the abundance of blue-green algae were reduced relative to previous years; however, algal biomass remained relatively high. The proliferation of the dinoflagellateCeratiumhirundinella, a vertical migrator, suggested that these algae may have obtained phosphorus from interflowing river waters having a high nutrient content.