During 1981 the seasonal growth and community composition of phytoplankton were investigated at six sampling stations in Eau Galle reservoir, a shallow eutrophic basin located in central Wisconsin, U.S.A. Eau Galle was strongly dominated by diatoms (spring and fall), and co-dominated by cyanophytes and dinoflagellates (summer). Three species (Stephanodiscus hantzschiiGrun.,Aphanizomenon flos- aquae(L.) Ralfs, andCeratium hirundinella(O.F.M.) Schrank) collectively contributed about 50% of total annual phytoplankton biomass. Phytoplankton response was quite similar at open water sampling stations, but differed at a station in the littoral zone and at another located near the mouth of the primary inflowing river. Depressed epilimnetic silica concentrations occurred during spring and fall periods of diatom growth. The development of nitrogen-fixing cyanophyte populations during the summer reflected reduced dissolved inorganic nitrogen availability. Midsummer mixing events reduced water column stability. Shifts in the phytoplankton species composition of Eau Galle appeared to occur in response to major changes in water column stability.