The dynamics of the phytoplankton community of a eutrophic reservoir are described for a two year period. Fifty-eight species were recorded, 25 of them common. Bacillariophyta dominated during the winter and early spring and Chlorophyta during late spring, to be replaced by a bloom of Cyanophyta. The mean and peak biomass of phytoplankton was 8.6 mg 1−1 and 40.8 mg 1−1 in 1981, and 8.3 mg 1−1 and 37.6 mg 1−1 in 1982. Temperature accounted for 67.3% and pH for 8% of the variation in total phytoplankton biomass over the two year period, using a multiple regression technique.