In the autumn of 2013, a cyanobacterial bloom outbreak happened again in Lake Erhai, which caused huge public outcry and concern of the local government.In order to characterize the variations in microcystins concentrations, phytoplankton composition and density during a cyanobacterial outbreak and to evaluate the impact of microcystins on the safety of water source, a large-scale survey across the lake was conducted in September and October of 2013.Results indicated that microcystins were detected in all samples although the concentrations were low compared with those of Lake Taihu or Lake Chaohu.Both the intracellular and extracellular MCs concentrations were far less than the standards recommended by WHO (<1 μg/L), suggesting Lake Erhai as a water source for Dali City is relatively safe.However, we still found the microcystins accumulation in the body of aquatic birds in the lake.During the study period, 39 genera phytoplankton belonging to 6 phyla were recorded, with microcystins (Cyanophyta) and Psephonema aenigmaticus (Chlorophyta) as the dominant genera(species).In September, the abundance of P.aenigmaticus was more than that of Microcystis, while the situation was opposite in October.The composition and abundance of toxin-producing cyanobacteria have impacts on the concentrations of microcystins in the bay.