Phytoplankton composition and density as well as some selected physical and chemical parameters were investigated between April 2008 and January 2009 in Lake Terkos and its inflowing streams. In total, 69 taxa were recorded belonging to 6 divisions: Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta, Dinophyta and Cryptophyta. Bacillariophyta showed the highest species richness in all the studied stations. The higher flow of Istranca Stream as well as its high contaminant load has a strong impact on the water quality of the lake. In addition, Karacakoy, Ciftlikkoy and Basakkoy streams, which receive untreated domestic wastewaters and animal wastes, were found to be rich in nitrate and phosphate. Therefore, in order to control water pollution in the catchment area of Lake Terkos, it is of utmost importance to provide the settlements surrounding the lake of adequate treatment plants and to rationalize the use of fertilizers, especially in the catchment area of the Istranca Stream.