Species composition, abundance and seasonality of phytoplankton, together with physicochemical variables were studied in Lake Sapanca from June 1997 to June 1998. The lake is temperate, monomictic, and poor in nutrients although some slight increases in nutrient levels were recorded. The phytoplankton was mainly composed of Bacillariophyta, Chlorophyta and Cyanobacteria. The dominating algae were Cyclotella ocellata and Fragilaria spp. in the upper water column and Planktothrix rubescens particularly in the metalimnion. Mougeotia sp. increased in late spring and early summer 1998. Members of four other divisions were rare and found only in small numbers. Density and biomass were low. The data suggest that the lake is still in an oligotrophic state slightly changing towards oligo-mesotrophy.