The phytoplanktonic algae were determined in samples taken from 7 different locations at the Sarιyar Dam Reservoir (SDR) from March 1996 to June 1997. The seasonal variation of phytoplankton and their chlorophyll a were determined and contents were discussed in relation to the physical and chemical parameters of the water. The divisions Bacillariophyta and Chlorophyta were dominant; however, Cyclotella, Navicula, Nitzschia and Synedra of Bacillariophyta, and Chlorella and Scenedesmus of Chlorophyta were abundant. Chlorophyll a reached its maximum levels during June and September 1996 and May 1997, corresponding to the maximum values of orthophosphate, sulphate, calcium and temperature in the lake. © TÜBİTAK.