The Keban Dam Reservoir (KDR), in eastern Anatolia, is one of the largest man-made reservoirs of the world with a storage capacity of >30 billion m(3). In the KDR, long-term water quality surveys have been carried out in order to determine the trophic status of this rather unique reservoir. In this paper, the focus is on the large phytoplankton species that were identified during the field studies conducted between 1991 and 1993. According to the results of the surveys, the overall phytoplankton density is low during the fall and winter months. However, species belonging to various groups started to become abundant in the spring when the water level began to rise. In terms of species diversity, the most dominant group was the Bacillariophyta. Other dominant species belonged to the Cyanophyta, Chloropyta, Pyrrophyta, Chrysophyta and Euglenophyta. The effects of various physicochemical quality parameters on the seasonal distribution and succession of the above taxa, as well as the interrelation with eutrophication, are also discussed.