Variations in the phytoplankton of Zayandeh-Rood Dam Lake were studied seasonally from September 2005 to August 2006 at 4 different stations. A total of 112 species and 53 genera belonging to 6 divisions were identified. The species belonging to Bacillariophyta, Chlorophyta, Cyanophyta, Euglenophyta, and Dinophyta were found frequently. Chrysophyta species were found in low numbers. Cyclotella meneghinina Kutz., a centric diatom, was the dominant species at all stations. The density of the phytoplankton was between 470 and 150,470 cells/cm(3). According to the Shannon-Weaver Index, the highest and the lowest diversities were determined in November and May, respectively. Regarding physico-chemical analysis and phytoplankton composition, it seems that Zayandeh-Rood Dam Lake is oligo-mesotrophic, and some species of phytoplankton can be used as indicators for evaluating water quality.