The seasonal variation of phytoplankton community and affecting some physico chemical factors of Sazlidere Dam Lake were investigated monthly from December 2003 to November 2005. Phytoplankton was constituted of Bacillariophyta, Charophyta, Chlorophyta, Cryptophyta, Cyanobacteria, Euglenozoa, Miozoa and Ochrophyta members. According to the functional group aproach, the phytoplankton community of the dam lake was formed by 19 groups (B, D, N, P, T, S2, X1, Y, E, F, J, H1, L-O, L-M, M, MP, R, W1 and W2) which were most common in mesotrophic and eutrophic waters. The seasonal variations of phytoplankton were most affected by light, temperature and nutrients. A highly significant positive correlation (r = 0.667) existed between phytoplankton density and orthophosphate concentrations. The Shannon-Weaver diversity index ranged from 0.035 to 1.059, Simpson dominance index changed between 0.096 and 0.973. According to regulations for the water quality of potable waters in Turkey, Sazlidere Dam is of class I in terms of temperature, pH, dissolved oxygen, nitrate, nitrite and orthophosphate concentrations. Chlorophyll-a concentrations (5.47-57.0mg/m(3)) showed that the lake has eutrophic characteristics. When the physicochemical and biologic features of the lake were evaluated, it was determined that the Sazlidere Reservoir showed a change towards from oligotrophic to mesotrophic characteristics.