Seasonal changes in phytoplankton community structure, chlorophyll a and some physico-chemical features of Lake Mogan were studied during one-year period, from October 2005 to October 2006. The collected data were compared with the data in previous and recent studies. The nutrient concentrations in the lake were observed to increase with inputs of domestic wastewaters in to the lake in the last years. Chlorophyta and Cyanobacteria in the phytoplankton community in the lake were identified to be dominant together. Merismopedia minima, Microcystis aeruginosa, Sphaerocystis schroeteri and Dictiyosphaerium pulchellum were dominant during the warm period (May - October). Cyanobacterial blooms in the lake were observed in May, which is characteristic for eutrophic lakes in northern temperate region. Changes in phytoplankton community structure, Chl a peak and nutrient concentrations indicated that the trophic level of the lake is still eutrophic.