Phytoplankton samples were taken fortnightly from alkaline and oligotrophic Lake Buyukcekmece,Marmara region, Turkey during 1996-1998. A total of 65 phytoplankton taxa belonging to Bacillariophyceae, Chlorophyceae, Cyanophyceae, Euglenophyceae and Cryptophyceae were encountered. Bacillariophyceae was dominant among the phytoplankton groups. Cyclotella ocellata Pantocsek, Navicula gracilis Ehrenberg, Cocconeis placentula Ehrenberg and Nitzschia palea (Kutz.) W. Smith were found to be widespread. Seasonal development of phytoplankton is characterized by a definite spring peak followed by a moderate summer peak. The relationship between abiotic factors and the dynamics of phytoplankton is discussed.