Seasonal variations of phytoplankton composition and density of Lake Sapanca, were investigated on the littoral and pelagic sample station points between December 1999 and February 2001. Samples were collected monthly from 4 stations which were chosen in the west part of the lake. At each station, water samples were obtained at three site. The first sites were at the closest point to the shore. In addition, there is 100 m distance between each sites. In total 54 phytoplankton species were recorded: Bacillariophyta 30, Chlorophyta 12, Cyanophyta 5, Euglenophyta 3, Dinophyta 2, Chrysophya 1 and Cryptophyta 1. Bacillariophyta, Chlorophyta and Cyanophyta constituted majority of the phytoplankton both in terms of species numbers and density. No apparent differences were found in phytoplankton compositon at three sampling sites of each station. In general, surface phytoplankton were found to be rather poor as it was in the previous quantative phytoplankton studies carried-out in the lake. Chlorophyll-a concentrations were measured higher (8.39-21.79 mg/m(3)) than the previous studies.