Phytoplankton composition and some ecological variables were investigated seasonally between November 2006 and August 2008 in the Gulf of Bandirma. From the analysis of phytoplankton community composition, 99 species of 5 different algal groups were identified: 56 dinoflagellates (56.6%), 38 diatoms (38.4%), 3 dictyochophyceans (3%), 1 euglenophycean (1%) and 1 prasinophycean (1%). Amphidinium sphenoides was reported for the first time in the Turkish coastal waters. Diatoms and dinoflagellates were the most dominant groups in terms of species number at 3 sampling stations. The maximum total phytoplankton abundance (640 x 10(3) cells L-1) was observed in the surface water of Station 2 in August 2008, with Cerataulina pelagica being the most dominant species of that month (600 x 10(3) cells L-1). During the sampling period, water temperature ranged between 6.5 and 26.0 degrees C, salinity between 21.4 and 38.5 ppt, dissolved oxygen between 3.50 and 15.62 mg L-1 the amount of NO2+NO3-N between 0.08 and 6.25 mu g-at N L-1 PO4-P 0.18-8.60 mu g-at P L-1, SiO4-Si 0.05-20.37 mu g-at Si L-1 and chlorophyll-a values between 0.22 and 14.70 mu g L-1. All through the study, the increase in depth caused an increase in the amount of nitrogen and silica, and a decrease in the amount of phosphorus.