Abstract In this study, a diatom increase in the phytoplankton community in the north-east part of Sea of Marmara was observed in winter (upper layer water temperature between 7-9oC and salinity between 22-24 psu). In the water samples were determined a total of 45 phytoplankton taxa from the four algae classes. Most of these taxa (94 %) were composed of dinoflagellates and diatoms and other organisms parts were silicoflagellates and euglenoid flagellate species. The most common species from diatoms were Proboscia alata f. alata (Brightwell) Sundström, from dinoflagellates Ceratium fusus (Ehrenberg) Dujardin. The diatom Proboscia alata f. alata was quantitatively the dominant species in the phytoplankton community. The highest cell density was measured about 185 x 103 cells l-1. It is noticeable that diatoms were more dominant than other phytoplankton species.