The phytoplankton species and seasonal variations of the nutrients were investigated in Buyukcekmece Bay, Sea of Marmara. The samples were collected monthly from different depths at five stations in Buyukcekmece Bay, from April 1998 to March 1999. A total of 125 phytoplankton species belonging to seven classes were identified, three of which were new to Turkish seas. The maximum values of total phytoplankton occurred at 0.5 m depth (292 x 10(3) cells 1(-1)) in March, for it was in this month that the diatom Skeletonema costatum bloomed. Dinoflagellates were present throughout the year and represented the majority of the population (52%), followed by diatoms (40%). The highest cell concentrations of dinoflagellates were recorded in May, and those of diatoms in March. Throughout the year, the amounts of nitrate + nitrite, phosphate and silicate were 0.05-10.79 mug-at N l(-1), 0.16-1.06 mug-at P l(-1) and 1.10-35.23 mug-at Si l(-1), respectively. The highest and lowest N:P ratios were found to be 11 at 30 m in April and 0.17 just below the surface in June, respectively. N:P ratios were below the normal value of 16:1, and N was a limiting nutrient. These findings show that the region examined is oligotrophic in nature.