This study was implemented to determine the environmental factors and causative organisms of the recent mucilage formation in the Marmara Sea. Samples were taken during the study from 7 different depths (0.5-30 m) of one sampling point of the Buyukada Island shore between January and June 2008. As a result, 62 phytoplankton species belonging to 5 different groups were identified. Dinoflagellates were dominant in terms of species number, and diatoms in terms of cell number. In January and February, mucilage formation was very dense, where 5 phytoplankton species (Clindrotheca closterium, Pseudo-nitzschia sp., Skeletonema costatum, Thalassiosira rotula (Bacillariophyceae) and Gonyaulax fragilis (Dinophyceae)) were reported as dominant organisms. Among them, Gonyaulax fragilis has never been reported in the Marmara Sea previously, thus that organism appeared firstly with the formation of dense mucilage and then when the mucilage decayed in May and June 2008, G. fragilis disappeared. Autofluorescent single-celled organisms were classified in three groups depending on their cell sizes (> 20 mu m, > 2 mu m, > 0.2 mu m) by membrane filtration and total count of bacteria were determined by epifluorescence microscope after dying with DAPI. The highest total bacteria was recorded in April at 25 m depth (6655 +/- 44.4 cells ml(-1)) while the lowest count was in June at 0.5 m depth (1077 +/- 26.1 cells ml(-1)). The seawater temperature ranged between 7.0 and 21.5 degrees C, salinity between 20.9 and 37.4 ppt and dissolved oxygen amount between 2.75 and 12.75 mg l(-1). The chlorophyll-a amount ranged between 0.10 and 6.35 mg l(-1), the higher values were recorded in January at 15 m depth (6.35 mu g l(-1)) and in April at 10 m depth (4.89 mg l(-1)). Among the nutrients, the amounts of nitrite + nitrate-N varied between 0.02 and 7.67 mu g-at N l(-1), phosphate-P between 0.11 and 0.96 mu g-at P l(-1) and silicate-Si between 0.37 and 8.93 mu g-at Si l(-1). The highest values were determined at a deeper layer where nutrients are accumulated. On the other hand, the N: P ratio interval was found as 0.1-11.3, Si:P ratio as 2.92-52.33 and N: Si ratio as 0.01-1.10 during the sampling period. Nitrogen was the limiting nutrient and the silica amount was enough to enable the development of diatoms.