The composition and abundance of phytoplankton and the corresponding environmental conditions were investigated during a mucilage event (October 2007-February 2008) in the Sea of Marmara. The mucilage producers Gonyaulax fragilis, Skeletonema costatum, and Cylindrotheca closterium were identified as abundant species. Thallassiosira rotula was also identified in large numbers. The maximum number of G. fragilis was observed at 83,600 cells $L^{-1}$ in November 2007 during the first sampling occasion on the surface layer of İzmit Bay, and T. rotula was the most abundant diatom species, with 131,040 cells $L^{-1}$ in the same period. G. fragilis was recorded at 96,250 cells $L^{-1}$ in the dense mucilagecontaining water samples collected from Değirmendere (İzmit Bay) in January 2008, and C. closteriumwas the dominant (161,250 cells $L^{-1}$) diatom species in the same sample. Species diversity values ($H' _{log2}$) increased from November to February, displaying maximum values of 2.5-3.5 in February at almost all stations (depths of 0.5 and 10 m), caused by the balanced increase of diatom species and their individual numbers in the total phytoplankton abundance. The surface layer waters of İzmit Bay exhibited severely low nitrogen to phosphorus ratio (N:P) values (0.1-14.4) compared to the Redfield ratio (16), which could be supported by the fact that the limiting nutrient for the Sea of Marmara is nitrogen. Even though there is a lack of data from before the mucilage period, the data obtained during the lifespan of the event indicated that a mixture of several phytoplankton species might have played a key role in the mucilage formation. The high organic carbon content of the mucilage (24% org-C) and the surrounding waters support the possibility that the event could have been related to organically rich cellular exudates of phytoplankton.