Stratigraphic analysis of Late Quaternary sediments of the Sea of Marmara Basin (SMB) indicates that it was a freshwater lake during the late glacial to ca 12,000 yr BP, depositing sediments with a Neoeuxinian fauna characteristic of the Black Sea Basin. At ca 12,000 yr BP, it was inundated by the Mediterranean waters and gradually converted into a marine realm as indicated by the presence above the Neoeuxinian sediments of a mixed layer, containing both marine and freshwater fauna. A sapropelic sediment layer was deposited between 10,600 and 6400 yr BP under suboxic bottom water conditions. This layer roughly corresponds in time to S1 sapropel unit of the eastern Mediterranean, suggesting a common origin. Its presence in the SMB, therefore, supports the hypothesis that a large influx of freshwaters from the Black Sea was an important factor in sapropel formation in the eastern Mediterranean. A second sapropelic layer formed in the SMB during 4750 to 3200 yr BP.