Surficial sediment samples collected from the northeastern and southwestern shelf regions of the Sea of Marmara, together with data available from other sources, indicate marked variations in sediment compositions resulting from differences in topographical, hydrological and biological conditions. In the strait channels of the Dardanelles and Bosporus, where strong undercurrents prevail, the floor was covered mostly by coarse-grained sediments (rich in sand and gravel). However, in areas of relatively low energy conditions, sediments contained appreciable amounts of mud, with a tendency towards an increase in the amount of clay towards the open sea. The effects of the strong undercurrents on the bedforms was also apparent in the southern Strait of Bosporus where sidescan sonar surveys revealed the presence of asymmetrical sand ripples. Although terrigenous mud is the principal sediment type in the two canyons (Dardanelles and South Bosporus), the sediments, in particular on the floor of South Bosporus Canyon, show a distinct contrast between the inner N-S and outer E-W trending parts: along its axis, where depths are greatest, the outer part of the canyon appears to contain much more coarse sediment (in part derived from the benthic communities) than the inner canyon. In general, both the topography-related current regimes and the biological activities in the study areas mostly determine the types and modes of sediment distribution.