The present study describes polychaete assemblages on three shallow-water benthic habitats, Ulva rigida, Cytoseira barbata and Mytilus galloprovincialis, in relation to space and time from the Sinop Peninsula (Turkish Black Sea coast). A total of 55 polychaete species and 21493 individuals belonging to 16 families were identified. Among the species, eight are reported for the first time from the Black Sea and 29 species from the Turkish Black Sea coasts. The polychaete density in the area ranged from 500 ind.m(-2) to 46775 ind.m(-2) (both on M. galloprovincialis), the polychaete biomass from 3.97 g.m(-2) (on U. rigida) to 72.01 g.m(-2) (on M. galloprovincialis). The highest diversity index values were calculated on M. galloprovincialis. Among the biotopes, C. barbata had the highest mean polychaete density in all seasons, whereas M. galloprovincialis possessed the highest mean biomass value in all seasons except in spring. Analysis of similarity revealed that polychaete community structure significantly differed among biotopes, sites and seasons.