The aim of the investigation was to describe the structure of benthic communities associated with dense assemblages of Polydora quadrilobata Jacobi (Polychaeta:Spionidae). Investigations were carried out in four different areas of the Kandalaksha and Onega bays in the White Sea on intertidal and subtidal sediments. It was shown that community structure depends on the density of Polydora quadrilobata assemblages. Only 21% of the species found were positively correlated with Polydora abundance. Community structure differs in the intertidal and the subtidal zones. The chironomid Cricotopus vitripennis, the gastropod Hydrobia ulvae and the oligochaete Tubificoides benedeni are associated with intertidal communities. Subtidal Polydora assemblages are inhabited by different species, including Terebellides stroemi, Scoloplos armiger, Aricidea nolani, Harmothoe imbricata, Eteone longa (Polychaeta), Pontoporeia femorata (Amphipoda) and Macoma balthica (Bivalvia). In all cases dense assemblages of P quadrilobata were found in places with strong organic enrichment (dead algae and Zostera mats, near mussel beds and mussel patches, in deep subtidal pits). Density of P quadrilobata in such places increases rapidly as is typical for opportunistic species. In mature assemblages of Polydora several species (mostly those with planktonic larvae or non-motile species) disappeared.