The surface deposit feeding species Pygospio elegans and Macoma balthica are dominant members of many sandy bottom communities of northern boreal regions. The feeding mode of both species and the tube-building of P. elegans are assumed to affect community structure by interactions with other species. The weight of tubes of P. elegans varied between 2 and 13 g DW/100 cm2 at the two stations investigated and during the year, which is equivalent to 230-1500 cm of tubes per 100 cm2 of sediment surface. Sediment stability may be affected directly or indirectly by the amount of tubes present. M. balthica shows a linear relation between the maximum size of particles which can be inhaled and animal length. In Kiel Bay, particles>0.5 mm are out of the range of this species. In summer, the potential feeding area (PFA) of a P. elegans population at one station in Kiel Bay was 1.8 times the available surface area. The PFA of three different populations of M. balthica in Kiel Bay exceeded the available surface area by factors of 2.6, 2.7, and 3.2. These findings indicate strong intra- and interspecific competition for food. Additionally, the feeding of both species may strongly affect the recruitment of benthic species via pelagic larvae. Experiments are proposed to evaluate the significance of the investigated behavioural aspects for community structure. © 1991 Biologische Anstalt Helgoland.