The factors influencing the composition, structure and density of benthic invertebrate communities in Yellowknife Bay in the Canadian subarctic were determined between June 1975 and April 1977. Attached algae, primarily Fragilaria vaucheriae, Fragilaria pinnata, Cymbella ventricosa and Cocconeis placentula were ingested in large numbers by several dominant species including the oligochaetes Rhyacodrilus sodalis, Lumbriculus variegatus and Limnodrilus claparendeianus and the mollusc Valvata sincera helicoidea. Although up to 7 × 105 cells/oligochaeta, representing 65% by volume of the gut contents, were recorded for littoral zone populations, specimens in the profundal zone contained < 7.5 × 104 cells in their guts. V. sincera helicoidea also ingested large numbers of attached algae (up to 2.3 × 104 cells/animal), these representing 15–18% by volume of the gut contents. Consequently, multiple regression analysis indicated that the availability of attached algae was the major factor influencing the benthic communities. While 31–64% of the population variability could be accounted for by this factor, other parameters such as phytoplankton availability, water depth, temperature, dissolved oxygen levels, total amount of food in the gut, and the organic content of the sediments had little if any direct effect on the benthic populations.