The structure of macrozoobenthos of Lake Syamozero was determined and updated through longterm studies. It comprises 182 taxa of invertebrates (7 types, 10 classes, 19 orders and 66 families). The Chironomidae family stood out for the greatest diversity (52 species and genera). The state of the main macrozoobenthos habitats in Lake Syamozero was studied. The highest biomass values were found in quiet overgrown littoral zones (8.7 g/m2) and profundal silts (7.3 g/m2); the lowest biomass values – in stonysandy shallow zones (5.7 g/m2) and sandy littoral zones (2.8 g/m2). The amplitude of longterm fluctuations of macrozoobenthos average biomass on soft substrates of Lake Syamozero in autumn varied from 0.9 to 5.4 g/m2. In terms of macrozoobenthos structure and biomass Lake Syamozero is a mesohumic mesotrophic lake of the chironomid group or, if placed on the trophic status scale – an αmesotrophic lake.