ABSTRACT: The influence of topographic heterogeneity (boulders of different sizes: 0.75, 1.5, 2.25 m) on intertidal soft-bottom community characteristics [diversity(H') richness, and biomass] was examined in the St. Lawrence Estuary, Canada. The macrofauna,sediment characteristics, and flow regime in the zones surrounding boulders were examined in 6 orientations (directions) and at 3 distances from boulders. Flow around boulders and control sites (no boulders) was estimated by the rate of plasterdissolution of standard plaster cylinders. Control sites (no boulders) showed the highest species diversity (H'_a: based on density) and richness. These parameters decreased significantly with boulder size and increased significantly withdistance from boulders. For both small and large boulder sizes, highest and lowest diversities were generally observed in opposite orientations. Overall, biomass was not influenced by boulder size. Plaster dissolution explained more variability indiversity, richness and biomass (up to 35, 30 and 25%, respectively) than did sediment characteristics (8, 6 and 27%, respectively). Species dominance was strong and probably explained the relative constancy of diversity among treatments.