The molluscan community structure (species composition, abundance, density and diversity) associated with common brown algae Halopteris spp. at seven subtidal sites on São Miguel island (Azores) wase studied and checked for differences between sites. A total of 8,921 specimens (29 species of Gastropoda, 7 species of Bivalvia) were recorded. Bittium sp., the most common species, representing 85.6% of the total number of molluscs sampled, was present at each site and the number of taxa found at each station ranged from 3 to 17. Four species (Bittium sp., Setia subvaricosa, Tricolia pullus azorica and Rissoa guernei) accounted for 96.6% of all the specimens sampled. The density of the molluscs reached 18,000 specimens/100 g of algal dry weight (ADW). No significant differences in species diversity were found between sites. Further multispecies analysis between sites were conducted with both clustering and ordination techniques, and showed some separation between samples from south and north locations. The species responsible for this separation were the endemic Azorean Rissoids Rissoa guernei and Setia subvaricosa.