Although dwarfed by the low saline conditions, the blue mussel (Mytilus edulis L.) is a biomass dominant in coastal areas of the Baltic Sea proper, owing to the lack of invertebrate predators and competitors for space. In the present study patterns of distribution of blue mussels on relatively smooth, moderately sloping rocky surfaces in the Asko area, northern Baltic proper, are described and interpreted. Biomass, growth rate, maximum shell length and recruitment of mussels decreased with water depths (from 3 to 15 m). No consistent differences in total abundance were, however, detected among depths. Shell growth, as measured by annual growth rings, was linear between about 2-8 yr. of age. Within this linear interval the estimated rate of shell growth was significantly faster (3.4 mm yr.(-1)) at 5 m than at 10 m depth (2.7 mm yr.(-1)) Although the observed vertical patterns in biomass distribution correlated both with recruitment and growth, the year round presence of large numbers of small mussel individuals (less than or equal to 2 mm) at all depths indicates that differences in somatic growth may explain the observed variability in biomass among depths. Potentially important processes such as intraspecific competition, food availability and food quality, which may generate the observed patterns in growth and biomass are discussed.