The extensive intertidal flats of the Minas Basin, Bay of Fundy, support some of the highest population densities of Macoma balthica ever recorded. Densities of Macoma are positively correlated with density of bacteria in the sediment; correlations with tidal elevation and organic carbon content of the sediment are not significant. Macoma density appears to be related to the amount of fine material present in the sediment. Although Macoma feeds on the bacteria within the sediment, it must supplement its diet by suspension-feeding during high tide in order to acquire sufficient protein.