Distribution and abundance of the molluscan fauna was studied in the intertidal and subtidal soft-bottoms of the Ensenada de San Simón (NW Spain). Depth, grain size, and total organic matter were the most important factors in determining distribution patterns of molluscs in this inlet. Three major malacological assemblages have been determined in the Ensenada de San Simón, two of them subdivided in two facies. In the intertidal area of the inlet, one facies (A1) was located in areas associated with seagrass meadows of Zostera spp. and was dominated by Hydrobia ulvae (Pennant, 1777) whereas the second facies (A2) had a high dominance of H. ulvae, Cerastoderma edule (Linnaeus, 1758), and Tapes decussatus (Linnaeus, 1758). An impoverished facies of this community was present in reduced, muddy bottoms (Group C). In the subtidal bottoms, one group (B1) was located in the central part of the inlet with H. ulvae, Rissoa labiosa (Montagu, 1803), Turboella radiata (Philippi, 1836), Parvicardium exiguum (Gmelin in Linnaeus, 1791), Loripes lacteus (Linnaeus, 1758), and Abra nitida (Müller, 1789) as characteristic species. A second facies (B2) was found in outer areas of the inlet, characterized by Thyasira flexuosa (Montagu, 1803), Mysella bidentata (Montagu, 1803), Abra alba (Wood, 1802), and Nucula nitidosa Winckworth, 1930.

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