The species composition and spatial distribution of macrozoobenthic assemblages in the subtidal, unvegetated areas of Arcachon Bay (a mesotidal coastal lagoon on the SW coast of France) were investigated through a quantitative survey of 18 stations, sampled during a winter period. Factorial correspondence analysis (FCA) of the distribution of main species among stations and automatic hierarchical classification identified seven species groups and five station groups or faunal assemblages, the latter being also characterized in terms of species constancy and fidelity. These assemblages were: (1) aGastrosaccus spinifer –Nephtys cirrosacommunity on clean, medium sands in the deep entrance channel; (2) a fine –medium sand assemblage characterized by the amphipodsBathyporeia sarsiandMelita palmatain the two main tidal channels, which penetrates deeply in the bay along the hydrological axes of the system; (3) a very localized assemblage of epi- and infauna on a heterogeneous substratum; (4) a modifiedAbra albacommunity in the muddy, transverse channels; and (5) a facies of the latter dominated by the oligochaeteTubificoides benedeni.Multiple discriminant analysis including seven environmental variables revealed that the most important factors in the ordination of stations were the sediment parameters, especially organic matter andZosteradebris, followed by the salinity range and water depth. The spatial organization of subtidal infauna in Arcachon Bay appears to be characteristic of a mesotidal coastal lagoon with important water exchanges with the ocean. Average abundance and biomass of macrofauna amounted to 1719 individuals m−2and 6 ·10 gm−2DW, respectively. Density, biomass and species richness were, on average, higher in muddy sands than in clean, sandy bottoms. Comparison of available quantitative data for Arcachon Bay shows that abundance and biomass of macrozoobenthos are lower in subtidal areas than in intertidal habitats.< Réduire