Arcachon Bay was the last important shell culture area of the French Atlantic coast without a stock assessment of the accidentally introduced Crepidula fornicata (Gastropoda). Following a stratified sampling strategy, 205 stations were sampled with a dredge. The total biomass of C. fornicata was estimated at 155 tn (confidence limits: 72 tn) scattered on 2·4 km2, i.e. about 5% of the infralittoral area. This biomass is very low compared to other exploited shellfish sites. Amongst mean environmental parameters (such as bathymetry, temperature, salinity, sediment grain-size, anthropic activity), the major keyfactors differentiating Arcachon Bay from other French sites were searched to explain the differences in C. fornicata colonization pattern. It seems that the low biomass of C. fornicata, 30 y after its occurrence was first recorded in the bay, could be correlated to: (1) the scarcity of subtidal muddy sediments which are suitable for C. fornicata settlement; (2) the presence of large Zostera spp. beds on both intertidal and subtidal areas; and (3) the absence of bottom trawl fishing.  Growth, production and productivity (P/B) ratios were compared between five stations characterized by different hydrological and sediment parameters. Growth and productivity (0·3 y−1) were not affected by mean biomass suggesting a lack of population regulation by intraspecific competition.