The Pacific oyster, Crassostrea gigas, well known throughout the world because of its ability to adapt to a wide range of environmental conditions, was introduced for cultivation into France on a massive scale in the 1970s. With global warming, the reproductive population, confined at the beginning to the south of the French Atlantic coast, became established at more northern latitudes (above 45 degrees 58'N), and wild C gigas began to colonize coastal areas such as our study site, Bourgneuf Bay (1 degrees-2 degrees W, 46 degrees-47 degrees N), an oyster-farming site. An original approach, based on orthophotograph analysis and in situ biomass sampling, revealed that, in the northern part of this bay, more than 70% of the total C gigas biomass was composed of wild oysters (i.e. C gigas not bred by oystermen). The analysis of the spatial distribution of wild oysters indicated that 75% of the stock consisted of wild oysters in natural beds (rocky areas) and on low retaining walls of former fisheries. Wild C gigas also colonized oyster-fanning structures with lower biomasses (21% of the stock composed of wild oysters), but locally they could reach densities of up to 55 kg.m(-1) i.e. 2.5 times the mean biomass of cultivated oysters. The economic and ecological consequences of this colonization by C gigas of an oyster culture site are discussed. Wild oyster seems to be the principal trophic competitor of cultivated oysters in Bourgneuf Bay. This may partly explain the decrease in growth of cultivated oysters observed in this bay during the last decade. Moreover, the trophic and spatial competition exerted by wild oysters may also affect the native biota and, in particular, the honeycomb worm Sabellaria alveolata. The results obtained in this study have led oyster farmers and regional authorities to modify oyster-farming practices and to destroy wild oyster stocks in concession areas. (c) 2006 Elsevier B.V. All fights reserved.