The distribution area of the nassariid gastropod Cyclope neritea (L.) includes the Mediterranean and the Black Sea, as well as the Atlantic coasts of the southern Iberian Peninsula. The species has spread north to the eastern Bay of Biscay (Arcachon Bay in 1976, Marennes-Oleron area and Morbihan Gulf in 1983-84). This spread might be explained either by (1) a natural spread favoured by environmental changes (e.g. an increase of temperature) or (2) a sudden range expansion due to the introduction of individuals from distant native populations. Molecular genetic analyses based on mitochondrial markers suggest that the present C. neritea population in Arcachon Bay has been introduced, probably unintentionally with oyster transfers, from several source populations, genetically similar to the populations analysed in this study, i.e. those in the western Mediterranean and in south Portugal. Within its new distribution area, C. neritea could potentially compete with the autochthonous nassariid Nassarius reticulatus, both species being scavengers. Although C. neritea tends to occur mainly in relatively clean sands in the intertidal and N. reticulatus in subtidal, organic rich sediments, the habitats of both species partially overlap. Laboratory experiments showed that in still water conditions C. neritea was more active and reached carrion faster than N. reticulatus, thereby having a competitive advantage over the latter; flow conditions (similar to1 cm s(-1)) appeared to stimulate the activity of N. reticulatus. Analysis of parasite load in both species in Arcachon Bay indicated that N. reticulatus was more heavily parasitized by digenean trematodes than C. neritea. This suggests that the spread and population dynamics of C. neritea along the French Atlantic coast has been favoured by the lack of parasites. Altogether, recurrent introduction, competitive ability and lack of heavy parasitic load might explain the successful settlement of C. neritea along the French Atlantic coast.