The Pacific oyster, Crassostrea gigas, native to northeast Asia, has been translocated widely for the purpose of aquaculture and is among the most important cultured shellfish. C. gigas has a complex history of introductions in Europe. In France, massive introduction was performed in the early 1970’s, allowing massive spatfall in favourable areas to become the basis of the local oyster industry. In the U.K, hatchery propagation was used to supply local production. Some seed was also exported to Northern Europe were demographically independent populations have been reported in the last decades. In the present study 22 sites were sampled in the British Isles, Denmark, France and Spain in order to assess the genetic diversity and population differentiation based on 11 microsatellite loci and a mitochondrial marker. Three hatchery-propagated stocks from two UK hatcheries were also studied. Two main genetic clusters were identified from pairwise genetic differentiation indexes, Bayesian clustering methods or neighbour-joining analysis: (1) North East cluster (East England, Ireland and Denmark; and UK hatcheries stocks) and (2) South West cluster (South Wales, South West England, France and Spain). There was a significantly higher allelic richness (A) and expected heterozygosity (He) in the South West cluster (A: 45.68, He: 0.928) than in the North East cluster (A: 26.58, He: 0.883). The Fst value between those two clusters was small but significant (Fst=0.017, 95% CI: 0.014-0.021). A 739-bp fragment of the Major Noncoding Region of the mitochondrial genome was sequenced for 12 of the studied samples, and for one from Miyagi prefecture (Japan). Eighty one haplotypes were found. Haplotypes frequencies confirmed our two clusters. The impact of human-mediated transport or natural dispersal will be discussed as potential explanations for the observed pattern of genetic differentiation, in combination with previous studied that focussed on other parts of Europe