The genetic diversity of the soft-shell clam Mya arenaria from seven locations in Europe (two stations in the southern Baltic Sea (the Gulf of Gdansk) and two in the North Sea (Veerse Meer and Oosterschelde), and three additional stations in the Denmark Straits and Bay of Biscay) was determined using starch gel electrophoresis of allozymes. The results showed a low level of genetic variability and a lack of genetic differentation among the populations studied. Basic polymorphism characteristics calculated for populations from the North Sea estuaries and the Gulf of Gdansk were: He 0·094–0·145, Ho 0·092–0·130, percentage of polymorphic loci 33 (0·95 criterion), mean number of alleles per locus 2·0–2·7. The mean value of FST was 0·0133 and not significant. It is concluded that in spite of a low level of genetic polymorphism the soft-shell clam is a successful colonizer. The genetic homogeneity among the populations reflects rapid population extension, alleles neutrality and a high gene flow.