A molecular genetic approach showed no difference between American and French specimens of E. compressa as well as Dutch individuals of 'E. casertana f. plicata', confirming the presence of this North-American pea clam species in The Netherlands. The analysis of this species in various collections revealed that E. compressa was introduced into Northeastern Germany before 1940, probably via the Elbe estuary and the Hamburg harbour. The apparent extension of the species over Western Europe since the 1990s suggests a second phase of introduction.