Pacific oysters have been introduced into the Wadden Sea by aquaculture in the 1980sand subsequently established. As oyster larvae need hard substrates for settlement, theyare overgrowing resident epibenthic mussel beds (Mytilus edulis). The impact of C.gigas on the native ecosystem was assessed by investigating the populationdevelopment and the scope for coexistence with M. edulis. The invasion of C. gigas inthe northern Wadden Sea started slowly with abundances remaining low for about 15years. Only a succession of three summers with anomalous high water temperatures ledto a massive increase in abundances. Field experiments revealed differential settlementof both species and faster growth of C. gigas compared to M. edulis. A high survivalrate of juvenile oysters was presumably caused by low predation pressure by the mainbenthic predators, shore crabs and starfish, which both strongly preferred mussels tooysters. As C. gigas is well adapted to the recipient ecosystem and competitive superiorto their native congeners, the development of massive oyster reefs on the expense ofmussel beds is expected. A massive increase of the oyster population may lead to foodlimitation of other suspension feeders, and to a decline of benthic predators.