In October-November 2002 juveniles of the introduced Pacific oyster Crassostrea gigas were observed settled on adult mussels Mytilus edulis along the Wadden Sea dike of the southern part of the Island of Texel, the Netherlands (fig. 1). These juveniles had reached a size of 2-22 mm (fig. 2), apparently the size they reach in their first winter here. Earlier settlement was observed here mainly of adult Crassostrea gigas (Cadée, 2001a). This new settlement on mussels means a direct competition for food with mussels, as both species are filter feeders, and may finally result in overgrowth and smothering of the mussels. This will cause another difficulty for the return of the original musselbeds in the Wadden Sea after their destruction around 1990 by shell-fisheries. Moreover, molluscivorous birds such as oystercatcher and eider will be hampered to find their favourite food (mussels) if musselbeds become replaced by beds of the Pacific oyster on which they cannot feed.