Recently, four observations of Marphysa sanguinea (Montagu, 1815) were made in the Eastern Scheldt in the southwestern part of the Netherlands. This is remarkable as this alien species has been found only once in the Netherlands, in 1990 at the North Sea coast. It seems that this large polychaete species, that previously was noted to have a northern distribution limit in the southern North Sea (English southern coast and the French west coast), has established a population in a semi-enclosed Dutch tidal bay. The species may have reached the region via shellfish transport associated with the numerous aquaculture sites in the region. Consequences of the settlement of this large at least partially predatory polychaete species, for the macrozoobenthic communities in this region are unclear. Species of the genus Marphysa are found to be successful in a variety of environments all over the world. So far, no other species of the genus Marphysa has been recorded as present in the Netherlands.