During a collecting trip (22-10-1993) in the waters around the Hollandse Biesbos (part of the river Rhine system) the occurrence of an anomalous Corbicula species was monitored in the Nieuwe Merwede. The collected specimens resemble very much to the description of C. fluviatilis (Müller, 1774) and C. radiata auct. However, the status of both species is still under discussion.