During a collecting trip (22-10-1993) in the waters around the Hollandse Biesbos (part of the river Rhine system), the occurrence of Pisidium casertanum plicatum Zeissler, 1962 was monitored in the Nieuwe Merwede. Up till now this anomalous form was only known from the Northern part of Germany (Brandenburg) and considered as a local subspecies. The form was probably introduced into the Netherlands in the early 1990s. A total of 300 specimens were collected all over the Nieuwe Merwede area, with a maximum local density of 140 specimens per 0.1 m2 off Spieringsluis. Historical data (Zeissier, 1971) as well as this recent quantitative analysis show however that P. casertanum and P.c. plicatum always appear together. Averaged percentages for the Nieuwe Merwede area: P.c. plicatum: 60%, P. casertanum (Poli, 1791): 35% and P.c. forma ponderosa Stelfox, 1918: 5%. According to this mixed occurrence, we therefore suggest the name: Pisidium casertanum forma plicata. However, future research on this population (development and changing of percentages) might give more clarity.