The distribution and ecology of the freshwater mussels of the family Unionidae was studied in the estuarine area of the rivers Rhine, Meuse and Scheldt. Only in the 10th century A.D. this area became inhabitable for unionids. Their distribution at present has been greatly influenced by the many inundations by salt and brackish water, most recently in 1953. Therefore the ecological conclusions must be based mainly on data from the freshwater area of the rivers. It is remarkable that in this area no type of species has evolved in the family Unionidae, that is adapted to the intertidal zone of the freshwater tidal area. The lower reaches of the river Rhine and its branches are too much polluted to support unionids, except for Unio tumidus, some specimens of which were found at a few particularly suitable places. A description is given of the distribution and ecology of Unio crassus batavus, U. tumidus, U. pictorum, Anodonta anatina, A. cygnea and Pseudanodonta complanata. These species seem to have different ecological ranges (fig. 12). It is suggested that these ecological differences may be partly caused by hostrelationships of the larvae to particular species of freshwater fishes.