A fauna of 112 species of bivalves (Mollusca) from the earliest part of the Oligocene (Grimmertingen Sand Member in Belgium and Klimmen Member in The Netherlands) is described and figured. Their relations with records from the North Sea basin, the Paris basin and other European regions is discussed. Nine species are recorded new for the Grimmertingen Sand Member. A neotype for Pinctada cf. wemmelensis (E. Vincent, 1893) is designated. Furthermore, four species new to science have been recorded at Grimmertingen: Glycymeris (Chevronia) caproensae nov. sp., Limopsis (Pectunculina) albrechtvalki nov. sp., Palindonaia oligocenica nov. sp., Planikellya nassauensis nov. sp., the last one also occurs in the Dutch Klimmen Member, together with another new taxon, Erycina johnjagti nov. sp. The new genus Striatonuculana nov. gen. is introduced. The fauna is compared with that of the Eocene and the Neogene and evolutionary lineages are discerned. Special attention is given to comparison with two other Belgian Rupelian deposits, the Kerniel Sand Member and the Ruisbroek Sand Member, without giving a complete inventory of the fauna of both Members. The bivalve molluscan fauna investigated has relatively few species in common with Eocene and Neogene, the largest part remains limited to the Member and its stratigraphic equivalents abroad. For this reason, the Grimmertingen Sand Member, Klimmen Member, Ratheim Member and (part of) the Latdorf Schichten could be separated into a distinct Early Oligocene unit, separated from the remaining pre-Chattian Oligocene deposits.