This paper studies the Palaeotaxodonta , Cryptodonta and Pteriomorphia (Bivalvia, Mollusca) from the Ledian (Upper Middle Eocene) of Belgium. The taxa are classified according to the « Treatise on Invertebrate Paleontology » (Part N, Vols.1 and 3, Mollusca 6, Bivalvia) and a synonymy is given. Stratigraphical and locality detailes are provided for each species, but these are mainly restricted to Belgian occurrences in the Ledian and strata immediately above and below it. Poorly known species are redescribed and figured and their nomenclatures revised. The catalogue numbers of all the figured and referred specimens are included. An alphabetically arranged list of localities has also been compiled. Forty-seven taxa are described in this first part, of which three are new species: Nucula smithsoni , Vulsellina (?) lediensis and Lyropecten (Aequipecten) walleri .