The osteology of the mosasaurs Prognathodon solvayi and P. giganteus is extensively described for the first time. P. solvayi is known from the holotype IRSNB R33, a complete skull and partially preserved postcranial remains as well as from fragmentary material from specimens IRSNB R107 and IRSNB R108. P. giganteus is based wholly on the fragmentary cranial and postcranial remains of the type specimen IRSNB R106. Both species are compared with the North American taxa P. overtoni (KU 950, the holotype), P.? overloni (SDSM 3393), P. rapax (AMNH 1490),? Prognathodon (FMNH PR 165) and "Prognathodon" crassartus (AMNH 1562). P. sotvayi and P. giganteus are less similar to each other than are P. giganteus and SDSM 3393, referred to P. overtoni by RUSSELL (1967), especially with regard to overall shape and size. Because of the very fragmentary holotype material of P. overtoni comparisons with it are restricted. Characters critical in the classification of Prognathodon (such as dentition, zygosphenes, zygantra and haemal arches) are re-assessed. Re-evaluation of these characters induces changes in the generic assignments of the taxa within the Plioplatecarpinae. "Prognathodon" crassartus is reassigned to Plioplatecarpus . The status of FMNH PR 165, referred to Prognathodon by RUSSELL (1970) is questioned on the basis of conflicting diagnostic characters; it is a large plioptatecarpine mosasaur probably more closely related to Selmasaurus or Plioplatecarpus . Dollosaurus IAKOVLEV, 1901 is synonymised wiih Prognathodon . The ecology of Prognathodon and certain other mosasaurs of Belgium is discussed and comparisons are made with other aquatic vertebrates.