The fish collection of the Royal Belgian Institute of Natural Sciences consists of more than 285,000 specimens. Oceanographer Gustave Gilson (director of the RBINS in 1909) started the collection during his study of the North Sea. It was the very first study of marine fauna in Belgium. The Institute has type material for 325 species. Central African fish species are well represented thanks to expeditions to the national parks in Congo. Missions by His Majesty Leopold III and RBINS biologist Jean-Pierre Gosse have also yielded many South American species.

The Royal Belgian Institute of Natural Sciences Pisces collection contains 9007 digitised specimens of 1896 taxa. The following classes are included: Actinopterygii, Cephalaspidomorphi, Cephalochordata Owen, Cephalopoda Cuvier, 1797, Chondrichtyes, Gastropoda Cuvier, 1797, Mammalia Linnaeus, 1758, Myxini, Sarcopterygii