In the last ten years, several fossil localities in Nakhon Ratchasima Province (northeastern Thailand) have yielded late Neogene turtle and crocodile taxa. Although not always well dated, the age of all these fossils is constrained between Middle Miocene and Pleistocene. Several crocodile taxa have been discovered: Crocodylidae, Alligatoridae and Gavialidae. In particular, the presence of Gavialis is attested in northeastern Thailand during the Pleistocene. Fossil turtles belong to several families: Geoemydidae, Testudinidae, and Trionychidae. In addition to the discovery of giant continental tortoises, some large aquatic trionychid and geoemydid turtles are present. The latter are only found in larger river systems and no longer encountered in the Mun or Chi Rivers of northeastern Thailand. Most fossil forms correspond to modern species and reveal interesting changes in biogeographical distribution patterns. These changes are likely related to changes in the size of the drainage and direction of the Mun River system. We suggest that the Mun River was probably not flowing from west to east to the Mekong River system as today but that it was flowing from east to west to the Chao Phraya River system.