Thailand has long held a key position in SouthEast Asia because of its location at the boundary of the Indochinese and Sundaic provinces, the major biogeographical regions of SouthEast Asia. These provinces are distinct climatically, floristically and faunistically. The present-day limit between them is located at the Kra Isthmus, in peninsular Thailand. Previous studies of the Javanese large mammal fossil faunas and the recent study of fossil large mammal faunas from Thailand strengthen the hypothesis of a continental migration route (in contrast with the insular hypothesis via Taiwan and the Philippines) during the Late Middle/Late Pleistocene period. Thailand was even part of this migration route. During the glacial periods, the faunal exchanges were favored by the emersion of a huge continental shelf called Sundaland (SouthEast Asian continental area connected to Borneo and Indonesia islands by land bridges), when the sea level was low. No geological, biogeographical or paleobiogeographical evidence supports the hypothesis of a migration route via Taiwan and the Philippines. Analysis of the extant and Late Middle Pleistocene large mammal faunas (Carnivora, Primates, Proboscidea and Ungulata) points out the antiquity of the Indochinese and Sundaic provinces. This idea is also supported by the fact that mainland faunas already displayed modern character, whereas the Javanese faunas were mainly composed of endemic forms. However, the occurrence of the extinct species, Elephas namadicus found in a Malaysian site (Tambun) indicates that the limit between the two provinces, if not close to the present place, was located more southward than today, possibly in peninsular Malaysia. It is also confirmed by the presence of northern species, such as Ailuropoda melanoleuca baconi and Crocuta crocuta ultima, in Late Middle Pleistocene sites of Thailand. In fact, because of climatic cooling that occurred in the northern hemisphere during the Pleistocene, the northern faunas had to move southward. To our knowledge, the hypothesis of the antiquity of the SouthEast Asian provinces with a boundary south of the present-day one is mentioned here for the first time. Previous biogeographical studies were based mainly on insular data from the Philippine Archipelago (geological and mainly present-day zoogeographical data) but also from Indonesia and Borneo (paleontological data). However, the recent discovery of Thai large mammal faunas from Late Middle Pleistocene allows to bridge the gap in the fossil record between the northern faunas of SouthEast Asia and the southern ones. These Thai faunas are characterized mainly by extant forms but some are today absent from the Thai territory (Ailuropoda melanoleuca, Crocuta crocuta, Pongo pygmaeus, Sus cf. barbatus, Rhinoceros cf. unicornis, Cervus eldii).