The cave of Tham Khuyen in Lang Son Province, northeastern Vietnam, has yielded a large mammalian fauna of probable late middle Pleistocene date. A series of isolated hominoid primate teeth, formerly allocated to the extant orangutan Pongo pygmaeus, has recently been reexamined and found to represent more than one species. These specimens are described in detail in this paper and are analyzed as follows. Some ofthe teeth are indeed clearly identifiable as those of Pongo pygmaeus, but the majority appear to belong to a species related to the orangutan but not identical with it
