This paper presents the results of a mammal survey conducted between 1995 and 1997 in the newly established National Park of Upper Niger in the Republic of Guinea, West Africa. Ninety-four species of mammals were recorded in the park area and its environs; 19 of these species were newly recorded or confirmed for Guinea. The fauna of the park includes about 50% of the known mammalian diversity of the country. Among the species found are West African endemics such as the Gambian mongoose Mungos gambianus. The park, although situated in the Guinea savannah belt, includes some remnant forest, which harbours tropical forest mammals such as Thomas's galago Galagoides cf. thomasi, hump-nosed mouse Hybomys planifrons, soft-furred rat Praomys rostratus and flying squirrel Anomalurops sp.. This National Park is a high priority area for the conservation of the vertebrate diversity of West Africa.