A mammal survey was carried out between 1984 and 1987 in southern West Sepik Province, Papua New Guinea. Eleven major collecting localities, as well as some more minor ones, lying at altitudes of between 120 and 3,200 m were investigated. Voucher specimens for 87 indigenous mammal taxa were obtained, but research suggests that mammal diversity in the area may be as high as 120 species. This is the highest mammal diversity recorded anywhere in Australasia. A similar high bird diversity suggests that the area may be one of exceptionally high biodiversity overall. The most diverse mammal assemblages in the study area are found in the midmontane oak forests (between 1,500 and 2,500 m). Seven species, which apparently have no ecological vicars elsewhere in PNG, inhabit these forests. Changing patterns of human exploitation endanger some species. Recommendations aimed at halting this decline are made. The effect of the introduction of cats in one area was assessed as cats were introduced in the middle of the survey period.