Allen's swamp monkey, Allenopithecus nigroviridis, is confined to the swamp and riparian forests of the Central African region. It occurs along the Congo River and its tributaries. Recent data show that it occupies suitable habitat at least 100 km to the northwest of its previously known distribution, in an area to the west of the Nouabalé-Ndoki National Park, in the northern Republic of Congo. The apparent absence of the swamp monkey from most of the interior of the Nouabalé-Ndoki National Park itself suggests that the species probably reached northwest Congo along the Sangha and Ndoki Rivers, rather than by dispersing across the watersheds from the Oubangui. Hunters find the species easy to shoot from canoes at night, as these monkeys like to sleep in overhanging vegetation beside the rivers. More details on its distribution in the region and of its conservation status are required.