In June 2007, a previously undescribed monkey known locally as ‘‘lesula’’ was found in the forests of the middle Lomami
Basin in central Democratic Republic of Congo (DRC). We describe this new species as Cercopithecus lomamiensis sp. nov.,
and provide data on its distribution, morphology, genetics, ecology and behavior. C. lomamiensis is restricted to the lowland
rain forests of central DRC between the middle Lomami and the upper Tshuapa Rivers. Morphological and molecular data
confirm that C. lomamiensis is distinct from its nearest congener, C. hamlyni, from which it is separated geographically by
both the Congo (Lualaba) and the Lomami Rivers. C. lomamiensis, like C. hamlyni, is semi-terrestrial with a diet containing
terrestrial herbaceous vegetation. The discovery of C. lomamiensis highlights the biogeographic significance and
importance for conservation of central Congo’s interfluvial TL2 region, defined from the upper Tshuapa River through the
Lomami Basin to the Congo (Lualaba) River. The TL2 region has been found to contain a high diversity of anthropoid
primates including three forms, in addition to C. lomamiensis, that are endemic to the area. We recommend the common
name, lesula, for this new species, as it is the vernacular name used over most of its known range.