The Mount Kilimanjaro guereza colobus monkey is endemic to northern Tanzania and southern Kenya, occurring on and near Mount Kilimanjaro/Mount Meru. Currently referred to as "Colobus guereza caudatus Thomas 1885", this monkey is geographically very isolated, and phenotypically distinct from all other taxa of guereza monkeys. As such, application of the "Phylogenetic Species Concept" resurrects the Mount Kilimanjaro guereza to specific rank as Colobus caudatus. The geographic range of C. caudatus is small (ca. 4030 km²) and in decline, as is the number of individuals and area of habitat. Colobus caudatus qualifies as an IUCN Red List globally "Endangered" species, as a nationally "Endangered" species in Tanzania, and as a nationally "Critically Endangered" species in Kenya. Colobus caudatus is Kenya’s most threatened species of primate. Recommendations for research and conservation actions are provided.