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Numerous field reports of hybrid monkeys and documented cases of persistent hybrid zones suggest that natural hybridization is common among African cercopithecines. Both theoretical considerations and a review of cases lead us to conclude that parapatric hybridization among closely related allotaxa is a widespread, usually natural process whose incidence may be modified by human influence. Sympatric hybridization, between species ecologically distinct enough to have overlapping ranges, is rarer, and in monkeys tends to occur in settings where natural or anthropogenic habitat edges restrict migration and hence access to unrelated conspecific mates. Although sympatric hybridization occurs in the absence of human disturbance, and may even have been a creative force in cercopithecine evolution, anthropogenic habitat fragmentation may increase its incidence. Hybridization with a more abundant form may increase the level of threat faced by a species whose numbers and range have been severely restricted, either naturally or artificially.