A new species of African forest monkey has been discovered in the lowland tropical rainforest of central Gabon. The previously undescribed monkey is a guenon, a member of the widespread African genus Cercopithecus. and belongs to the Cercopithecus lhoesti superspecies, whose two previously recognized species live in widely separated populations, C. preussi in Cameroon, and C. lhoesti east of the Zaire basin. The new guenon occurs as a small isolated population separated by 600 km and 1600 km, respectively, from the other two. Distinctive external characteristics are its white throat-ruff, extensive chestnut-orange saddle, and yellow-orange tip to the tail. Chromosomal analysis confirms its close phylogenetic affinities with C. preussi and C. Ihoesti, but reveals a unique karyotype. The guenon is described as a new species, Cercopithecus solatus, the sun-tailed monkey.