De Brazza's monkey (Cercopithecus neglectus), like other guenons, shows marked sexual dimorphism in an array of features. While strong sexual dimorphism is generally associated with a polygynous mating system, populations of De Brazza's monkeys in Gabon are reportedly monogamous. An explanation of this unique phenomenon is offered here. Patterns of sexual dimorphism are examined for morphology, growth and development, behavior, and ecology, and field and captive studies on the social organization and mating system of De Brazza's monkey and congeneric guenon species are reviewed. Based on the findings, it is postulated that 1) De Brazza's monkeys are not strictly monogamous, but exhibit interpopulational variation in their mating system, from facultative monogamy to mild polygyny; 2) marked sexual dimorphism most likely reflects the effect of the historical-phylogenetic factor; ie, it represents a holdover of a degree of dimorphism established earlier in evolutionary history when the degree of polygyny Was higher; and 3) lessening in the degree of polygyny and a tendency toward monogamy represents a consequence of selection toward small group size. Small group size, a unique antipredator strategy, and failure to form polyspecific associations are ultimately most likely the result of intragroup and interspecific competition and predation pressure.