Sympatric guenons in East Africa associate with one another in mixed-species groups to an intermediate degree. This makes it possible to compare a given group's ecology and behavior when it is part of an association to when it is unaccompanied, and to correlate association tendency with various ecological parameters. Two studies incorporating these approaches have been made of sympatric Cercopithecus ascanius and C. mitis monkeys in the Kakamega Forest, Kenya, and the Kibale Forest, Uganda. The pattern of and reasons for association in each site are reviewed. Compared to Kakamega, association between C. ascanius and C. mitis in Kibale occurs less often, and the species appear to benefit relative to one another in different ways. These results suggest that the particular ecological setting greatly influences the nature of the interaction between species, through its effect on population structure, dietary overlap, food distribution, and community composition. The major ecological differences between the two study areas probably reflect post-Pleistocene history and possibly climate, but they have important consequences for the present-day population structure and feeding ecology of each species, and hence affect relations between them as well.