Red colobus living in gallery forests along the Tana River, Kenya, are usually found in groups of 12–30 animals, containing one, or at most two, adult ♂♂. These groups are much smaller than those at some rain forest sites and have a more disparate ratio of ♂♂ to ♀♀. A single study group censused regularly over 28 months varied in size from 16 to 33 animals. There were two replacements of adult ♂♂ and at least 34 migrations to or from the group by adult or subadult ♀♀. The incidence of extra-group animals in the study area is described. Although ♂♂ were quite commonly found outside reproductive groups, there appeared to be at least 6 times more adult ♀♀ in the population than adult ♂♂. It is hypothesized that the sex ratio within reproductive groups is influenced by the mortality rate amongst ♀♀ that leave those groups.