Mangabey groups studied in the Kibale Forest Reserve, Uganda, in 1971 were studied again in 1991 using similar data collection protocols. The results were used to assess the effect of group size on activity budgets and travel costs, and to document the effects of habitat changes on mangabey density and demography. Larger mangabey groups traveled longer distances per day than smaller groups. Time budgets were less clearly influenced by group size. Mangabey population density increased over the 20 year period. This increase in population density paralleled habitat changes, particularly an increase in tree density, and was accompanied by increased use of regenerating forest. © 1994 Wiley-Liss, Inc.