We documented and assessed the influence of chimpanzee activity on group size and composition of Colobus guereza in Kyambura Gorge, southwest Uganda, from July to September, 1994 and in February and March 1996. The population density of colobus is very high: 347 individuals per km2. Density differed outside activity centers of chimpanzees (525 individuals per km2) and within the centers (186 per km2). We identified a total of 24 colobus groups, ranging between 3 and 13 individuals. Of the 24 groups, 22 were one-male groups. Groups were smaller and the percentage of subadults and juveniles was lower in groups within chimpanzee activity centers. Estimates of home ranges are between 1.7 and 6.2 ha, but neighboring home ranges overlapped ≤80%. Only a territory of approximately 0.5 ha was defended by the alpha male.