To investigate relationships between ecology and social organization, observations were made on several populations of Colobus guereza in East Africa in 1970-74. Study time was concentrated in the Kibale Forest and the Kabalega National Park, Uganda. Findings on social behaviour are reported in this paper. Assessments of group composition were made. The repertoire of communicatory acts was analysed. During five-day continuous observation periods at Kibale (12 consecutive months) and at Kabalega (two months) all social interactions between members of one habituated group, and between this group and others, were recorded. 6 Kibale groups were monitored over a four-year period to examine changes in group structure. The results are compared with those from other studies and the adaptive significance of guereza social behaviour is discussed, emphasizing differences in male and female strategies.