This paper presents information collected during an 18-month study of four groups of Propithecus verreauxi living in the north-west and south of Madagascar. Various aspects of the social organization and ecology of this species are discussed, including group size and composition, patterns of group dispersion, ranging and home-range utilization, diet and feeding behaviour, daily activity patterns and intra-individual relationships within each group. Particular attention is given to the nature and extent of regional, seasonal and local variation in these parameters, and to the possible underlying environmental correlates.