Study done party in the Kalahari Gemsbok Park and party with captive birds in the USA. The behaviour of the Sociable Weaver Philetairus socius was studied briefly in South Africa in 1969, and more extensively in aviaries in southern California for over three years (1972-75). The species was bred successfully in captivity, apparently for the first time. Building of the compound nest in nature is reviewed in the light of preceding accounts, both by ourselves and by others. The optimal preference for nest materials was experimentally determined. Male and female appear identical and there seems to have been no previous detailed account of the behaviour of Sociable Weavers based on distinctively colour-ringed individuals of known sex. Among the sex differences in behaviour are the amount and nature of building, dominance, malespecific song, and sex differences in relative share of incubation and in the care of nestlings and fledglings. In addition, we describe in detail for the first time certain rarely observed behavioural acts including precopulatory courtship and copulation. Allopreening and water bathing are very rare in this species, and the birds do not take dust baths.