This paper reports flocking, communal feeding and other aspects of sea-based social behaviour in the jackass penguin (Spheniscus demersus). Penguins tend to occur within about 15 km of the mainland, but range farther afield from the islands used for breeding and/or roosting. Relatively large groups of 50 and more birds occur more than 50 km from the nearest island, but seldom more than 15 km from the mainland. The majority of the sea-going population consists of birds occurring in groups. Mean group size is eight birds. In any one particular group the members all tend to perform the same behaviour at the same time. The tendency to form foraging groups and the highly synchronized diving and cohesion of these groups indicate that this behaviour is socially facilitated, suggesting that it is adaptive in terms of both enhanced prey location and capture. Feeding penguins do not submerge for long and do not dive deeply. Birds in diving groups perform head-dipping movements which might signal readiness to dive and thus promote synchronous activity. The paper points out how little is known about jackass penguins at sea - the environment in which they probably spend the majority of their time. Lack of information on the birds at sea precludes proper interpretation of many land-based events attending the biology of the bird and its conservation.