The biology of two species of diving petrel (Pelecanoides georgicus and P. urinatrix exsul) was studied at Bird Island, South Georgia. Existing criteria (bill shape and morphology, wing length) for distinguishing these species are reviewed, and several new characters are recognised. For adults bill depth, the colour of the posterior part of the tarsus, and vocalisations are distinctive; chicks have down of different colours (pale grey in P. u. exsul, dark grey in P. georgicus). At South Georgia the species breed in different habitats and at different times – P. u. exsul nests in steep tussock slopes in very early summer, P. georgicus in fine scree slopes in midsummer. P. georgicus lays a proportionately larger egg, and has an incubation period of 46 days (c.54 days in P. u. exsul) and a chick fledging period of 46 days. The fledging period of P. u. exsul is 54 days, very similar to recorded values for P. u. urinatrix (53.5 days) and P. u. chathamensis (56 days). Data on feeding frequency and feed size were derived from daily chick weighings and from twice-daily weighing during 30 days preceding fledging. In both species chicks are fed every night, and often by both parents. In P. georgicus true mean chick feed size is c.37 g; in P. u. exsul it may be slightly less. Analysis of chick stomach contents suggests that P. u. exsul feeds extensively on copepods, whereas P. georgicus largely takes krill (Euphausia superba). P. u. exsul breeding adults commence moulting before their chicks have fledged; P. georgicus moults exclusively in the non-breeding season. Ectoparasites were collected, and the feather louse Pelmatocerandra setosa was found to be restricted to P. u. exsul, and Pelmatocerandra enderleini to P. georgicus. P. georgicus, which breeds later and whose chicks fledge faster, is suggested to be better adapted to the climatic and marine environmental conditions than P. u. exsul, which may be the more recent colonist of these high latitudes.