SUMMARYJohnstone, G. W. 1979. Agonistic behaviour of the giant-petrelsMacronectes giganteusandM. hallifeeding at seal carcasses. Emu 79: 129–132.At the islands where they breed, the two giant-petrels derive only a small part of their diet from seals that die ashore. Both species behaved similarly when feeding on seal carcasses at Macquarie Island. However,M. halli, the less abundant species, adapted to the Subantarctic, was more frequently recorded at this source of food than the more numerousM. giganteus, adapted to the Antarctic; yet in disputes at the carcasses,M. giganteuswas more aggressive and more successful. This may be explained by the far greater availability of dead seals in the Subantarctic than the Antarctic. Consequently,M. hallihas a greater tendency thanM. giganteusto eat this food but selection for aggressiveness while doing so may have been greater inM. giganteus.