A field study of the ecology of feral cats, with particular emphasis on diet, was conducted at Macquarie I. from December 1973 to March 1975. Diet was determined by the examination of scats and gut contents. Rabbit remains were found in 82% of scats and 71 % of gut samples; rabbits weighing < 600 g (about 10 weeks old) accounted for 81 % of all rabbits eaten. Antarctic prion and white-headed petrel collectively occurred in 45 % of the scats and 49 % of the gut samples and formed the second major component of the cats' diet. Rats, mice and wekas were eaten in small numbers. Cats also scavenged on dead elephant seals and penguins on the beaches, especially during winter. Cats were found over c. 65 % of the island and were common in all habitats except for the plateau feldmark, where they were either sparse or absent. The adult population was estimated at between 250 and 500. Availability of food in winter appeared to be an important factor limiting population.