Gentoo Penguins (Pygoscelis papua) were studied for 3 months at four separate colonies on Macquarie Island. No external characters were found for sexing and ageing adult birds, but first-year birds could be distinguished by plumage. Breeding success varied between colonies from an estimated 0.36 to 1.14 chicks per pair. Chicks joined creches by 5 weeks cf age, and they were fully feathered by 11 weeks, when the mean weight was 5.4 kg. An unusual temporary partial retention of body feathers by moulting adults was observed, and a weight loss of 3.3% per day during moult was similar to that of other penguin species.