The diets of adult Macaroni penguins Eudyptes chrysolophus chrysolophus and Southern rockhopper penguins E. chrysocome chrysocome were analysed quantitatively at Marion Island, southern Indian Ocean, throughout two successive chick-rearing seasons. The diets were broadly similar. Crustaceans were the predominant prey type comprising, overall, 90% by mass and 98% by numbers in Macaroni penguins and 96% by mass and 99% by numbers in rockhopper penguins. Nauticaris marionis was the predominant crustacean eaten by both penguin species in 1983–84, but Euphausia vallentini and Thysanoessa vicina predominated in 1984–85. Themisto gaudichaudii was present in appreciable numbers only in Macaroni penguins. Fish was not found in measurable quantities in either species in 1983–84, but contributed 5% and 4% of the mass of the diet in Macaroni and rockhopper penguins, respectively, when calculated in terms of the original biomass of food ingested. In 1984–85, however, fish comprised 10% and 6% of observed mass and c. 25% and 14% of original biomass ingested in Macaroni and rockhopper penguins, respectively. Pelagic myctophids, predominantly Krefftichthys anderssoni, Protomyctophum tenisoni and P. normani between 0·01 and 8·3 g, were the most commonly identified fish prey, but Macaroni penguins took an appreciable number of Electrona carlsbergi in 1983–84. Cephalopods made up between 1 % and 3% of the diet by mass in both penguin species and between 5% and 13% of original biomass ingested. Predominant cephalopods eaten were Kondakovia longimana and an unidentified octopus species. The relative proportions of each prey type change throughout chick-rearing, with pelagic fish and cephalopods comprising a larger proportion later in the season when the penguins were assumed to be foraging farther from their breeding sites. Dietary segregation of the two species appears to be related to the difference in the timing of the breeding season, which begins three to four weeks earlier in Macaroni penguins.