Diets of diurnal and overnight foraging chinstrap penguins (Pygoscelis antarctica) breeding at Seal Island during chick-brooding period of 1988/89, 1989/90, 1993/94 and 1994/59 were analyzed. Diet mass estimated from digested and intact stomach contents indicates that krill were predominant in their diets but fish constituted 0-1 % and 14-45 % of the diet of diurnal and overnight foragers respectively. The highest intake of fish was observed in 1993/94 when krill abundance was very low. Calorific values of the estimated diet mass were 4085 kJ for diurnal foragers and 4809 kJ for overnight foragers, which were comparable with daily average energy requirements calculated for a diurnal forager (4853 kJ) and an overnight forager (5732 kJ) based on available input parameters. A comparison of krill in the diet and those in the foraging areas indicates that diurnal foragers took krill in the inshore region, where krill tended to be smaller and less matured. On the other hand, overnight foragers took krill in the offshore region, where krill tended to be larger and matured, and also in the inshore region. Overnight foraging penguins tended to select larger female krill, but diurnal foragers tended to select subadult male krill; both foragers took very few adult male krill and juvenile krill. It is, therefore, considered that both foragers tend to take larger krill of the least capable of avoiding capture in their respecitve foraging areas.