The diving behavior and diet composition of King penguins were examined during summer 1995 at Kerguelen Islands. This was in relation to real-time estimations of diel prey availability during 2 sampling sessions totaling 10 d at sea. During daylight hours King penguins performed medium to deep dives of 120-250 m. At night, they dived no deeper than 60 m. Daytime, dusk and night-time sampling of prey was performed in the depth ranges corresponding to the depths of the penguins' dives in this study. The diversity in mesopelagic fish was found to be highest during the night at 0-50 m (15 vs 9 species during the day), and their number was up to 20-fold higher at these depths at night than during the day at the 150-250 m depth layer. The 3 myctophid species Electrona antarctica, Gymnoscopelus fraseri and G. braueri which were present in large sub-surface numbers during the night were virtually absent from the penguins' diet. The 2 species dominant in their diet, Muraenolepis marmoratus and Krefftichtys anderssoni (56.5 and 32.9 % by number, 30.3 and 31.6 % by biomass, respectively), were scarcely detected in the penguins' diving range during the night. In contrast, these 2 species, of which only the latter is bioluminescent, were significantly present during the day in the 0-300 m depth range (16.5 and 30.0 % of the diurnal catches by number, respectively). In terms of biomass, these 2 prey types constituted only 6.3 and 12.6 % of the total daylight trawls, which were dominated by the genus Protomyctophym (38.6 % of the catches). The overlap between the penguins' diet and trawl content was the most significant with daylight sampled data, King penguin feeding success seems mostly dependent on deep dives during the day, despite lower prey availability than during the night. Ambient light levels, which were found to be higher at deeper depths (150-180 m) during the day compared to the sub-surface (0-60 m) at night-time, therefore appear to better determine King penguin foraging success than diel migration of prey to shallow depths.