(1) Radio-telemetry was used to study the activity patterns of gentoo penguins, Pygoscelis papua, at Bird Island, South Georgia, throughout the breeding season, to assess variation in foraging effort (foraging trip duration, frequency and proportion of time spent at sea per day). (2) Except during chick-brooding, more than 80% of foraging trips consisted of birds departing early in the morning (75% before 07.00 h, local time) and arriving back in the afternoon (90% after 12.00 h); 96% of all trips were completed in the same day. During brooding, 45% of all trips were started after 08.00 h, compared to only 6%, 10% and 12% for the pre-breeding, incubation and creche periods. (3) Only 4% of all trips were overnight, all during chick-rearing, but these accounted for about one-third of the total variation in trip duration. (4) Mean trip duration varied significantly between breeding periods, being shortest during brooding (6.96 h) and longest during incubation (10.5 h). Foraging trip frequency and time spent at sea increased throughout the season, being greatest during chick-rearing. (5) Trip duration did not vary significantly with sex or brood size, but decreased with departure time for all breeding periods. Trip duration increased with chick age throughout chick-rearing, probably because of increased food demand from the growing chick during brooding, and due to adult food requirements for moult during creching. (6) Differences between individual birds accounted for 9-13% of total variation in trip duration. (7) The implications of these results for the use of penguin foraging trip duration as an environmental monitoring parameter are discussed, and examples of suitable sampling protocols suggested.