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We studied activity rhythms at a gentoo penguin (Pygoscelis papua) colony at Cierva Point, Antarctic Peninsula, during the 1992-93 summer. We counted the number of penguins crossing a specific point on their route to and from the colony. Penguins showed a strong daily rhythm of activity, with a two-peak pattern for those leaving the colony and a one-peak pattern for those returning. The peak of penguins departing to sea was at dawn, with a secondary peak in the afternoon which was coincident with the peak of returns. Although this behaviour could be explained by nest relief schedules, the pattern remained once crèches had formed. The main peak of departures strongly correlated with sunrise, which might support the existence of a light signal synchronizing activity. Even though an external factor could be triggering movements, an endogenous circadian clock might drive both patterns.