Abstract: Crèching behaviour in penguins is defined as the rearing of chicks by their own parents in large flocks called 'crèches'. Although several hypotheses have been proposed to account for the behaviour, the factors inducing chicks to aggregate remain relatively poorly understood, in particular for colonial seabirds. We studied crèching behaviour in the king penguin, Aptenodytes patagonicus, by looking at the dynamics of crèche formation and possible costs and benefits associated with this strategy. Crèches increased in size but declined in number throughout the austral winter. They were located preferentially in the central parts of the colony. Lone chicks suffered the most aggression from unrelated adults, whereas chicks in a crèche suffered the least. Chicks attacked by unrelated adults preferentially joined a crèche. Adult aggression appeared to be a major factor inducing crèching behaviour. Chicks at the periphery of a crèche were more vigilant while sleeping, as measured by eye openings. Crèches seemed to occasion intense competition among chicks for access to the centre. Chicks in poor condition were attacked and pushed to the periphery of the crèche, where they were preyed on by giant petrels. During harsh weather conditions, chicks amalgamated into larger crèches, tolerated lower interindividual distances and turned their backs to the wind and rain. Our results accord with the idea that crèching behaviour in king penguins is a strategy that protects chicks from adult aggression, predation and severe weather.