Agonistic interactions between Great Cormorants (Phalacrocorax carbo) in a roost have been studied to detect possible social dominance causing spatial segregation in relation to age. Adults tended to dominate first- and second-winter birds, and were found relatively higher in the roost, where they are probably the safer from human intruders. Because of the high proportion of adults, attacks frequently involved two adults. No differences were found between the frequency and success of attacks in relation to the age of the birds arriving to the roost, perhaps indicating that pre-attack display represented an efficient form of agonistic communication. The number of attacks was not correlated with the number of cormorants in the roost, probably due to a constant density of birds, unaffected by the size of the roost.