The first case of cooperative polyandry in the Common Tern (Sterna hirundo) is reported. During the breeding season of 2000 two males, originating from the same brood, bred together with a female, who was paired in the previous season with one of the males. Both males copulated with the female. All three terns incubated the eggs, brooded and fed the chicks, but in different proportions. Despite poor food supply and bad weather conditions during the breeding season 2000, this polyandrous trio reared two fledglings, one of which has returned to the native colony in 2002. In another case, a two-year old, unrelated male incubated a clutch belonging to a pair of terns for a few days. Possible reasons for this deviation from the common monogamous mating system are discussed.