THE apex of animal social organization is eusociality, which has three characteristics: overlapping generations, reproductive division of labour, and cooperative care of young1,2. So far, eusociality has been recognized only among social insects and the African mole-rats3-5. Here I report the first case of eusociality in a marine animal. The sponge-dwelling shrimp Synalpheus regalis lives in colonies that may have >300 individuals, but that contain only one reproductive female. Direct-developing juveniles remain in the natal sponge, and allozyme data suggest that most colony members are full sibs. In laboratory experiments, larger colony members, most of whom apparently never breed, defended the colony against heterospecific intruders. Ecological similarities among mole-rats, termites and these sponge-dwelling shrimp, all of which are diploid animals, strengthen arguments that eusociality is favoured by gradual metamorphosis, parental care, and occupation of protected, expansible niches6.