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The beadlet sea anemone Actinia equina (L.) shows a well-documented sequence of aggressive responses towards conspecific individuals. Aggression is also shown towards sea anemones of certain other species. A study was carried out to assess aggressive responses of A. equina to other anemones over a wide range of levels of genetic divergence from genetically identical individuals (clonemates) to various other species, all of which were potentially sympatric. The other species used were the dahlia anemone Urticina felina (L.), the gem anemone Bunodactis verrucosa (Pennant), the snakelocks anemone Anemonia viridis (Forskål), the plumose anemone Metridium senile (L.) and the strawberry anemone Actinia fragacea Tugwell. Intraspecific aggression was also studied in A. fragacea. A. equina exhibited high levels of aggression to all the other species and to unrelated (i.e. non-clonal) individuals of its own species, but was never aggressive to clonemates. The levels of aggression shown by A. equina were found to correlate with the genetic divergence of the other anemone. It was also noted that A. equina only left damaging acrorhagial peels on conspecific individuals, whereas A. fragacea never left a peel on other A. fragacea, but produced peels during all successful Œfights¹ against A. equina. It is suggested that the non-self recognition system, which triggers the acrorhagial application behaviour in A. equina, is not species-specific, although the occurrence of acrorhagial peeling may be species-specific. Inglés