The activation sequence and related factors of the prophenoloxidase activating system in crustaceans was compared with the equivalent system in tunicates. Both solitary and colonial ascidians present in their hemolymph a copper‐dependent phenoloxidase activity that may be inhibited by tropolone and phenylthiourea. Carbohydrates are able, to various extents, to trigger proPO system which requires serine protease cleavage for activation to phenoloxidase (PO). In some ascidians, hemocytes called ≪morula cells≫ show PO activity, while inCiona intestinalisthe ≪univacuolar refractile granulocytes≫ are positive after cytochemical staining with L‐dopa. The relationships between proPO system and defence reactions (opsonic, cytotoxic) of ascidians are discussed.