Most of the Mollusca and Crustacean are economic species.The diseases related to environmental deterioration and germ plasm degeneration have caused great economic loss.More and more studies have been focusing on the immune defense system of Mollusca and Crustacean.As a key part of prophenoloxidase-activated system,phenoloxidase(PO) plays an important role in initial immune defense of invertebrates.The function,tissue location and expression,gene cloning and sequence analysis,as well as phylogenetic analysis of PO in Mollusca and Crustacean are reviewed in this paper.The results from sequence and phylogenetic analysis evidently indicate that although the tyrosinase from Mollusca and the PO from Crustacean are isoenzymes,their encoded genes are quite different.This evidence challenges to the widely accepted view that PO belongs to invertebrates,while tryosinase belongs to vertebrates.Therefore,PO only should be specified to arthropod,while the tryosinase should be referred to mollusca and other non-arthropod as well as vertebrates.