ABSTRACTTwo phenoloxidase (PPO) isoforms were purified from Taiwanese black tiger shrimp (Penaeus monodon) purified via ammonium sulfate precipitation and high performance hydrophobic interaction chromatography on phenyl Sepharose CL‐4B. Optimal DL‐ã‐3, 4‐dihydroxyphenylalanine (D, L‐DOPA) oxidation by these isoforms was observed to occur at pH 6.0, and at 45°C. The isoforms showed both mono‐ and di‐PPO activities, and preferential medabolism of both monoand di‐phenolic substrates lacking a carboxyl group either directly attached to the ring, or in the side chain moiety of the p