Melanosis developed in shrimp (Penaeus vannamei)is mainly initiated by polyphenoloxidase (PPO),thus understanding of the characteristics of PPO in shrimpis important for controlling the melanosis of shrimp. Theshrimp cephalothorax turns black most rapidly amongst allthe tissues during the chilled storage. Crude PPO extractedfrom this cephalothorax has an optimal pH of 6.0 and anoptimal temperature of 50 C. PPO is relatively stable underneutral and weak alkaline conditions (pH 5.5–9.0) andthe temperature range of 25–35 C. The kinetic parametersKm and Vmax were recorded as 3.02 mM and 54.3 U/mg ofprotein, respectively, using L-Dopa as a substrate. Themolecular weight of PPO was estimated as 200–220 kDaby an activity staining test. A hydroxypyridinone derivative,5-hydroxy-1-octyl-4-oxo-1,4-dihydropyridine-2-carbaldehydeO-ethyl oxime, was demonstrated to efficientlyinhibit the PPO, indicating that this compound might findapplication as a shrimp preservative.