Along French coasts, the abalone Haliotis tuberculata is affected by mass mortality events caused by both immune depression and the pathogen Vibrio harveyi. During this immune depression various immune parameters have been monitored including phenoloxidase (PO) activity, which significantly decreases. Nevertheless the basal PO activity level, to date, has not been characterized biochemically and the PO subclass involved has not been identified. The aim of this study was to use various substrates and inhibitors, specific to one or more PO subclasses, to biochemically characterize enzymatic activity. Among specific and non-specific substrates, PO had the highest affinity for dopamine (K-m = 1.92, V-max = 0.086). Tyrosinase-specific substrates were not oxidized whereas both laccase-specific substrates tested were oxidized. Results obtained with the different inhibitors tested are in agreement with the results obtained with substrates. In fact, the laccase-specific CTAB totally inhibited PO activity and the IC50 for this inhibitor was the lowest of any tested. The molecular weight has been estimated between 83 and 106 kDa.