The enzyme phenoloxidase from the periostracum of the bivalvePerna viridis (Linnaeus 1758) was extracted and the substrate specificity was studied spectrophotometrically. The enzyme is solubilised by sodium dodecyl sulphate and is activated by trypsin. It shows high activity with the phenolic substrates pyrocatechol, dopamine (3–4 dihydroxyphenylethylamine) and dopa (3–4 dihydroxyphenylalanine). As the enzyme shows catalytic activity with many substrates, it may exist in a multiple form. A qualitative analysis of the phenols occurring in the mantle reveals the presence of dopa and dopamine which may play a role in the tanning of the periostracum. The nature and properties of the enzyme phenoloxidase from the periostracum ofPerna viridis differ from that occurring in its byssal gland.