Tyrosinase (monophenol, L-DOPA:oxygen oxidoreductase) was isolated from the ink of the squid, Illex argentinus. Squid tyrosinase, termed ST94, was found to occur as a covalently linked homodimeric protein with a molecular mass of 140.2 kDa containing two copper atoms per a subunit. The tyrosinase activity of ST94 was enhanced by proteolysis with trypsin to form a protein, termed ST94t, with a molecular mass of 127.6 kDa. The amino acid sequence of the subunit was deduced from N-terminal amino acid sequencing and cDNA cloning, indicating that the subunit of ST94 is synthesized as a premature protein with 625 amino acid residues and an 18-residue signal sequence region is eliminated to form the mature subunit comprised of 607 amino acid residues with a deduced molecular mass of 68,993 Da. ST94 was revealed to contain two putative copper-binding sites per a subunit, that showed sequence similarities with those of hemocyanins from mollusks, tyrosinases from microorganisms and vertebrates and the hypothetical tyrosinase-related protein of Caenorhabditis elegans. The squid tyrosinase was shown to catalyze the oxidation of monophenols as well as o-diphenols and to exhibit temperature-dependency of o-diphenolase activity like a psychrophilic enzyme.