Phosphatase I purified from a psychrophile (Shewanella sp.) [Tsuruta et al. (1998) J. Biochem. 123, 219-225] dephosphorylated O-phospho-L-tyrosine and phospho-tyrosyl residues in phosphorylated poly(Glu₄, Tyr₁) random polymer (polyEY) and phosphorylated myelin basic protein (MBP) but not phosphoseryl and/or phosphothreonyl residues in phosphorylated histone H 1, casein and phosphorylase a, indicating that the enzyme showed protein-tyrosine-phosphatase (PTPase, EC 3. 1. 3. 48)-like activity in vitro. The enzyme was remarkably inhibited by diethylpyrocarbonate (DEPC), monoiodoacetic acid (MIAA), and monoiodoacetamide (MIAM). Binding of 1 mol of DEPC to 1 mol of the enzyme caused complete inhibition of the enzyme; and 0.88 mol of 1-carboxymethylated histidine per mole of the enzyme was found when 90% of enzyme activity was lost by modification with ¹⁴CMIAA. These results indicated that this psychrophilic enzyme was a PTPase-like enzyme with histidine as its catalytic residue.