The phosphatase of a psychrophile (Shewanella sp.) was purified by ammonium sulfate fractionation, followed by sequential column chromatographies. The purified enzyme was electrophoretically homogeneous on native- and SDS-PAGE. Its molecular weight was 41,826 by its amino acid composition. The enzyme had its optimal pH for the activity at 9.8, and a broad substrate specificity to dephosphorylate ATP, pyrophosphate, glycerophosphate, and so on. Its activity was increased by metal ions including Mg(2+), Mn(2+), and Co(2+). The maximal activity was observed at 40°C, and the enzyme at 0°C showed 39% of activity at 40°C. The enzyme, however, tended to lose its activity at 20°C and pH 9.8. These results indicated that purified enzyme was an alkaline phosphatase with characteristics; high catalytic efficiency at low temperature and gradual inactivation at an intermediate temperature.