ATP-sulfurylase purified from Porphyra yezoensis showed maximal activity at pH 8.2, and 37°C. It was shown that the actual substrate was Mg2+-ATP complex, and that ree-ATP was an inhibitor of the forward reaction. APS was a potent product inhibitor, competitive with respect to Mg2+-ATP and a mixed type inhibitor with respect to molybdate. The Km values for Mg2+-ATP, molyb-date, PPi, and APS were 0.67mM, 6.7mM, 33μM, and M, respectively. Initial velocity studies showed that the mechanism of the molybdolysis reaction was a sequantial type. The molybdolysis reaction was inhibited by several nucleotides and anions of a sulfate analog, but was not affected by p-chloromercuribenzoate.