1. An improved method is given for preparation of pyruvate,phosphate dikinase from Bacteroides symbiosus. 2. The bacterial enzyme is stable, free from interfering enzyme activities, and does not require thiol compounds to maintain stability during storage or assay. 3. New direct assays of enzyme activity are based on acid evolution or consumption as measured at constant pH in a pH-stat. 4. The optimum rate of reaction in the direction of pyruvate formation occurs at about pH6.4; in the direction of phosphoenolpyruvate formation, it is at pH7.2-7.8. 5. Newly determined substrate K(m) values for the enzyme are: AMP, 3.5x10(-6)m; ATP, 1x10(-4)m; pyruvate, 8x10(-5)m; P(i), 6x10(-4)m. 6. K(+) may substitute for NH(4) (+) in activating the reaction catalysed by the B. symbiosus enzyme. 7. In the direction of pyruvate formation the bivalent metal ion requirement of the enzyme is fulfilled by salts of nickel, manganese, magnesium and cobalt. In the other direction only magnesium salts were effective. 8. The nucleotide specificity of the enzyme is strictly limited to the adenine nucleotides. CTP and ITP strongly inhibit the reaction in the direction of phosphoenolpyruvate formation.