Isocitrate dehydrogenase (IDH) of Escherichia coli is regulated by a bifunctional protein, IDH kinase/phosphatase. In addition to the kinase and phosphatase activities, this protein catalyzes an intrinsic ATPase reaction. The initial velocity kinetics of these activities exhibited extensive similarities. IDH kinase and phosphatase both yielded intersecting double-reciprocal plots. In addition, we observed similar values for the kinetic constants describing interactions of the kinase and phosphatase with their protein substrates and the interactions of all three activities with ATP. In contrast, while the maximum velocities of IDH kinase and IDH phosphatase were nearly equal, they were 10-fold less than the maximum velocity of the ATPase. Although the IDH phosphatase reaction required either ATP or ADP, it was not supported by the nonhydrolyzable ATP analogue 5'-adenylyl imidodiphosphate. The kinetic properties of wild-type IDH kinase/phosphatase were compared with those of two mutant derivatives of this protein. The mutations in these proteins selectively inhibit IDH phosphatase activity. Inhibition of IDH phosphatase resulted from three factors: decreases in the maximum velocities, reduced affinities for phospho-IDH, and a loss of coupling between ATP and phospho-IDH. These mutations also affected the properties of IDH kinase, increasing the maximum velocities and decreasing the affinities for ATP and phospho-IDH. The intrinsic ATPase activities also exhibited reduced affinity for ATP. These results are discussed in the context of a model which proposes that all three activities occur at the same active site.