Response regulators are activated to elicit a specific cellular response to an extracellular stimulus via phosphotransfer from a cognate sensor histidine kinase to a specific aspartate residue. Phosphorylation at the conserved aspartate residue modulates the activity of the response regulator. Methylesterase CheB is a two-domain response regulator composed of a regulatory domain and an effector domain with enzymatic activity. CheB functions within the bacterial chemotaxis pathway to control the level of chemoreceptor methylation. In its unphosphorylated state, the regulatory domain inhibits methylesterase activity of the effector domain. Phosphorylation of the regulatory domain leads to an enhancement of methylesterase activity through a relief of inhibition and a stimulatory effect on catalysis. CheB is a useful model protein for understanding the effects of phosphorylation of the regulatory domain on interdomain interactions and stimulation of enzymatic activity of the effector domain. Kinetic analyses of CheB activation indicate that the basis for the nearly 100-fold methylesterase activation upon phosphorylation is due to a change in the catalytic rate constant for the methylesterase reaction. It is also shown that the P2 domain of histidine kinase CheA inhibits the methylesterase activity of CheB and that this inhibition is decreased upon phosphorylation of CheB. Finally, studies of methylesterase catalysis by the free catalytic domain in the presence and absence of the regulatory domain have enabled detection of an association between the two domains in the absence of the linker.