PhoB is a response regulator of PhoR/PhoB two-component system from Escherichia coli that is involved in the environmental phosphate regulation. It has been reported that the N-terminal receiver domain (PhoB(N)) forms a dimer using the alpha 1-alpha 5 face in the apo state and a dimer using the alpha 4-beta 5-alpha 5 face in the active state investigated by X-ray crystallography. However, it is not clear whether the conformational switch of the dimer is dependent on phosphorylation. Here, we report the NMR studies of PhoB(N) in solution in its apo form. We observed that the secondary structural fragments of apo PhoB(N) characterized by NMR are almost the same as those determined by crystallography, but the NMR spectrum of PhoB(N) shows inhomogeneous amide signals. Concentration dilution experiments and backbone relaxation parameters showed that PhoB(N) exists in equilibrium between monomer and dimer states. Using paramagnetic relaxation enhancement experiments, we demonstrated that the dimer of apo PhoB(N) forms several transient dimer interfaces in solution. This finding suggested that, in addition to the monomer-to-dimer exchange, the inactive conformation of PhoB(N) has different domain arrangements, which are independent of phosphorylation. It provides an experimental data for the conformational selection mechanism of the phosphorylation of PhoB(N).