RsbP, a regulator of RNA polymerase sigma(B) activity in Bacillus subtilis, is a phosphatase containing a Per-Arnt-Sim (PAS) domain in its N-terminal region that is expected to sense energy stresses such as carbon, phosphate or oxygen starvation. Energy-stress signals are transmitted to the PAS domain and activate the C-terminal phosphatase domain of RsbP, leading to activation of the downstream anti-anti-sigma(B) factor RsbV. Finally, the general stress response is induced to protect the cells against further stresses. The recombinant PAS domain of RsbP was crystallized by the sitting-drop vapour-diffusion technique using 40% PEG 400 as a precipitant. The crystals belonged to space group P2(1), with unit-cell parameters a = 55.2, b = 71.7, c = 60.2 A, beta = 92.1 degrees . Diffraction data were collected to a resolution of 1.6 A.