Sensing external stress in the bacterial periplasm and signal transduction to the cytoplasm are important functions of the CpxAR, Bae and sigma(E) signalling pathways. In Escherichia coli, the sigma(E) pathway can be activated through degradation of the antisigma factor RseA by DegS and YaeL. The periplasmic protein RseB plays an important role in this pathway by exerting a direct or indirect negative effect on YaeL cleavage efficiency. RseB from E. coli, missing the periplasmic signal sequence (RseB(DeltaN)), was cloned, expressed, purified and crystallized. Crystals were obtained in two different forms belonging to space group P42(1)2 (form I) and C222(1) (form II) and diffracted to 2.8 and 2.4 A resolution, respectively. In crystal form I two copies of the protein were located in the asymmetric unit according to heavy-atom analysis, while crystal form II contained three copies.