CysB protein is a positive regulator of transcription of genes involved in cysteine biosynthesis in bacteria and a member of the LysR family of DNA binding proteins. A 233-residue N-terminal chymotryptic fragment of the protein, with DNA and N-acetylserine binding activity, has been crystallized in the presence of monomethyl-polyethylene glycol 750. The crystals diffract to 2.5 A (1 A = 0.1 nm) spacing on a rotating copper anode X-ray source and to 2.1 A spacing using synchrotron radiation and are suitable for structural studies. The space group is P2(1)2(1)2 with unit cell dimensions of a = 68.8 A, b = 109.9 A and c = 33.5 A. On the assumption that the asymmetric unit comprises one monomer, the crystals have a solvent content of approximately 50%.