A 110-residue protein encoded by the TM1442 gene of Thermotoga maritima shows amino-acid sequence similarity to Bacillus subtilis anti-anti-sigma factors RsbV and SpoIIAA. It has been overexpressed in Escherichia coli and the recombinant protein exists primarily as both a monomer and a dimer in solution. The dimeric form has been crystallized using polyethylene glycol (PEG) 8000 as a precipitant. Native X-ray diffraction data have been collected at 100 K to 2.0 A resolution. The crystals are monoclinic, belonging to the space group P2(1), with unit-cell parameters a = 31.54 (13), b = 116.83 (37), c = 31.39 (7) A, alpha = 90, beta = 119.84 (9), gamma = 90 degrees. The asymmetric unit contains two monomers of the recombinant polypeptide, with a corresponding V(M) of 2.24 A(3) Da(-1) and a solvent content of 45.0%.