A small DNA-binding protein of 87 amino-acid residues from the hyperthermophilic archaeon Methanococcus jannaschii (Mja10b) was cloned and overexpressed in Escherichia coli. The protein was crystallized and the crystals belong to the space group P6(1)22/P6(5)22, with unit-cell parameters a = b = 50.85, c = 124.02 A, alpha = beta = 90, gamma = 120 degrees. The crystals diffracted to a maximum resolution of 2.2 A at 100 K using Cu Kalpha radiation. The presence of one molecule per asymmetric unit gives a crystal volume per protein mass (V(M)) of 2.4 A(3) Da(-1) and a solvent content of 49% by volume. A full set of X-ray diffraction data was collected to 2.2 A from the native crystal.