N-utilization factor G (NusG) from Aquifex aeolicus (Aa) was overexpressed in Escherichia coli, purified and crystallized using the hanging-drop vapor-diffusion technique. The drops consisted of 2.5 microl protein solution (approximately 30 mg ml(-1) in 20 mM Tris-HCl pH 8.0, 200 mM NaCl, 2 mM EDTA and 10 mM DTT) and 2.5 microl reservoir solution (0.085 M Na HEPES pH 7.5, 15% glycerol, 11% 2-propanol and 20% PEG 4000) derived from condition number 41 of the Hampton Cryo Screen. The crystals grew at 291 +/- 1 K and reached dimensions of 0.2 x 0.1 x 0.05 mm in 5-7 d. The crystals, which diffracted to 2.45 A resolution, belonged to space group C222(1), with unit-cell parameters a = 65.95, b = 124.58, c = 83.60 A. One AaNusG molecule is present in the asymmetric unit, corresponding to a solvent content of 59.80% (Matthews coefficient = 3.06 A(3) Da(-1)). Crystal structure determination is in progress.