Recombinant archaeal histone from the hyperthermophile Pyrococcus horikoshii OT3 (HPhA) was crystallized by the hanging-drop vapour-diffusion method. Crystals grew at 291 K in 200 mM (NH(4))(2)SO(4), 100 mM sodium acetate buffer pH 4.6, 19% PEG 4000. Diffraction data were obtained to a resolution of 2.3 A from a single frozen crystal, which belonged to space group P2(1) with unit-cell parameters a = 34.99, b = 46.89, c = 35.02 A, alpha = gamma = 90, beta = 104 degrees. The asymmetric unit contained two molecules and had a solvent content of approximately 35%.