The histone protein HPhA from the hyperthermophilic archaeon Pyrococcus horikoshii, shows hyperthermostability, as required for optimal growth of the organism at 95℃. The structure of recombinant P. horikoshii HPhA has been determined to 2.3A resolution by molecular replacement, and refined to Rwork and Rfree values of 20.7% and 27.3%, respectively. The HPhA monomer structure is characterized by the histone fold and assembles into a homodimer like other archaeal histones. There are four anions found in the dimer structure, giving rise to clues as to where DNA might bind. A detailed comparison of four known structures of archaeal histones, which evolve and exist under different temperatures, shows that the thermophilic archaeal histone HPhA has a larger hydrophobic contact area, an increased number of hydrogen bonds and a reduced solvent-accessible area. We also observe a unique network of tyrosine residues located at the crossover point of the two HPhA monomers, which locks them together and stabilizes the dimer. These factors together account for the increased thermal stability.