The crystal structure of peroxiredoxin from the anaerobic hyperthermophilic archaeon Pyrococcus horikoshii (PhPrx) was determined at a resolution of 2.25 angstrom. The overall structure was a ring-type decamer consisting of five homodimers. Citrate, which was included in the crystallization conditions, was bound to the peroxidatic cysteine of the active site, with two O atoms of the carboxyl group mimicking those of the substrate hydrogen peroxide. PhPrx lacked the C-terminal tail that forms a 32-residue extension of the protein in the homologous peroxiredoxin from Aeropyrum pernix (ApPrx).