Neelaredoxins are a type of superoxide reductase (SOR), which are blue 14 kDa metalloproteins with a catalytic nonhaem iron centre coordinated by four histidines and one cysteine in the ferrous form. Anaerobic organisms such as Archaeoglobus fulgidus, a hyperthermophilic sulfate-reducing archaeon, have developed defence mechanisms against toxic oxygen species in which superoxide reductases play a key role. SOR is responsible for scavenging toxic superoxide anion radicals (O(2)(*-)), catalysing the one-electron reduction of superoxide to hydrogen peroxide. Crystals of recombinant A. fulgidus neelaredoxin in the oxidized form (13.7 kDa, 125 residues) were obtained using polyethylene glycol and ammonium sulfate. These crystals diffracted to 1.9 A resolution and belonged to the tetragonal space group P4(1)2(1)2, with unit-cell parameters a = b = 75.72, c = 185.44 A. Cell-content analysis indicated the presence of a tetramer in the asymmetric unit, with a Matthews coefficient (V(M)) of 2.36 A(3) Da(-1) and an estimated solvent content of 48%. The three-dimensional structure was determined by the MAD method and is currently under refinement.