Thioltransferase from the hyperthermophilic archaeon Pyrococcus furiosus is a unique protein disulfide oxidoreductase. The recombinant protein expressed in Escherichia coli was crystallized by the sitting drop vapor diffusion method, using polyethylene glycol 550 monomethyl ether as the precipitant. The crystals belong to the hexagonal space group P6(1)22 or P6(5)22, with unit cell dimensions of a = b = 110.6 Angstrom and c = 68.4 Angstrom and with one monomer in the asymmetric unit. The crystals diffracted beyond 2.0 Angstrom and a complete native data set was collected to 2.3 Angstrom resolution. The solution of the crystal structure by multiple isomorphous replacement is in progress. (C) 1997 Academic Press.