6-Pyruvoyl tetrahydrobiopterin synthase (PTPS) catalyses the conversion of dihydroneopterin triphosphate to 6-pyruvoyl tetrahydropterin, the second of the three enzymatic steps in the synthesis of tetrahydrobiopterin from GTP. PH0634, a 13.51 kDa archaeal PTPS homologue from Pyrococcus horikoshii OT3, was overexpressed as native and selenomethionine-substituted protein and the purified protein was crystallized by the oil-microbatch method at 295 K. X-ray diffraction data were collected to 2.1 A resolution from the native crystal using synchrotron radiation at 100 K. The crystal belongs to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 35.83, b = 95.71, c = 105.65 A. Threefold noncrystallographic symmetry was identified from self-rotation calculations. Assuming the presence of a trimer in the asymmetric unit, the solvent content is 45% (V(M) = 2.24 A3 Da(-1)). The selenomethionine-substituted crystal is isomorphous to the native crystal and diffracts X-rays to 2.9 A.