H+ -transporting ATP synthase (H+ -ATPase) is a multi-subunit complex which acts to produce ATP molecules. The catalytic subunit A of the archaeal-type H+ -ATPase from Pyrococcus horikoshii OT3 was cloned, expressed in Escherichia coli, purified and crystallized by the hanging-drop vapour-diffusion method with MPD as a precipitant. X-ray intensity data were collected to 2.55 A resolution at beamline BL41XU of SPring-8. The crystals belong to the tetragonal space group P4(1)2(1)2 or P4(3)2(1)2, with unit-cell parameters a = b = 128.0, c = 104.7 A, and contain one molecule per asymmetric unit.