Recombinant aspartate aminotransferase from an extremely thermophilic bacterium, Thermus thermophilus HB8, has been crystallized in two different crystal forms. The crystals of both forms are orthorhombic and belong to space group P212121 with cell dimensions a = 124.3, b = 113.6 and c = 61.6 Å for form I and a = 197.3, b = 109.7 and c = 80.3 Å for form II. The crystals of form I and II diffract to 2.1 and 2.5 Å resolution, respectively, on a conventional laboratory rotating-anode source. Two heavy-atom derivatives have been identified for form I.