Leucyl-tRNA synthetase from Thermus thermophilus (LeuRSTT) is the first LeuRS to be crystallized. Two crystal forms of the native enzyme have been obtained using the hanging-drop vapour-diffusion method with ammonium sulfate as a precipitant. Crystals of the first form belong to space group I422 and have unit-cell parameters a = b = 312.4, c = 100.4 A. They diffract anisotropically to 3.5 A resolution in the c-axis direction and to only 6 A resolution in the perpendicular direction. Crystals of the second form, which can be obtained native or with leucine or a leucyl-adenylate analogue bound, belong to space group C222(1) and have unit-cell parameters a = 102. 4, b = 154.1, c = 174.3 A. They diffract to 1.9 A resolution and contain one monomer in the asymmetric unit. Selenomethionated LeuRSTT has been produced and crystals of the second form suitable for MAD analysis have been grown.