The gene encoding LysX, an essential component of the lysine-biosynthesis pathway in Thermus thermophilus (molecular weight ≃ 31 000 Da), was cloned and expressed and the purified protein was crystallized by the hanging-drop vapour-diffusion technique in two different space groups, C2 (unit-cell parameters a = 124.7, b = 51.4, c = 103.6 Å, β = 122.8°) and R3 (a = b = 122.6, c = 97.6 Å). Crystals improved by macroseeding diffracted to beyond 2.3 and 3 resolution for the C2 and R3 crystal forms, respectively. Complete diffraction data sets were collected for the C2 and R3 crystal forms at 2.5 and 3.1 Å resolution, respectively. Crystals of selenomethionine-containing LysX protein were obtained by cross-microseeding, using the native microcrystals as a seed. Structure determination is now in progress.