Lysyl-tRNA synthetase from Thermus thermophilus has been cocrystallized with either its cognate tRNAlYS or Escherichia coli tRNAlys using ammonium sulfate as precipitant. The crystals grow from solutions containing a 1:2.5 stoichiometry of synthetase dimer to tRNA in 18–22% ammonium sulfate in 50 mM Tris-maleate buffer at pH 7.5. Both complexes form square prismatic, tetragonal crystals with very similar unit cell parameters (a = b = 233 Å, c = 119 Å) and diffract to at least 2.7 Å resolution. However the homocomplex is of space group P4212 and the heterocomplex of space group I422. © 1995 Wiley-Liss, Inc.