Overexpressed dimeric E. coli aspartyl-tRNA synthetase (AspRS) has been crystallized in its free state and complexed with yeast tRNA(Asp). Triclinic crystals of the enzyme alone (a = 104.4, b = 107.4, c = 135.0 A, alpha = 102.9, beta = 101.0, gamma = 106.3 degrees ), have been grown using ammonium sulfate as the precipitant and monoclinic crystals (a = 127.1, b = 163.6, c = 140.1 A, beta = 111.7 degrees ), space group C2, have been grown using polyethylene glycol 6000. They diffract to 2.8 and 3.0 A, respectively. Crystals of the heterologous complex between E. coli AspRS and yeast tRNA have been obtained using ammonium sulfate as the precipitant and 2-propanol as the nucleation agent. They belong to the monoclinic space group P2(1) (a = 76.2, b = 227.3, c = 82.3 A, beta = 111.7 degrees ) and diffract to 2.7 A.