Histidyl-tRNA synthetase from Escherichia coli was over-expressed and purified by Q Sepharose and hydroxyapatite chromatography. Crystals of the complex containing histidyl-tRNA synthetase, ATP and histidine have been grown by vapor diffusion against reservoirs containing 0.1 M Tris (pH 7.4), 0.5 M NaCl and 10% polyethylene glycol 6000. Under these conditions, two crystal forms are obtained. The triclinic form has unit cell dimensions a = 61.3 A, b = 108.5 A, c = 110.2 A, alpha = 115.1 degrees, beta = 90.2 degrees and gamma = 97.2 degrees. The monoclinic form, space group P2(1), has cell dimensions a = 61.2 A, b = 109.7 A, c = 196.7 A and beta = 98.1 degrees. Both crystal forms diffract up to 2.7 A and are stable in the synchrotron beam. Assuming a dimeric mass of 96,000 daltons and Vm value of 3.4 A3/dalton, the asymmetric unit in both forms contains two dimers with a solvent content of approximately 60%. A 3.7 A resolution native dataset with an Rmerge on intensities of 7.9% has been collected from the monoclinic crystal form.