Amino acids are building blocks of proteins, while aminoacyl-tRNA synthetases (aaRSs) catalyze the first reaction in such building: the biosynthesis of proteins. The E. coli arginyl-tRNA synthetase (ArgRS) has been crystallized in complex form with tRNA(Arg) (B. stearothermophilus), at pH 5.6 using ammonium sulfate as a precipitating agent. Two crystal forms have been identified based on unit cell dimension. The complete data sets from both crystal forms have been collected with a primitive hexagonal space group. A data set of Form II crystals at 3.2 A and 94% completeness has been obtained, with unit cell parameters a = b = 98.0 A, c = 463.2 A, and alpha = beta = 90 degrees , gamma = 120 degrees , being different from a = b = 110.8 A, c = 377.8 A for form I. The structure determination will demonstrate the interaction of these two macromolecules to understand the special mechanism of ArgRS that requires the presence of tRNA for amino acid activation. Such complex structure also provides a wide opening for inhibitor search using bioinformatics.