We have determined the nucleotide sequence of a cDNA coding human cysteinyl-tRNA synthetase. The predicted protein sequence of 638 amino acids has substantial sequence similarity to the E. coli cysteinyl-tRNA synthetase in the nucleotide-binding fold region that constitutes the potential active site. The results are consistent with the hypothesis that evolution of the full complement of aminoacyl-tRNA synthetases occurred prior to the divergence of prokaryotes and eukaryotes.