cDNA clones for rat cytosolic aspartate aminotransferase (cAspAT, L-aspartate:2-oxoglutarate aminotransferase) [EC 2.6.1.1] were isolated from a rat cDNA library, and the primary structure of the gene for cAspAT was deduced from its cDNA sequence. Rat cAspAT consists of 412 amino acids and its molecular weight is 46,295. The deduced amino acid sequence of rat cAspAT was compared with the sequences of AspATs from other species. The degree of sequence identities of rat/mouse cAspAT, rat/pig cAspAT, rat/chicken cAspAT, rat/pig mAspAT, and rat/Escherichia coli AspAT were 97.1, 89.6, 81.7, 48.1, and 41.2%, respectively. A coding region of rat cAspAT cDNA was inserted into E. coli expression vector pUC9, and enzymatically active cAspAT was expressed as a beta-galactosidase-cAspAT hybrid protein. This hybrid protein represented about 18% of the soluble proteins in E. coli and its kinetic properties were comparable with those of cAspAT preparations purified from rat liver.