A novel aspartate aminotransferase inhibitor and cyclic amino acid named gostatin was isolated as crystals from the culture filtrate of Streptomyces sumanensis NK-23. Its structure was established to be 5-amino-2-carboxy-4-oxo-1,4,5,6-tetrahydropyridine-3-acetic acid by X-ray crystallographic analysis (K. Ibata et al., manuscript in preparation). Gostatin shows a strong inhibitory activity against aspartate aminotransferases (EC 2.6.1.1, GOT) of various origins, and exhibits time-dependent inhibitory action against aspartate aminotransferases and alanine aminotransferase. Both pyridoxal phosphate- and pyridoxamine phosphate-linked enzymes are sensitive to the inhibitor. This paper reports the isolation and some chemical and biochemical properties of gostatin.