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The natural occurrence of optically active isocitric acid was first demonstrated by Nelson' and Bruce(2) in blackberry fruits, and a somewhat large accumulation of the acid was found by Pucher et al. 3~ in Bryophyllum leaf tissue. This acid has also been recognized as a member of the tri-carboxylic acid cycle in many animals, plants and microorganisms. All isocitric acid, found before in nature, was the same stereoisomer, the free acid of which was dextrotatory in aqueous solution and its lactone having strong levorotatory activity. Therefore this has been given the prefix of d-. Since isocitric acid contains two asymmetric centers, its diastereoisomer must exist, i.e., allo-isocitric acid, and then, the occurrence of four stereoisomers, i.e., d-, l-isocitric acid and d-, l-allo-isocitric acid, would be expected. The presence of dl-allo-isocitric acid in the synthetic acid was first demonstrated by Pucher et but they could not obtain it in a pure state. Recently, the stereochemistry of the a-carbon of isocitric acid was elucidated by Greenstein et al.c5~,cs>. All four stereoisomers were obtained in a pure state and configuration of the x-carbon of these isomers were decided. d-Isocitric acid was proved to belong to the Ls-series.