Abstract: In our first report, the yellowish crystalline constituent, m. p. 275, C₁₇H₁₁O₇N (Ⅰ) was isolated from A. debilis S. et Z. and it was proposed to be isoaristolochic acid (Ⅱ), which was originally isolated from A. indica by Krishnaswamy et al. (J. Ind. Chem. Soc., 12, 476, 1935). However, during our further studies of its structure, it was found that the compound formed derivatives identical to those derived from aristolochic acid (Ⅲ), such as decarboxylated compound (Ⅳ) C₁₀H₁₁O₅N, m. p. 208, and a hydrogenated compound (V), C₁₇H₁₁O₄N, m. p. 313. Furthermore, the UV-spectra of (Ⅳ) was practically the same as the decarboxylated compound obtained by Pailer et al. from A. clementis (Monat., 87, 249, 1956). Neither the aristolochic acid methyl ester nor its decarboxylated compound showed depression in melting point when mixed with their respective crystals from A. clementis. According to these facts, isoaristolochic acid and aristolochic acid ought to be identical. The authors wish to express their gratitude to Dr. M. Pailer of the University of Vienna, Austria for his kindness to send us the aristolochic acid methyl ester and its decarboxylated compound.