A new aristolochic acid derivative, 3-hydroxy-4-methoxy-10-nitrophenanthrene-1-carboxylic acid methylester [1], was isolated from the stems of Aristolochia liukiuensis in addition to a cinnamide, another aristolochic acid, aristololactams, dioxoaporphine, and isorhamnetin-3-O-robinobioside. This is the first report of this flavonol glycoside in Aristolochia species. The genus Aristolochia (Aristolochiaceae) is found in wide areas from the tropics to temperate zones and consists of about 300 species. Some species have been used in the form of crude drugs as anodynes, antiphlogistics, and detoxicants, especially in China. Five species are native to Japan, namely Aristolochia kaempferi, Aristolochia onoei, Aristolochia abilis, Aristolochia contorta, and Aristolochia liukiuensis (1). A. liukiuensis has recently been segregated from A. kaempferi from a taxonomic standpoint, but these species are morphologically very similar. The chemical constituents of A. liukiuensis were investigated. A new aristolochic acid derivative 1 and a flavonol glycoside were isolated from the stems. The present paper deals with the structure elucidation of 1. Compound 1, mp 164-165°, was isolated as pale yellow needles from a CHCl₃ extract after repeated separation with Si gel cc. In the eims, the [M]⁺ was observed at m/z 327, which agreed with a molecular formula of C₁₇H₁₃NO₆ by hrms (found 327.0720, calcd 327.0743). The uv [λ max 214, 257, 295, 356, 374 nm], the ir [ν 3470 (OH), 1720 (C=O), 1525 (NO₂)], and the ms [m/z 281, a fragment caused by the elimination of a nitro group] suggested that 1 was one of the typical aristolochic acids. In the ¹H-nmr spectrum, two three-proton singlets at δ 3.92 and 3.82 were observed. The former corresponded to a methoxyl group, and the latter was due to a methyl group of a methylester moiety, indicating that 1 was a methylester of an aristolochic acid derivative. With reference to the data described in the literature (2), two one-proton singlets at δ 7.83 and 8.41 were assignable to H-2 and H-9, which implied 3,4-dioxygenation on the A ring. The complicated signal pattern in the aromatic region indicated the C ring to be unsubstituted. The position of the methoxyl group was deduced to be at C-4 because of its low chemical shift (δ 60.6) in the ¹³C-nmr spectrum. Therefore, the structure of 1 was proposed to be 3-hydroxy-4-methoxy-10-nitrophenanthrene-1-carboxylic acid methylester. By usual acetylation or methylation, 1 gave a monoacetate or a monomethylether of 1. The respective spectral data supported the structure thus proposed. In addition to 1, isorhamnetin-3-O-β-L-rhamnopyranosyl (1→6)-β-D-galactopyranoside, N-(p-hydroxyphenyl)-p-ethyl-p-hydroxycinnamide, aristolochic acid I, aristololactams A II, B II, and A III, and 2-hydroxy-1-methoxy-4,5-dioxo-6,7-dehydroaporphine were also isolated. These known structures were characterized by means of their spectral properties. All aristolochic acid derivatives in Aristolochia species have a methylenedioxy group on the A ring; in contrast, in the methyl ester 1 this group is lacking, which suggests that 1 has a possibility to be a species-specific marker useful to characterize the species. To investigate its occurrence in other Aristolochiaceae, the constituents in other species are now being examined.