A new xanthone C-glucoside, C₁₉H₁₈O₁₁·1/2H₂O, decomposing over 260° without melting, [α]²¹_D+5.5° (pyridine), was isolated in a pure state along with mangiferin (I) from the aerial parts of Anemarrhena asphodeloides BUNGE (Liliaceae) and named isomangiferin. On the basis of chemical and spectral data, it has been assigned the structure 4-C-β-D-glucopyranosyl-1, 3, 6, 7-tetrahydroxyxanthone (XV), the first position isomer of I to be found and isolated. The relation between XV and I, in structures and properties, is corresponding to that between flavonoid 8-C- and 6-C-glucosides such as orientin and isoorientin.