Swertisin, C₂₂H₂₂O₁₀, m.p. 243°(decomp.), was isolated in a pure state from the whole herb of Swertia japonica MAKINO (Gentianaceae), and identified as 6-C-β-D-glucopyranosylgenkwanin. Isoswertisin, C₂₂H₂₂O₁₀, m.p. 295°(decomp.), was obtained by the acid-treatment of swertisin, and formulated as 8-C-β-D-glucopyransylgenkwanin. At the same time, it was found that they were interconvertible into each other, reminiscent of the interrelationship between vitexin and isovitexin.