A new phthalide isoquinoline alkaloid with an unusual ring B structure decumbenine-C, isolated from the roots of Corydalis decumhens (Thunb) Pers, a traditional Chinese medicinal herb is reported. Decumbenine-C, C_(19)H_(11)NO_6, m. p. 211—213℃, forms brown woolly crystal. Its structure was established by a combination of UV, IR and ~1H NMR spectral study and shown in the Chinese text. Every proton in decumbenine-C was assigned by a combination of the double irradiation and NOED spectral studies. Three mg o...