Three new C-20 diterpene alkaloids (talassamine (I), talassimidine (II), and talassimine (III)) and the known alkaloid lappaconitine have been isolated from the epigeal part of Aconitum talassicum (Ranunculaceae). The structures of the new alkaloids proposed on the basis of spectral characteristics and chemical transformations have been confirmed by an x-ray structural analysis of talassimine. © 1991 Plenum Publishing Corporation.