The new diterpene alkaloid septedine has been isolated from the roots of Aconitum septentrionale Koelle. On the basis of spectral characteristics and x-ray structural analysis (diffractometer, CuKα radiation, 1537 reflections, direct method, R = 0.055) it has been established that septedine has an atisine skeleton including an oxazolidine ring with a C14–C20 bridge. © 1996, Plenum Publishing Corporation. All rights reserved.