The alkaloids of Aconitum gigas LEV. et VAN. which was collected in Hokkaido, were reexamined. In addition to the reported lycaconitine, two known compounds, lycoctonine and atisine hydrochloride, and a new base, named gigactonine, were isolated. The structure of gigactonine was deduced from a few chemical reactions and its ¹³C-nuclear magnetic resonance spectrum, and it was established that 18-O-methylgigactonine was identical with the known diterpenoid alkaloid, delsoline. From oxidation reactions and the above-emntioned correlation, the structure of gigactonine was established. Furthermore, bases I and V were also isolated from the same plant as the artifacts of lycaconitine.