The structures of two new diterpenoid alkaloids, secojesaconitine (1__∼) and subdesculine (5__∼), from (Aconitum)___- (japonicum)___- Thunb. were determined on the basis of spectral and chemical evidence. The structure of an unusual epoxide (1__∼) was confirmed by X-ray analysis and was the first example of an epoxy ring between C₃ and C₁₇. Treatment of the epoxide (1__∼) with aceteic acid and acetylation were found to afford the normal C<19>-aconitine type alkaloid.