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 C₇ and C₁₇. Treatment of the epoxide (1) with acetic acid and acetylation were found to afford the normal C₁₉-aconitine type alkaloid.