A novel norditerpenoid alkaloid, brachyaconitine (3), which possesses a trans-3-hexenoyl side chain in the molecular structure, has been isolated fl om aphids Brachycaudus aconiti feeding on Aconitum napellus, along with 11 known alkaloids. Aphids Brachycaudus napelli living on A. napellus also yielded brachyaconitine (3) along with 12 known alkaloids. The structure of the new alkaloid 3 was derived from its spectroscopic data and was confirmed by semisynthesis from aconitine.