A novel norditerpenoid alkaloid, brachyaconitine (3), which possesses a trans-3-hexenoyl side chain in the molecular structure, has been isolated from 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. We now describe the isolation of these alkaloids, the structure determination and semisynthesis of brachyaconitine (3), and the identification of the known alkaloids. Spectroscopic data of the previously known 8-O-acetyl-15Rhydroxyneoline (9) and complete 1H-NMR spectral assignments for the known 14-O-acetylneoline (2) are reported for the first time in this paper.