From the roots of Aconitum septentrionale, a new alkaloid, septentriosine [la] (hetisan- lp,2a,9p, 19a-tetrol), was isolated, and its structure was determined by spectroscopic and X-ray data.The presence of alkaloids in Aconitum septentrionale Koelle (Ranunculaceae) was recorded almost a century ago (1). In 1967, Marion et al. (2) reported the isolation and characterization of seven alkaloids of which two were the known C 19-diterpenoid alkaloids, lappaconitine and deacetyllappaconitine. The structures of the remaining have not been determined (2). In previous communications, we have reported the isolation of lapaconidine (3) and also the structures of two C19-diterpenoid alkaloids, septentriodine and septentrionine (4). In the present paper, we wish to report the isolation and structure of a minor alkaloid, septentriosine, obtained in 0.015% yield from the roots of A. septentrionale.