Fukinotoxin (I), a novel pyrrolizidine alkaloid, was isolated from young scape of Petasites japonicus and its structure has been shown to be (12R,13R,15R,3’R)-12-hydroxy-4,12,13-trimethyl-4,8-secosenec-l-enine-15-spiro-2’-(3’-methyl)oxiran with the aid of chemical and X-ray crystallographic analyses. © 1976, The Pharmaceutical Society of Japan. All rights reserved.