A novel norditerpene was isolated from the flower of Eupatorium adenophorum, named (4aR,7R,8S,8aR)- 1,2,4a,5,6,7,8,8a-octahydro-8-[3-methylenebut-4-alyl]-4,4a,7,8-tetramethylnaphthalen-2(1H)-one (1). Its structure was established by extensive NMR experiments. Based on the diversity of the side chains, a possible biodegradation pathway for the compound from the clerodane skeletone is proposed.