The structure of thalicsessine, a new C20-diterpenoid alkaloid isolated from Thalictrum sessile Hayata (Ranunculaceae), together with a known diterpenoid alkaloid spiradine A, was elucidated on the basis of spectral and chemical evidence. This is the first report on diterpenoid alkaloids obtained from the plant of the genus Thalictrum. © 1987.