A new stemona alkaloid, tuberostemonone (C22H31NO6), has been isolated from the roots of stemona tuberosa Lour, and its molecular structure determined by MS, IR, NMR spectra and X-ray crystallography (R=0.046 for 1746 observed Mo-Kα data). Besides retaining the characteristic stemona skeleton, which is composed of an α-methyl-γ-lactone and an N-methylene azepine ring linked at the γ-position of this lactone, the tuberostemonone molecule possesses a nine-membered azacycle which is hitherto not found in other known stemona alkaloids. © 1991 Plenum Publishing Corporation.