Protostemonamide (1), a new protostemonine-type alkaloid, and 12 known compounds were isolated from the roots of Stemona sessilifolia. Their structures were elucidated by 1 D and 2 D NMR spectral and other spectroscopic studies. The main alkaloidal constituents, protostemonine (2), stemospironine (4), and maistemonine (7), showed significant antitussive activity in a citric acid-induced guinea pig cough model following peripheral administration; stemonamine (11) had antitussive activity following i. c. v. administration.