In a reinvestigation of the basic components of Stephania japónica, a new hasubanalactam alkaloid, named oxostephasunoline (1), was isolated from the roots. Its structure was established from consideration of spectral properties and by chemical conversion to 16-oxometaphanine (5) via 16-oxoprometaphanine (4). Oxostephasunoline (1), having a carbonyl group at C-16, is the sixth example of this type of compound isolated from natural sources. © 1984, American Chemical Society. All rights reserved.