A new ester-ketal hasubanalactam alkaloid, oxostephabenine (1), was isolated from a methanolic extract of the fruits of Stephania japonica (Menisperamaceae). Alkaline hydrolysis of oxostephabenine (1) gave a new hasubanalactam alkaloid, N,O-dimethyloxostephine (3) and benzoic acid. The structures of the new alkaloids, 1 and 3, were elucidated by 1H nmr, 15C nmr, and mass spectral arguments. Further, permanganate oxidation of stephabenine (2), a known ester-ketal hasubanan alkaloid, gave a γ-lactam identical with the naturally occurring 1. © 1985, American Chemical Society. All rights reserved.