A new ester-ketal hasubanalactam alkaloid, oxosrephabenine (l), 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 'H nrnr, I3C nmr, and mass spectral arguments. Further, permanganate oxidation of stephabenine (2), a known ester-ketal hasubanan alkaloid, gave a y-lactam identical with the naturally occurring 1. In the first paper of this series (l), we reported the isolation and structure elucidation of a new hasubanan ester-ketal alkaloid, stephabenine (2), present in a petroleum ether extract from the fruits of Stepbanza japonica Mien (Menispermaceae). In the continuation of our studies on alkaloids of the fruits of this plant, we have recently isolated a new hasubananlactam ester-ketal alkaloid named oxostephabenine (1). The present paper deals with the structures of the new alkaloid, oxostephabenine (l), and its derivative, N,O-dimethyloxostephine (3).