Prostephanaberrine (I), a new hasubanan alkaloid, was isolated from the fresh fruits of Stephania japonica. The new alkaloid and its acetate 2 were converted into the metaphanine-type base stephanaberrine (3). The reaction sequence and spectral data showed that the structures of 1 and 3 are related to prometaphanine (7) and metaphanine (10). In the previous papers of this series (1,2), we reported the structures of three new hasubanan alkaloids, stephabenine (4), oxostephabenine (5), and N, O-dimethyloxostephine (6), together with two unidentified alkaloids tentatively named Base-A and Base-B. The unidentified alkaloids were isolated from a petroleum ether extract and we briefly reported their physical and spectral data (2). In a continuing search for this plant material, Base-B, isolated also from a methanolic extract, now named prostephanaberrine (l), was shown to be a new hasubanan alkaloid.In the present work, we report the isolation and characterization of the new alkaloid prostephanaberrine (1) and its derivative, stephanaberrine (3).