A new hasubanan ester-ketal alkaloid, named stephabenine, was isolated from the fresh fruits of Stephania japnica MIERS (Menispermaceae). Alkaline hydrolysis of the new base gave benzoic acid and a basic component, whose proton nuclear magnetic resonance spectrum was identical with that of authentic N,O-dimethylstephine.Although eleven hasubanan alkaloids2) have been isolated from the stems and roots of Stephania japonica MIERS (Menispermaceae) together with many other alkaloids, as far as we know no research has been carried out on alkaloids in the fruits. Recent investigation of the fruits collected in Amakusa, the southwestern region of Japan, resulted in the isolation of a new hasubanan ester-ketal alkaloid, named stephabenine (1), together with two unidentified alkaloids. The present paper deals with the structure of the new alkaloid, stephabenine (1).