From the leaves of Stephania cepharantha HAYATA (Menispermaceae) cultivated in Japan, a new morphinane alkaloid, cephasugine (1), was isolated together with 14 known alkaloids. The structure of 1 was elucidated as the N- methyl derivative of sinococuline (17) by comparison of spectroscopic data with those of cephakicine (16). This is the first report of the alkaloidal constituents of the leaves of S. cepharantha.