Two new secobisbenzylisoquinoline alkaloids, secocepharanthine (base J) (3) and O-methyl-punjabine (base K) (4), were isolated from Stephania sasakii Hayata (Menispermaceae). Their structures were established and the structures of two other alkaloids [base B (1) and base C (2)] were also established as dihydrosecocepharanthine (1) and O-methyldeoxopunjabine (2). In addition, two known bisbenzylisoquinoline alkaloids, obaberine (5) and thalrugosine (6), were newly isolated from the same plant. © 1985, The Pharmaceutical Society of Japan. All rights reserved.