Keramaphidin B (1), a novel cytotoxic alkaloid possessing a 1,4-etheno-2,7-decahydronaphthylidine core with two macrocyclic rings has been isolated from an Okinawan marine sponge Amphimedon sp., and the structure was elucidated by extensive 2D NMR investigation as well as a single crystal X-ray analysis. Keramaphidin B (1) might he a plausible biogenetic precursor of manzamine alkaloids and a very important key compound to clarify the biogenetic path.