A new macrocyclic diamine alkaloid, named isosaraine-3 (9), has been isolated from the Mediterranean sponge Reniera sarai. The structures of both this new compound and saraine-3 (6), until now only partially clarified, have been completely elucidated by extensive spectroscopic studies. The absolute stereochemistry of the quinolizidone systems present in the co-occurring major alkaloids, saraine-1 (4) and saraine-2 (5), has been determined by applying advanced Mosher's method to the alcohol derivatives (10-13). Copyright (C) 1996 Elsevier Science Ltd