A new bisguanidinium alkaloid, stellettadine A (1), containing a norsesquiterpene unit has been isolated from a marine sponge Stelletta sp, as a metamorphosis-inducing compound. Its structure including the absolute stereochemistry was elucidated on the basis of spectral data and chemical degradation. Compound 1 induced metamorphosis of the ascidian Halocynthia roretzi larvae with an ED(100) value of 50 mu M. Copyright (C) 1996 Elsevier Science Ltd