A new manzamine-related tetrahydro-beta-carboline alkaloid with a methylene carbon bridge between N-2 and N-27, ma'eganedin A (1), has been isolated from an Okinawan marine sponge Amphimedon sp., and the structure including absolute stereochemistry was elucidated on the basis of spectroscopic data. The most stable conformation of 1 was deduced from molecular mechanics calculation. (C) 1998 Elsevier Science Ltd. All rights reserved.