Seven new manzamine-related alkaloids have been isolated together with known manzamine alkaloids from the Okinawan marine sponges of the genera Ircinia and Amphimedon. These alkaloids seem to be biogenetically related to manzamines A ∼ C. We propose a plausible biogenetic path of manzamines A ∼ C on the basis of the structures and ratios of all the manzaminerelated alkaloids isolated from the Amphimedon sponge. The structures and biogenesis of other manzamine-related alkaloids from marine sponges are also described.