A new 5,s-diazabenz[ul]azulene alkaloid, aaptosamine, was isolated from the marine sponge Aaptos aaptos collected in the southern Caribbean. The structure was determined by 2D NMR spectroscopy using HMQC and HMBC experiments. The known alkaloid 9-demethyloxyaaptamine which was isolated from an earlier collection of A. aaptos, underwent an unprecedented base-catalyzed rearrangement to give a new furanonaphthyridine derivative.