Four novel alkaloids of the aaptamine class have been isolated in addition to the known aaptamine, isoaaptamine, demethyl(oxy)aaptamine and its dimethylketal from an unidentified species of Indonesian marine sponge of the genus Xestospongia. Their structures were elucidated on the basis of detailed 1D and 2D NMR spectroscopic data. Their antimicrobial activity was tested towards Gram (+) (S. aureus), Gram (-) (E. coli, V. anguillarum) bacterial strains, a fungus (C. tropicalis); their cytotoxic activity was evaluated against KB cells. © 2003 Elsevier Ltd. All rights reserved.