Four alkaloids,demethyl(oxy)aaptamine(1),aaptamine(2),isoaaptamine(3)andaaptosine(4),have been isolated from the Formosan marine sponge Aaptos aaptos.The structures of these compounds were established by spectral methods.Biological screening revealed that all compounds except 4 showed potent cytotoxicitiesagainst P-388,KB16,A549 and HT-29 tumor cells.