Two new aaptamine alkaloids, demethylaaptamine (2) and demethyl(oxy)aaptamine (3), have been isolated from the Okinawan marine sponge Aaptos aaptos as cytatoxic and antimicrobial substances. The structures of demethylaaptamine (2) and demethyl(oxy)aaptamine (3) were determined by interpretation of spectral data and results of chemical degradation experiments. The yellow colour of the marine sponge Aaptos aaptos changes to dark brown after collection and the sponge contains a large amount of yellow compounds. The 70% ethanolic extract of the sponge showed a-adrenoceptor blocking, antimicrobial, and cardiotonic activity. We have previously reported the isolation and structural determination of aaptamine (1) which has a-adrenoceptor blocking activity. Aaptamine (1) belongs to a new type of alkaloid having a 1H-benzo[de][1,6]naphthyridine skeleton. Recently, the structure of aaptamine (1) was confirmed by total syntheses. We have now isolated two further aaptamine alkaloids from this sponge. In this paper, we describe the structural elucidation of demethylaaptamine (2) and demethyl(oxy)aaptamine (3).