Indonesian specimens of the marine sponges Hyrtios erectus and H. reticulatus were found to contain 5-hydroxytryptamine-derived alkaloids. Their structures were determined on the basis of their spectral properties. H. erectus contained hyrtiosulawesine (4), a new beta-carboline alkaloid, together with the already known alkaloids 5-hydroxyindole-3-carbaldehyde (1), hyrtiosin B (2), and 5-hydroxy-3-(2-hydroxyethyl)indole (3). H. reticulatus contained the novel derivative 1,6-dihydroxy-1,2,3,4-tetrahydro-beta-carboline (11) together with serotonin (5), 6-hydroxy-1-methyl-1,2,3,4-tetrahydro-beta-carboline (7), and 6-hydroxy-3,4-dihydro-1-oxo-beta-carboline (9).