The crude extract from the marine sponge Aaptos sp. collected in Abrolhos, Bahia, Brazil, was unpalatable to a natural assemblage of reef fishes. Preliminary assays also indicated the presence of anti HSV-I compounds in this crude mixture. Successive chromatographic purification afforded a new alkaloid, 8,9-dimethoxy-4-methyl-4H-benzo[de][1,6]naphthyridine (2,4-methylaaptamine) and the known demethyloxyaaptamine (1), which structures were determined mainly by 1D and 2D NMR. Both compounds showed potent antiviral activity against herpes simplex virus type I and low toxicity to Vero cells, suggesting that they may be selectively targets to inhibit virus replication.