Dactylomelatriol (1), obtained from the sea hare Aplysia dactylomela, is the first naturally occurring omphalane-derived sesquiterpene from the marine environment. From this species the known chamigrene and modified bisabolene sesquiterpenes 2-6 were also isolated. The structure and relative configuration of 1 were established by spectroscopic evidence. Its chemical structure is related to omphalic acid, the unique terrestrial-derived omphalane sesquiterpene isolated from a liverwort. A biogenetic route for this compound is proposed. The antimicrobial activities of compounds 1-6 were evaluated against a panel of microorganisms.