This paper reports the chemical study of a sample of Fasciospongia sp. collected along the Atlantic Portuguese coast. Three new isomeric furanosesquiterpenes, isomicrocionin-3 ( 1), (-)-microcionin-1 ( 2), and (-)-isomicrocionin-1 ( 3), were isolated along with the known (-)- ent-pallescensin A ( 4) and (-)-pallescensin-1 ( 5) from the ethyl acetate-soluble portion of the methanolic extract. The structures of the new compounds were elucidated by extensive spectroscopic studies. (-)-Microcionin-1 ( 2) tested positive against several Gram-negative bacteria.