Three new antifungal sesquitetpenoids, cavemoisonitrile (l), cavemothiocysnate (2). and lOctisocyan&amorphene (3), have been isolated from the marine sponge Acanthella cf. cavertwsa. and their stfuctums were elucidated on the basis of spectroscogic data. The nudibranch Phyllidia ocelkzta also mcained 2 and 3. Marine sponges of the genera Acanthelkz and Axinella and nudibranch molluscs of the genus Phyllidia which prey on these sponges often contain sesquiterpene isocyanides which possess antimicrobial, cytotoxic. and ichthyotoxic activities.~ In the course of our study on bioactive substances from Japanese marine invertebrates, the matine sponge Acanthelld cf. cavertma collected off Hachijo-Jima Island showed strong antifungal activity. Bioassay-guided isolation afforded several sesquitepene isocyanides including two new compounds and a new sesquiterpene thiocyanate. The nudibranch molhtsc Phyllidia ocellata which likely preyed on this sponge also gave rise to sesquiterpenes, which are identical with the sponge metabolites. This paper deals with isolation and structure elucidation of thme new metabolites.