Three novel phenolic metabolites, tridentatols A-C (l-3), with an uncommon sulfurcontaining functional group have been isolated from the marine hydroid Tn'dentata marginato. The structures of the compounds were determined by interpretation of spectral data and a single crystal Xray diffractIon study on tridentatol C. Ecological studies showed that tridentatol A deters feeding by a common hydroid predator. These metabolites also may function to protect T. marginala from damaging solar ultraviolet radiation.