Three Hypselodoris species, H. villafranca, 14. cantabrica, and H. tricolor, have been studied in order to further investigate the defensive strategies of Chromodorididae mollusks from Iberian coasts. All animals possess large amounts of furanosesquiterpenoids, probably derived from their diet of sponges. The products are transferred and stored in specific sites of the mantle (mantle dermal formations, MDFs) to be used as chemical deterrents against predators. Chemical analysis of Cantabrian Hypselodoris species has led to the characterization of eight sesquiterpenoids. Among these, six have been found previously in both sponges and nudibranchs, while two are new natural products. Hypselodoris species cooccurring in the same area contained comparable mixtures of sesquiterpenoids. However, two populations of H. cantabrica from different sites of the Cantabrian sea showed different metabolic pattems. All sesquiterpenoids induce feeding deterrence in Carassius auratus and two are also toxic to Gambusia affinis.