In a study of structural diversity in the total pool of phlorotannins in brown algae, we have shown, for the first time in any alga (by means of solvent partition and NMR experiments), that Fucus spiralis has simultaneous production of two types of polymeric phlorotannins of the fucol and fucophlorethol classes, respectively. These components exhibited a greater antioxidant activity than ascorbic acid and the monomer phloroglucinol, a constitutive of the polymeric phlorotannins. No significant differences in antioxidant activity were observed between fucol and fucophlorethol.