The brown seaweed Sargassum vulgare C. Agardh abounds along the southeast coast of Rio de Janeiro State, Brazil, creating unique habitats, and its polyphenols play an important ecological role as antifouling agents. In order to understand more precisely this defensive strategy of S. vulgare, we collected this macroalga at Ilha de Itacuruçá in May and October 2009. Thalli were separated according to tissue type: pneumatocysts, receptacles, leaflets, and axis. Phenolic extracts from each specific part of the plant were obtained and the associated antifouling activity was tested in order to assess whether a tissue specialisation/antifouling activity pattern exists. Preliminary separation process was carried out on a phenolic extract to separate the polyphenols from the remaining compounds. Such fractionation allowed us to study the involvement of strictly phenolic compounds in antifouling activity. Within-thallus variation in both phenolic content and antifouling activity were highlighted, and seasonal variation in both those characters was also observed. For both experiments, pneumatocyst and leaflet phenolic extracts were the most active against the attachment of Perna perna, but no correlation was observed between phenolic concentration and antifouling activity. On the contrary, the extract containing only phenolic compounds was twice less active than the extract from which 80 % of the polyphenols were removed. These results led us to hypothesise that polar compounds other than polyphenols are involved in the antifouling activity.