ABSTRACT – Organic extracts of eleven species of macroalgae, collected from northern coasts of Tunisia, belonging to the class Phaeophyceae were studied for their antibacterial and algicidal activity (Cutleria multifida, Dictyopteris membranacea, Dictyota fasciola, Padina pavonica, Cystoseira compressa, Cystoseira sp., Sargassum vulgare, Colpomenia sinuosa, Scytosiphon lomentaria, Cladostephus hirsutus and Stypocaulon scoparium). For antibacterial activity, the highest inhibitions were obtained for the Cutleriales and Dictyotales. Scytosiphonales species on the other hand gave the best inhibition against the diatom Chaetoceros calcitrans. These results are encouraging for a possible use of Tunisian seaweeds as a source of bioactive compounds.