A diverse bacterial community, dominated by Gram negative populations, was found associated with biofilms of the fouling diatom,Amphora coffeaeformis. In the characterization of some members of this culturable bacterial community, emphasis was placed upon their ability to enhance growth of the diatom in elevated Cu2+concentrations. Some selected bacterial consorts, particularly those belonging to the generaPseudomonasandRhodococcus, enabledA. coffeaeformisto grow in artificial sea water medium containing normally inhibitory Cu2+concentrations, demonstrating their ability to establish functional interactions with the diatom leading to enhanced copper tolerance. Owing to the specific nature of the beneficial effect (only produced by certain consorts and under copper stress), the establishment of such interactions could be regarded as a mechanism of copper resistanceof A. coffeaeformis.