This study analyzed the bioactivity of extracts from 25 Antarctic demosponge species against 20 bacterial isolates and 1 diatom species collected from the waters off the western Antarctic Peninsula. All sponge species had lipophilic and hydrophilic extracts assayed at two concentrations (1× and 3× natural concentration) against 16 strains of Gamma Proteobacteria, 1 Flavobacterium, and 3 unidentified species of bacteria isolated from sympatric sponges. The majority of the bacterial isolates had no growth inhibition with only one isolate found to consistently have any growth inhibition due to sponge extracts. The sponges also had lipophilic and hydrophilic extracts assayed at three concentrations (0.3×, 1×, and 3× natural concentration) against the chain-forming pennate diatom Syndroposis sp. Almost every sponge's lipophilic extract (96%) resulted in significant diatom mortality at the estimated natural concentration with the majority of the extracts (60%) still resulting in significant mortality at 30% of the natural concentration. The hydrophilic extracts of 60% of the sponges resulted in significant diatom mortality at the natural concentration. Even at 30% of the natural concentration, 24% of the hydrophilic sponge extracts resulted in significant diatom mortality. These sponges appear to have stronger defenses against diatom fouling than bacterial growth suggesting that there may be more selective pressure for chemical defenses against diatoms. © 2009 Springer-Verlag.