Antiherbivore effects of phlorotannins from eight common Antarctic brown algae were tested with three omnivorous sympatric grazers, the fish Notothenia coriiceps, the sea star Odontaster validus, and the amphipod Gondogeneia antarctica. Incidence of grazing deterrence overall was relatively low, with only a few algal species deterring each of the grazers, and with phlorotannins from different algae active against different grazers. Most activity was found in Desmarestia menziesii and Cysto-sphaera jacquinotii, which deterred two out of the three grazers, while D. anceps and Himantothallus grandifolius deterred only one grazer type each. Only weak antimicrobial effects were detected in all phlorotannin extracts against four Antarctic bacterial strains, with phlorotannins of D. anceps and Phaeurus antarcticus having the strongest effects. Phlorotannin extracts of all brown algal species tested caused strong in vitro mortality in a fouling diatom. The ecological significance of this activity remains to be tested. While the bulk tissue phlorotannin concentrations likely overestimate naturally occurring surface concentrations encountered by foulers in situ, fouler settlement may be deterred at much lower concentrations than those eliciting mortality. Overall, Antarctic phlorotannin deterrent properties were specific both to the algal species as well as the grazer, bacteria and diatom taxa tested. For grazer deterrence, we propose that, for each of the tested grazers, complex interactions between phlorotannin chemical properties and grazer digestive characteristics may determine the effectiveness of specific phlorotannins. Larger generalizations of phlorotannin-grazer patterns such as distinction between meso- and macrograzers may not be valid, at least not for Antarctic brown algal phlorotannins. © 2009 by Walter de Gruyter.