Organic extracts from the antarctic soft corals Alcyonium paessleri May and Gersemia antarctica Kukenthal exhibited antimicrobial and antifoulant activity. A third antarctic soft coral, the stoloniferan Clavularia frankliniana Roule, exhibited no such bioactivity. Marine bacterial attachment was inhibited in the presence of chloroform and aqueous methanol extracts of A. paessleri and G. antarctica, but not in the presence of solvent controls. Similarly, inhibition of microbial growth in three sympatric species of antarctic marine bacteria occurred in response to aqueous methanol extracts of these two soft corals, but not to the controls. Antifoulant activity of chloroform extracts of A. paessleri and G. antarctica was detected in a month-long field assay measuring inhibition of benthic diatom settlement. Observed invertebrate recruitment rates were too low to allow an evaluation of antifoulant bioactivity. Laboratory growth experiments, employing the antarctic diatom Navicula sp. and larvae of the antarctic sea urchin Sterechinus neumayeri Meissner, confirmed the presence of growth-inhibiting metabolites in the soft corals A. paessleri and G. antarctica. These results suggest that both A. paessleri and G. antarctica, which do not appear to be fouled in the field, possess bioactive compounds with antifoulant activity. In contrast, C. frankliniana, which is often heavily fouled, appears to lack antifouling compounds. © 1995.