Prior studies have documented predator deterrence and antifouling activities for organic extracts of the Antarctic soft corals Alcyonium paessleri and Gersemia antarctica under surficial contact situations. Field observations and subsequent seawater collections support the release of water-borne allelochemicals by these 2 species. A. paessleri and G. antarctica released organics totaling 1.54 and 1.26 mg l-1 of seawater extracted, respectively. The compounds from A. paessleri were comprised of 86% sterols by mass and they diluted to non-detectable levels within 1 to 2 cm of the colony surface. The absolute production rates for the sterols varied positively with colony size and ranged from 3.4 to 6.6 mg l-1 seawater extracted d-1. Three of the 4 sterols isolated from A. paessleri deterred seastar predators; included amongst these compounds was the primary metabolite, cholesterol. In contrast, G. antarctica released many classes of compounds and few in any major quantities. Nonetheless, the organic fraction of seawater collected near this soft coral showed potent antibacterial activity against 3 sympatric microbes. Bioautography was used to guide isolation of the antimicrobial fraction which contained homarine, trigonelline, and a minor metabolite. Homarine was responsible for most of the bioactivity noted during the bioautography assay.