Crude extracts of Antarctic macroalgae were examined for toxicity to model strains of sympatric diatoms that could potentially foul them in nature. This is an initial step in determining the potential for ecologically relevant macroalgal defenses against fouling. Extracts of two species collected from McMurdo Sound were toxic to diatoms at or below concentrations originally present in the macroalgal thalli (the "natural concentration"). At least one extract type of all 22 western Antarctic Peninsula macroalgae was toxic at three-times the natural concentration and at least one extract type in 15 of these species was toxic at the natural concentration. © 2005 by Walter de Gruyter.