Insoluble crude extract fractions of 11 species of Antarctic macroalgae were screened for algicidal activity against sympatric diatoms. Medium pressure liquid chromatography (MPLC) fractions of soluble extracts of four species were also screened. Levels of activity in insoluble fractions were lower than in previously examined corresponding soluble fractions. Combined MPLC fractions had lower activity than unfractionated extracts in three species, but in two, specific fractions with activity were identified. Combined soluble fractions of a fourth species retained strong activity but no individual fraction had activity even at tenfold higher concentrations, indicating that activity probably results from synergistic interactions between multiple compounds. © 2012 by Walter de Gruyter • Berlin • Boston.