The algae Enteromorpha prolifera, Enteromorpha compressa, Cladophora sericea, Pylaiella litoralis, Ceramium rubrum, Nemalion multifidum and Furcellaria fastigiata contain extractable auxin. After paper chromatography in different solvents, the Triticum section-test and Avena curvature-test showed that the main activity was due to IAA. This result was supported by colour tests with indole reagents after paper and thin layer chromatography.In Ceramium rubrum, Enteromorpha prolifera and Enteromorpha compressa the low IAA level was correlated with a high content of inhibitors.Only Furcellaria fastigiata contained an auxin in the nonacidic fraction. As yet an identification was unsuccessful.Alkali-hydrolysis of the algae using N NaOH liberated large amounts of auxins. Also in this case, IAA was the main auxin. With thin layer chromatography 2 or 3 other indole derivatives could be detected.