In recent years many sterols with potent biological activity have been identified from marine sources. Here we report the isolation of stigmast-5,24-dien-3-ol (fucosterol) as a major metabolite from the bioactive hexane-fraction of Sargassum tenerrimum using different chromatographic techniques. Moreover, the chemical defense of brown algae Sargassum tenerrimum was investigated using feeding assay performed in an aquarium and in the field.