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Different solvent extracts of a red algae, Hypnea flagelliformis, and two brown algae, Cystoseira myrica and Sargassum boveanum, which were collected from the Persian Gulf coast were subjected to different bioassays, including: 1,1-diphenyl-2-picrylhydrazyl (DPPH) free radical scavenging assay, antibacterial and antifungal activity by thin layer chromatography (TLC)-bioautography, agar disc diffusion (ADD) and nutrient-broth micro-dilution (NBMD) bioassays. The water extracts were found to have the most antioxidant activity. The antibacterial minimum inhibitory concentrations (MIC) of the active extracts were determined for the susceptible organisms, Staphylococcus aurous and Bacillus subtilis, using NBMD bioassays. The active substances were identified as free fatty acids (FFA), by using gas chromatography-mass spectrometry (GC-MS). After derivatization to their methyl esters, their concentrations were measured by using GC- lame ionization detection (GC-FID). In addition to the fatty acids, fucosterol, cholesterol and 22-dehydroxychlosterol were detected as the major sterols in S. boveanum extract using GC-MS analyses