The widespread uses of antibiotics have been resulted in resistant strains of microorganisms and increasing of worldwide antibiotic resistance. Thus the investigations on new natural antibacterial agents as new drugs are important. According to the previous researches, some multicellular marine algae have significant antibacterial properties. In the present study, antibacterial effects of organic and aqueous extracts of Sargassum glaucescens (collected from Chabahar’s coast, Oman Sea, Iran) were tested on three strains of Gram-negative bacteria: E. coli, Proteus vulgaris, Vibrio cholerae and two strains of Gram-positive bacteria: Listeria monocytogenes and Staphylococcus aureus. Extractions were obtained by immersion method after 48 hours. Antibacterial effects were investigated by the disk diffusion method and serial dilutions in tube to determine the minimum inhibitory concentration. The ethanolic extract showed the largest impact on the L. monocytogenes with significant difference than that by the neomycin. Yet, the aqueous extract showed no effects. Ethanolic extract of algae had no effects on the Proteus vulgaris. The results of present study demonstrated that Ethanolic extract of S. glaucescens had reliable antibacterial effects against L. monocytogenes, Vibrio cholerae, E. coli and Staphylococcus aureus
