The Israeli Journal of Aquaculture - Bamidgeh, IIC:63.2011.667, 5 pages * Corresponding authors. Tel.: +63-33315-8090, e-mail: fnbaleta19@yahoo.com (F.N. Baleta), jhunlaureta@yahoo.com (L.V. Laureta) Biological Activity of Extracts of Sargassum oligocystum (Magnaye) against Aquaculture Pathogenic Bacteria Francis Nuestro Baleta 1,2* , Liberato V. Laureta 1* , Mary Jane S. Apines-Amar 1 , Philip Ian P. Padilla 3 , Gerald F. Quinitio 4 1 Institute of Aquaculture, College of Fisheries and Ocean Sciences (CFOS), University of the Philippines Visayas (UPV), 5023 Miagao, Iloilo, Philippines 2 College of Fisheries, Agriculture and Agribusiness, Isabela State University Roxas Campus, Rang-ayan, Roxas, Isabela, Philippines 3 Division of Biological Sciences, College of Arts and Sciences, UPV, 5023 Miagao, Iloilo, Philippines 4 Institute of Marine Fisheries and Oceanology, CFOS, UPV, 5023 Miagao, Iloilo, Philippines (Received 20.11.10, Accepted 29.1.11) Key words: antibacterial agent, brown algae, fish pathogenic bacteria, Sargassum oligocystum, sequential extraction Abstract Sargassum oligocystum (Phaeophyceae) was collected from the coastal area of Sta. Ana, Cagayan, Philippines, and used in in vitro antibacterial assays against six pathogenic bacteria commonly occurring in aquaculture. The extracts (methanol, n-hexane, dichloromethane, ethyl acetate, aqueous) were screened against Vibrio harveyi, V. parahaemolyticus, V. alginolyticus, Flavobacterium aurantiacum, Streptococcus faecalis, and Pseudomonas aeruginosa. The methanol extract showed strong antibacterial activity against V. harveyi, S. faecalis, and P. aeruginosa and moderate activity against the rest of the test pathogens. In general, V. harveyi was the most susceptible strain to all the extracts. This study suggests that extracts of S. oligocystum may be promising sources of antibacterial agents for use in aquaculture. The IJA appears exclusively as a peer-reviewed on-line open access journal at http://www.siamb.org.il Sale of IJA papers is strictly forbidden.