other پژوهشي Orginal research papers <em>Aeromonas hydrophila</em> was isolated from diseased Siamese fighting fish <em>Betta splendens</em> Regans.&nbsp; The virulence of <em>A. hydrophiala</em> to Siamese fighting fish was conducted by intraperitoneal injection of two doses of bacterial suspensions with 7.5&times;10⁷ and 7.5&times;10⁵ cfu per 0.05 ml. The cumulative mortality was observed in high and low dose groups which showed 98.33 and 20 percent, respectively. Moreover, five medicinal plants extracts namely <em>Centella asiatica</em>, <em>Morinda citrifolia, Melissa officinalis</em>,<em> Piper sarmentosum</em> and <em>Terminalia catappa</em> were determined for antimicrobial activities by broth dilution method. <em>Terminalia catappa</em> extract showed highest antimicrobial effect of MIC and MBC (25 and 12.5 mg mL^-1), followed by <em>M. officinalis</em> and <em>P. sarmentosum</em> with the same values of MIC and MBC (12.5 and 25 mg mL^-1).&nbsp; Finally, <em>Centella asiatica</em> and <em>Morinda citrifolia</em> were similarly MIC and MBC (50 and 100 mg mL^-1).&nbsp; It was concluded that five medicinal plants can use to inhibit and kill <em>A. hydrophila</em> in fish.&nbsp; &nbsp; Aeromonas hydrophila, Betta splendens, Antibacterial activity, Medicinal plants 349 354 http://jifro.ir/browse.php?a_code=A-10-2095-2&slc_lang=other&sid=1 C. Kanchan cmunchan@yahoo.com 100319475328460029855 100319475328460029855 Yes Program in Aquaculture Technology, Faculty of Agriculture Technology, Rajabhat Maha Sarakham University, 44000, Thailand Program in Aquaculture Technology, Faculty of Agriculture Technology, Rajabhat Maha Sarakham University, 44000, Thailand P. Imjai 100319475328460029856 100319475328460029856 No Program in Aquaculture Technology, Faculty of Agriculture Technology, Rajabhat Maha Sarakham University, 44000, Thailand Program in Aquaculture Technology, Faculty of Agriculture Technology, Rajabhat Maha Sarakham University, 44000, Thailand N. Kanchan 100319475328460029857 100319475328460029857 No Animal Production Technology Program, Faculty of Agro-Industrial Technology, Kalasin University, 46000, Thailand Animal Production Technology Program, Faculty of Agro-Industrial Technology, Kalasin University, 46000, Thailand K. Panchai 100319475328460029858 100319475328460029858 No Biology Program, Faculty of Science and Technology, Nakhon Ratchasima Rajabhat University, 30000, Thailand Biology Program, Faculty of Science and Technology, Nakhon Ratchasima Rajabhat University, 30000, Thailand K. Hatai 100319475328460029859 100319475328460029859 No Borneo Marine Research Institute, University Malaysia Sabah, Malaysia, 88400 Kota Kinabalu, Sabah, Malaysia Borneo Marine Research Institute, University Malaysia Sabah, Malaysia, 88400 Kota Kinabalu, Sabah, Malaysia