en Biochemistry Biochemistry پژوهشي Orginal research papers Streptococcosis is an infectious disease with significant economic and sanitary repercussions for trout farms in the world. The objective of this study was characterize the chemical constituents and <em>&nbsp;in vitro</em> antibacterial properties of <em>Mentha piperita</em> L., <em>Satureja khuzistanica</em> Jamzad, <em>Matricaria recutica</em> L., <em>Zataria multiflora</em> Boiss and <em>Rosmarinus officinalis</em> L. extracts against <em>streptococcus iniae.</em> The chemical components of the selected hydroalcoholic extracts were also analyzed by GC/MS. The most constituents were menthol (24%) in the <em>M. piperita</em> L., carvacrol (85.88%) in the <em>S. khuzistanica</em> Jamzad, guaiazulene (25.6%) in the <em>M. recutica</em> L. carvacrol (51.12%) in the <em>Z. multiflora </em>Boiss and &alpha;<em>-</em>pinene (12.5%) in the<em> R. officinalis</em> L. Among plant extracts under study <em>Z. multiflora</em> had the lowest MIC (4.896 mg ml^-1) and MBC (9.792 mg ml^-1) and the maximum MIC and MBC were belonged to <em>M. piperita</em> (18.55 mg ml^-1) and <em>R. officinalis</em> (33.645 mg ml^-1) respectively. Also, the inhibition zone diameter of bacteria had determined by disc diffusion method and compared to erythromycin. The highest and the lowest Inhibition zone diameter were belonged to<em> Z. multiflora </em>(14.43&plusmn;0.55 mm) and <em>M. recutita</em> (13.23&plusmn;0.35 mm) respectively. Results showed that <em>Z. multiflora</em> extract exhibited highest antibacterial effect against <em>S. iniae </em>among all plant extracts in this study. Herbal extracts, Antistreptococcal property, Minimum inhibitory concentration, Minimum bactericidal concentration 1373 1383 http://jifro.ir/browse.php?a_code=A-10-2570-1&slc_lang=en&sid=1 A.A. Tafi aliakbartafi@gmail.com 100319475328460034092 100319475328460034092 Yes Department of Fisheries, Faculty of Natural Resources, Urmia University S. Meshkini s.meshkiniy@gmail.com 100319475328460034093 100319475328460034093 No Department of Food Hygiene and Quality Control, Faculty of Veterinary Medicine, Urmia University A. Tukmechi atokmachi@gmail.com 100319475328460034094 100319475328460034094 No Department of Microbiology, Faculty of Veterinary Medicine, Urmia University M. Alishahi alishahimoj@gmail.com 100319475328460034095 100319475328460034095 No Department of Clinical Science, Faculty of Veterinary Medicine, Shahid Chamran University F. Noori Yalda.45@gmail.com 100319475328460034096 100319475328460034096 No Department of Aquaculture, Artemia & Aquaculture Research Institute