en fish disease fish disease پژوهشي Orginal research papers This study aimed to identify the chemical composition of essential oil of some Iranian medicinal plants and their antifungal activity against <em>Saprolegnia parasitica</em> in comparison to formalin as positive control under <em>in vitro</em> conditions. The essential oils of <em>Eryngium campestr</em><em>e, </em><em>Pimpinella affinis, </em><em>Mentha piperita, Achillea wilhelmsii </em>and <em>Cuminum cyminum </em>were analyzed for their activity by disk diffusion, Minimum Inhibitory Concentration (MIC) and Minimum Fungicidal Concentration (MFC) methods. Also, the oil constituents of investigated plants were analyzed by gas chromatography/mass spectrometry (GC/MS). The major constituent of the surveyed essential oils was bornyl acetate (17.9%) in <em>E. campestre</em>, Pregeijene (27.3%) in <em>P. affinis</em>, menthol (48.5%) in <em>M. piperita,</em> 1,8-cineol (25.2%) in <em>A. wilhelmsii,</em> and &alpha;-Pinene (29.1%) in <em>C. cyminum</em>. The MIC values for the surveyed essential oil were was 0.5 &micro;g ml^-1 for <em>C. cyminum</em> followed by <em>M piperita</em> and <em>E. campestre</em> both having 1 &micro;g ml^-1, for <em>P. affinis</em> 2 &micro;g ml^-1 and was 4 &micro;g/mL for <em>A. wilhelmsii</em>. The MFC for the mentioned essential oil were with 0.5 &micro;g ml^-1 again lowest for <em>C. cyminum</em>, followed by <em>M. piperita</em> and <em>E. campestre</em> with 2 &micro;g ml^-1, for <em>P. affinis</em> MFC was 4 &micro;g ml^-1 while it was with 8 &micro;g ml^-1 highest for essential oils from <em>A. wilhelmsii</em>. The results indicate that the essential oils of <em>C. cyminun</em>, <em>E. campestre</em> and <em>M. piperita </em>could be potential candidates for new plant based antifungal components in aquaculture against <em>S. parasitica</em><em>.&nbsp; </em> Iranian medicinal plants, Chemical composition, Antifungal activity, Saprolegnia parasitica 3239 3254 http://jifro.ir/browse.php?a_code=A-10-335-7&slc_lang=en&sid=1 M. Adel miladadel85@yahoo.com 100319475328460038410 100319475328460038410 Yes Department of Aquatic animal Health & Disease, Iranian Fisheries Sciences Research Institute (IFSRI), Agricultural Research Education and Extension Organization (AREEO), P.O. Box: 13185-116, Tehran, Iran M. Dadar dadar.m77@gmail.com 100319475328460038411 100319475328460038411 No Center of Biotechnology and Biology Research, Shahid Chamran University, Ahvaz, Iran M.J. Zorriehzahra zorrieh@yahoo.com 100319475328460038412 100319475328460038412 No Department of Aquatic animal Health & Disease, Iranian Fisheries Sciences Research Institute (IFSRI), Agricultural Research Education and Extension Organization (AREEO), P.O. Box: 13185-116, Tehran, Iran. R. Elahi Razea.elahi@yahoo.com 100319475328460038413 100319475328460038413 No Department of Aquatic animal Health and Diseases, School of Veterinary Medicine, Shahrekord University, P.O. Box 115, Shahrekod, Iran T. Stadtlander 100319475328460038414 100319475328460038414 No Research Institute for Organic Agriculture, Frick, Switzerland