en fish disease fish disease پژوهشي Orginal research papers The economic losses due to fungal contamination in development of sturgeon and the side effects of malachite green in the environment are very important. Therefore, it is essential to investigate and introduce suitable natural substances instead of malachite green. The aim of this study was to compare antifungal effect of <em>Thymus vulgaris </em>with malachite green on fungi isolated from Siberian sturgeon eggs obtained from International Sturgeon Research Institute. The samples were inoculated in culture media (SDA) and Glucose-Peptone Agar at 25&deg;C for 5-7 days and then isolated fungi were identified. The antifungal effect of <em>T. vulgaris </em>extract was evaluated by disc diffusion method and MIC and MFC of fungi was measured. In this study, <em>Saprolegnia </em>spp<em>.</em><em>, Fusarium </em>spp<em>.</em><em>, Rhodotorula </em>spp<em>.</em><em>, Penicillium </em>spp<em>.</em><em>, Trichoderma </em>spp<em>.</em><em>, Chrysosporium </em>spp<em>.</em> and sterile hyphae were isolated and identified from fertilized eggs of <em>A. baerii</em>. According to the results, <em>T. vulgaris </em>extract had a significant effect on all isolated fungal species. The most effective sample was observed on <em>Saprolegnia </em>and the least effective ones were <em>Fusarium </em>and <em>Trichoderma.</em> The MIC and MFC of alcoholic extract for <em>Saprolegnia </em>spp<em>., Fusarium </em>spp.<em>, Rhodotorula </em>spp<em>.</em><em>, Penicillium </em>spp<em>.</em><em>, Trichoderma </em>spp<em>.</em><em>, Chrysosporium </em>spp<em>.</em>, sterile hyphae were &ge;0.75, 3, 1.5, 1.5, 0.75, 1.5, &le;0.75 mg/ml, and &ge;1.5, &ge;3, 3, 3, 1.5, 3 and 1.5 mg/ml, respectively. Due to high frequency of <em>Saprolegnia </em>spp<em>. </em>in <em>A. baerii </em>eggs, and the significant effect of alcoholic extract of <em>T. vulgaris</em>, it could be used as a good alternative for malachite green to reduce the frequency of fungal contamination and its economic losses during reproductive seasons of <em>A. baerii</em>. Acipenser baerii, Thymus vulgaris, Antifungal effects, Malachite green 218 229 http://jifro.ir/browse.php?a_code=A-10-3482-1&slc_lang=en&sid=1 M. Shahrani masood.shahr@yahoo.com 100319475328460039214 100319475328460039214 No Health and Aquatic Diseases Department, Science and research Branch, Islamic Azad University, Tehran, Iran G. Azari Takami takami85@hotmail.com 100319475328460039215 100319475328460039215 Yes Professor of Health and Aquatic Diseases, science and research Branch, Islamic Azad University, Tehran, Iran M. Sharif Rohani mostafasharif@yahoo.com 100319475328460039216 100319475328460039216 No Professor of Aquatic Diseases, National Institute of Fisheries Research, Agricultural Research and Training Organization, Tehran, Iran A. Motallebi 100319475328460039217 100319475328460039217 No Science and Research Branch, Islamic Azad University, Tehran, Iran M. Yazdani Sadati 100319475328460039218 100319475328460039218 No Caspian Sea International Sturgeon Research Institute, Agricultural Research Education and Extension Organization (AREEO), Rasht, Iran