en Biology & physiology Biology & physiology پژوهشي Orginal research papers The growth of the gastrointestinal tract of the larvae and juvenile of <em>Alburnus chalcoides</em> was studied after histological analysis using light microscopy, haematoxylin-eosin staining and the mouth development was scanned via electron microscopy. This study focuses on the morphology and histology of the mouth growth and digestive tract of <em>A. chalcoides</em> larvae to test the best weaning time for providing practical diet for fry based on the grade of their morphological aspects. It was observed that on the fifth day after hatching, the larvae mouth was opened. On the eighth day, the yolk sac was absorbed by two-thirds. On the same day, food is fed manually. Yolk sac was completely absorbed in 10th day. The histological base of the esophagus was formed by day 3. At day 3, the formation of enterocytes started. Also, the larvae hepatopancreas was formed on 5th day. According to the results, it was observed that after 8 days onwards, larvae of these fish can have a proper diet. At this day, the size of the mouth was 84 &micro;m. when the lips were formed within 20 days and for the mouth angles 45&ordm;and 90&ordm;, the food size for mouth, was 168 and 307 &mu;m, respectively. Alburnus chalcoides, Ontogenetic, Digestive system, Larvae, Juvenile 2159 2172 http://jifro.ir/browse.php?a_code=A-10-2610-1&slc_lang=en&sid=1 M. Zakeri Nasab m.zakeri_mzn@yahoo.com 100319475328460035761 100319475328460035761 No Department of Marine Biology, Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran Sh. Jamili shahlajamili45@yahoo.com 100319475328460035762 100319475328460035762 Yes Iranian Fisheries Sciences ResearchInstitute, Agricultural Research,Education and Extension Organization, Tehran, Iran A.R. Valipour valipour40@gmail.com 100319475328460035763 100319475328460035763 No Inland Waters Aquaculture Research Center, Iranian Fisheries SciencesResearch Institute, Agricultural Research,Education and Extension Organization (AREEO), Bandar Anzali, Iran S.M.R. Fatemi reza_fatemi@hotmail.com 100319475328460035764 100319475328460035764 No Department of Marine Biology, Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran E. Ramezani Fard eramezanifard@gmail.com 100319475328460035765 100319475328460035765 No Department of Marine Biology, Faculty of Natural Resources and Environment, Science and Research Branch, Islamic Azad University, Tehran, Iran