It is now known that thyroid hormones can be transferred into eggs and larvae, suggestthyroid hormones may play an important role during early larval development of fish.Attempts to the eel fry has extremely heavy mortalities occurring during the 2 or 3 dayfrom hatching and just can servival a few days . The aim of this study is to investigatethe effects of thyroxine on egg hatching and larval development in Japanese eel.