Effects of nonylphenol (NP) on gonadal development and growth of olive flounder, Paralichthys olivaceus were investigated. NP treatment was carried out to fry fish (during 55 to 64 days after hatching) using oral adminstration at nominal NP concentrations of 50, 100 and 150 tig/g BW Gonad before NP treatment was sexually undifferentiated as observed with mostly gonia cells. At 159 days after hatching, ovarian lamella of ovary were filled with oogonia and perinucleolus oocytes. On the other hand, testicular lobules of the testis were occupied by spermatogonia, spermatocytes and spermatids. Histological observation of ovary and testis development was any different between the control and NP treated groups. However, growth was significantly increased in NP treated groups than control groups (P< 0.05). These results considerate that NP has any effect for sex differentiation and gonadal development, but act for early growth in olive flounder.