The purpose of this study was to determine the effects of bisphenol A (BPA), which is known to have estrogenic activity, on the early development of medaka fish (Oryzias latipes). The fertilized eggs of medaka were treated with BPA at different concentrations for 3 weeks. Embryonic growth, deformation, hatching success, and gonadal differentiation were determined to observe the effects of this chemical. Also we tried to measure the estrogenic activity of bisphenol A using ELISA and RT-PCR methods. By using this techniques, we evaluated the induction of vitellogenin, an estrogen-regulated gene from the whole body-homogenates of larvae. At results, a reduced blood circulation was seen in embryos and peritoneal edema and hindrance of yolk-sac absorption were observed in larvae of treated group. However, BPA at the concentrations tested (2～200 ㎍/L) did not have severe adverse effects on the early life-stages. According to the observation of gonadal histology, inter-sex or sex -reversal was not found in all test fish. After the exposure was ended, vitellogenin mRNA and protein levels were measured in larvae and then their levels were found to be increased in treated group with 200㎍/L. These results indicate that BPA can induce the expression of vitellogenin in early life-stages as well as in adult male fish.