The anal fin in Japanese medaka, Oryzias latipes, is a typical sexual secondary character. In the present study, we focused on this organ and examined the effects of low doses of a natural estrogen, 17β-estradiol (E₂), and an environmental xenoestrogen, bisphenol A (BPA), in vivo by monitoring estrogen receptor (ER) α gene expression. Groups of adult male and female medaka were immersed in 10 ⁻⁹ M E₂ or 10 ⁻¹⁰ to 10^{− 8} M BPA and the levels of ERα gene transcripts in the anal fins were measured by semi-quantitative reverse transcription polymerase chain reaction (RT-PCR). One day of treatment with each concentration of BPA examined and 10 ⁻⁹ M E₂ increased the levels of ERα mRNA in female anal fins by 3-fold as compared with controls. In the male specimens, neither 10 ⁻⁹ M E₂ nor 10 ⁻¹⁰ M BPA showed remarkable effects on the anal fins as compared with the results in females, but 10⁻⁹ and 10 ⁻⁸ M BPA increased the levels of ERα mRNA by 2.3- and 3.3-fold with 1 day of exposure, respectively. The present results showed that medaka anal fins may be a sensitive bio-indicator for screening of environmental estrogenic chemicals.