Abstract: In teleost fish, bone morphogenetic protein bmp2b is important for the development of skeletons, including the development of secondary sex characters of fins. Sex steroid hormones are also necessary for the development of those characters. In this study, bmp2b expression levels in fins of adult Japanese medaka (Oryzias latipes) after treatment with sex steroid hormones were examined using semi-quantitative RT-PCR. In males, bmp2b expression levels in the posterior part of anal fins were increased at 6 days after 11-ketotestosterone (11-KT) treatments. In contrast, bmp2b expression levels in the posterior part of anal fins were decreased at 6 days after 17β-estradiol (E2) treatments. The bmp2b expression levels in the pelvic fin of males were substantially increased at 3 and 6 days after E2 treatment. No significant difference in the dorsal, anterior part of the anal, pectoral and caudal fins of male was found among time-related courses after 11-KT and E2 treatments. In females, bmp2b expression levels in the dorsal, anterior and posterior part of anal fins were further increased at 3, 12 and 3 days, respectively after 11-KT treatment. Conversely, there was no difference in bmp2b expression levels of pectoral, pelvic and caudal fins of females among time-related courses after treatment with 11-KT and E2. Present results, therefore, suggest that bmp2b regulation in the fin may be involved in 11-KT and E2-mediated regulation depending on the sex-dependent characters of fin morphology in adult Japanese medaka.