In the genus Oryzias, the morphologies of the dorsal and anal fins are typical secondary sex characters. In the Japanese medaka (Oryzias latipes) and Thai medaka (Oryzias minutillus), androgen receptor (AR) expression levels in the dorsal, anal, and pectoral fins were higher in males than in females. Conversely, in both species estrogen receptor (ER) β expression levels in the dorsal and anal fins were higher in females than in males. AR and ERβ expression levels in the dorsal and anal fins of sex-undeterminable individuals of Thai medaka were intermediate between those in normal male and female Thai medaka. There was no difference in the bone morphogenic protein (Bmp) 2b expression level between male and female Japanese medaka. In contrast, the Bmp2b expression level in the dorsal fin of sex-undeterminable individuals was lower than in normal male and female Thai medaka. It is thus clear that androgen and estrogen regulate the sex-dependent characters of fin morphology in both Oryzias species. In sex-undeterminable individuals of Thai medaka, the low levels of Bmp2b expression in the dorsal fin are evidence that androgen and estrogen are necessary for adequate expression of Bmp2b in the normal development of at least the dorsal fin.