The biliary bile salts of the medaka, the Japanese rice fish (Oryzias latipes) were isolated and identified. Only bile acids were present, and all were N-acylamidated with taurine. Three bile acids, constituting 98% of total bile acids, were isolated by chromatography and their structure inferred from their properties compared to those of synthetic standards when analyzed by liquid chromatographytandem mass spectrometry. The dominant bile acid was the 25R-epimer (82%) of 3α,7α,12α-trihydroxy-5β-cholestan-27-oic acid. The 25S-epimer was also present (11%), as was cholic acid (5%). Complete ¹H and ¹³C NMR signal assignments of the C-25 epimers were made by using a combination of several 1D- and 2D-NMR techniques. The ¹H and ¹³C NMR chemical shifts and spectral patterns of the hydrogen and carbon atoms, being close to the asymmetric centered at C-25, provided confirmatory evidence in that they distinguished the two epimeric diastereomers. The medaka is the first fish species identified as having C₂₇ biliary bile acids as dominant among its major bile salts.