A nucleoside catalyzing the oxidoreduction of NADH and K3Fe(CN)6 was isolated from Torula yeast RNA and also obtained in 0.05% yield by a series of steps: SDS-phenol extraction, nuclease P1 digestion, alkaline phosphatase digestion, anion exchange chromatography, and HPLC on an ODS column. Its chemical structure was clearly determined at 5-hydroxycytidine, from the results of FAB-MS and 1H and 13C NMR spectroscopies. The mass spectra, chromatographic behavior, UV spectra, and NMR spectra of this nucleoside from natural and synthetic sources were identical. This is the first report of an RNA catalyst having catalytic activity except for the cleavage and ligation of phosphodiester bonds of RNA. That an RNA has oxidoreduction activity indicates new possibilities for RNAs as "living molecules". 5-Hydroxycytidine may be a vestige of RNAs that formerly possessed metabolizing ability.