An unknown flavin compound was found in the culture solution of Rhizopus oryzae containing riboflavinylglucoside. This flavin was suggested to be a new type of nucleotide, namely a riboflavinylglucoside mononucleotide, because it was revealed chemically and enzymatically that the compound consisted of a riboflavinylglucoside of which ribityl radical was linked with a phosphate.