Incubation of quercetin with Streptomyces rimosus subsp. rimosus ATCC 10970 yielded an unusual glycosylated derivative. The structure of the product was determined to be quercetin-7-O-β-4″-deoxy-hex-4″-enopyranosiduronic acid based on the spectral data. Quercetin was completely converted into the glycoside in 72 h.