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-beta-4 ''-deoxy-hex-4 ''-enopyranosiduronic acid based on the spectral data. Quercetin was completely converted into the glycoside in 72 h. (C) 2012, The Society for Biotechnology, Japan. All rights reserved.