The microflora of the bovine rumen under anaerobic conditions rapidly degraded bioflavonoids such as rutin, quercitrin, naringin, and hesperidin to water-soluble products. Phloroglucinol was detected as a transitory intermediate in the fermentation of rutin, quercitrin, and naringin, but not hesperidin. Other unknown phenolic compounds were also produced. Protocatechuic acid was decarboxylated to yield catechol, and catechol was not degraded.