1. Flavone and isoflavone were subjected to microbial transformation studies. Screening with 80 fungal species showed that flavone was transformed by a large number of microorganisms and in higher yields than isoflavone. 2. Large-scale fermentations of flavone followed by isolation and characterization of metabolites using spectroscopic analysis showed the formation of 4'-hydroxyflavone, 3',4'-dihydroxyflavone and two cleavage products identified as o-hydroxyphenyl-hydroxymethyl ketone and 1-(o-hydroxyphenyl)-1,2-ethanediol. However, fermentation of isoflavone gave only 4'-hydroxyisoflavone and 3',4'-dihydroxyisoflavone.