Chorismate mutase of Brevibacterium flavum, a common enzyme in phenylalanine and tyrosine biosynthesis, was separted into two different component, A and B, with molecular weights of 250,000 and 25,000, respectively, by ammonium sulfate fractionation or gel-filtration. Both components were essential for the enzymatic activity. In the presence of the reaction substrate, chorismate, the two components associated reversibly to give an active enzyme complex with a molecular weight of 320,000. Binding sites of the feedback inhibitors, phenylalanine and tyrosine, on the enzyme were localized on component A as determined by hybridization experiments with the wild-type and mutant components. Tyrosine repressed the synthesis of component B much more strongly than that of component A, while phenylalanine did not show any significant repressive effect on either component. The wild-type strain No. 2247 had four times more component A than component B. Elution patterns in gel, DEAE-cellulose or hydroxyapatite column chromatography as well as the disc-gel electrophoretic pattern of chorismate mutase component A and 3-deoxy-D-arabino-heptulosonate 7-phosphate (DAHP) synthetase activities completely overlapped, suggesting the presence of a bifunctional protein having the two activities. In accord with this suggestion, chorismate mutase as well as DAHP synthetase was insensitive to feedback inhibition by phenylalanine and tyrosine in all the 3-fluorophenylalanine-resistant mutants tested that excreted both phenylalanine and tyrosine. All the phenylalanine and tyrosine double auxotrophs defective in chorismate mutase lacked component B but not A.