Assays for the enzymes in Aerobacter aerogenes converting chorismate into 2,3-dihydroxybenzoate (isochorismate synthetase, 2,3-dihydro-2,3-dihydroxybenzoate synthetase and 2,3-dihydro-2,3-dihydroxybenzoate dehydrogenase) have been developed. Isochorismate synthetase is stimulated by Mg2+ and 2,3-dihydro-2,3-dihydroxybenzoate dehydrogenase requires NAD for activity. Isochorismate synthetase has been separated from the other two enzymes by column chromatography on DEAE-cellulose. The addition of iron to the culture medium of A. aerogenes represses each of the enzymes. An experiment with a multiple aromatic auxotroph of Escherichia coli indicates that iron alone, not complexed with any 2,3-dihydroxybenzoate derivative, represses the enzymes concerned with 2,3-dihydroxybenzoate synthesis. © 1969.