The control of the synthesis of certain key enzymes of aromatic amino acid biosynthesis was studied. Tyrosine represses the first enzyme of the 3-deoxy-d-arabino heptulosonic acid 7-phosphate pathway, DAHP synthetase, as well as shikimate kinase and chorismate mutase about fivefold in cultures grown under conditions limiting the synthesis of the aromatic amino acids. A mixture of tyrosine and phenylalanine represses twofold further. Tryptophan does not appear to be involved in the control of these enzymes. The specific activity of at least one early enzyme, dehydroquinase, remains essentially constant under a variety of nutritional supplementations. Two enzymes in the terminal branches are repressed by the amino acids they help to synthesize: prephenate dehydrogenase can be repressed fourfold by tyrosine, and anthranilate synthetase can be repressed over 200-fold by tryptophan. There is no evidence that phenylalanine represses prephenate dehydratase. Regulatory mutants have been isolated in which various enzymes of the pathway are no longer repressible. One class is derepressed for several of the prechorismate enzymes, as well as chorismate mutase and prephenate dehydrogenase. In another mutant, several enzymes of tryptophan biosynthesis are no longer repressible. Thus, the rate of synthesis of enzymes at every stage of the pathway is under control of various aromatic amino acids. Tyrosine and phenylalanine control the synthesis of enzymes involved in the synthesis of the three aromatic amino acids. Each terminal branch is under the control of its end product.