The independent control of regulatory isoenzymes by different metabolites constitutes one well-known pattern of control in branched metabolic pathways. This pattern was previously found to be widely distributed in the aromatic amino acid pathway of microorganisms in the case of the first enzyme of the sequence, 3-deoxy-d-arabino-heptulosonate 7-phosphate (DAHP) synthetase. The comparative stability of the isoenzymes as well as the effect of aromatic amino acids in the growth medium upon the levels of the individual isoenzymes were shown for Salmonella typhimurium. Several lines of evidence are discussed to demonstrate the strong reliance of Escherichia coli upon the phenylalanine-sensitive isoenzyme for the ordinary biosynthetic needs of wild-type strains. The frequent occurrence of "dominant" isoenzyme species which resist repressive effects of the inhibitory end products was noted. The lack of an obligatory correlation of the level of an isoenzyme activity and the synthesis of the end product which specifically controls its activity is used to discount the possibility that each isoenzyme might feed a unique and separate metabolic pool of end-product precursor. An isoenzymic DAHP synthetase sensitive to feedback inhibition by low levels of tryptophan was fractionated from tyrosine- and phenylalanine-sensitive isoenzymes in cell-free extracts of Neurospora crassa.