Streptomyces antibioticuspossesses a tryptophan-inhibitable 3-deoxy-D-arabino-heptulosonic acid 7-phosphate (DAHP) synthetase whose synthesis is also repressed byL-tryptophan. Studies of the DAHP synthetase obtained by ammonium sulfate fractionation of a crude extract derived fromS.antibioticusrevealed that the enzymic activity was only partially inhibited by tryptophan. Inhibition of the DAHP synthetase activity was strongly pH dependent at values below 7.0. A number of tryptophan analogues was noted to inhibit the enzyme; by contrast, other aromatic amino acid end products failed to affect DAHP synthetase activity. Chorismic acid, a key intermediate in aromatic amino acid biosynthesis, was ineffective as an inhibitor when used alone; however, if supplied withL-tryptophan, a further reduction of DAHP synthetase activity (15–25%) was routinely observed.