Regulation of expression of aroH, the structural gene for the tryptophan-sensitive 3-deoxy-D-arabinoheptulosonic acid-7-phosphate synthetase, by the tryptophan repressor and its corepressor, L-tryptophan, was studied in vivo by using aroH-lacZ fusions. Protein and operon fusions were constructed on multicopy plasmids and subsequently crossed in single copy to the bacterial chromosome via the specialized transducing bacteriophage lambda RZ5. Analysis of the resulting lysogens demonstrated that aroH-lacZ expression in a trpR mutant strain varied four- to fivefold relative to an isogenic trpR+ strain under fully repressing conditions. In trpR+ strains containing either fusion, a modest (ca. 50%) change in activity was seen in response to the addition of L-tryptophan to the culture medium. These data demonstrate that aroH gene expression is only moderately regulated by the tryptophan repressor and that this regulation is at the level of transcription. Addition of L-phenylalanine, L-tyrosine, or Casamino Acids (Difco Laboratories, Detroit, Mich.) to the cell culture medium resulted in a tryptophan repressor-dependent derepression of aroH expression. We believe that this effect is caused by L-tryptophan limitation as a result of repression and feedback inhibition of the tyrosine- and phenylalanine-specific 3-deoxy-D-arabinoheptulosonic acid-7-phosphate synthetase isoenzymes. Derepression of aroH expression by the L-tryptophan analogs, 3-beta-indoleacrylic acid and indole-3-propionic acid, is also documented.