We constructed a trpR-lacZ gene fusion that specifies a hybrid protein that has full beta-galactosidase activity. The gene fusion was associated with the unaltered trpR transcription and translation control region; thus, hybrid beta-galactosidase production was an indicator of expression of the trp aporepressor (trpR) operon. To facilitate in vivo expression studies, a DNA segment containing the trpR-lacZ gene fusion and the trpR controlling region was transferred to bacteriophage lambda and subsequently inserted into the bacterial chromosome. Analyses of hybrid beta-galactosidase production showed that the trpR operon is regulated autogenously but that the rate of synthesis of aporepressor varies only 4- to 5-fold in response to changes in the intracellular concentration of tryptophan. Under comparable conditions, the trp operon is regulated by trp repressor approximately 70-fold. Therefore, the operators of the trp operon and the trpR operon must have very different affinities for trp repressor in vivo. The promoter controlling trpR expression was found to be moderately active. Nevertheless, there are only about 50-300 molecules of trp aporepressor per cell. The low aporepressor level appears to be due to inefficient translation of trpR mRNA.