Using the Casadaban Mu d1 phage (Casadaban and Cohen 1979) we fused cis-acting regulatory sites for the Salmonella typhimurium glnA gene, the structural gene encoding glutamine synthetase, to lacZ so that transcription of lacZ was controlled by the glnA promoter-operator. Activities of beta-galactosidase in two glnA::Mu d1 fusion strains were high, approximately 25% and 125% the induced level of beta-galactosidase when transcription of lacZ is under control of the lac promoter, indicating that glutamine synthetase is not required to activate transcription of its own structural gene. Introduction of nitrogen regulatory mutations ntrA::Tn10 or ntrC::Tn10 into fusion strains resulted in greatly decreased synthesis of beta-galactosidase indicating that the positive regulatory factors encoded by ntrA and ntrC activate glnA expression at the level of transcription. Comparison of beta-galactosidase activities in fusion strains with those in fusions carrying ntrC or ntrA mutations indicated that: 1) the magnitude of activation of glnA expression is at least 43-fold; 2) the magnitude of repression is approximately 13-fold and repression occurs at the level of transcription; 3) the degree of modulation of glnA expression by ntr products is at least 560-fold (13 X 43); and 4) glutamine synthetase is not required for repression of transcription of its own structural gene. In contrast to strains carrying non-polar mutations in glnA, strains carrying glnA insertion mutations, including glnA::Mu d1 fusions, are apparently defective in activating expression of some nitrogen controlled genes other than glnA. Defects cannot be accounted for by the absence of glutamine synthetase protein or catalytic activity; they appear to be due to decreased expression of nitrogen regulatory genes ntrB and/or ntrC, which are adjacent to glnA.