By using the 'lacZ gene, the activities of the nirI, nirS, and norC promoters were assayed in the wild type and in NNR-deficient mutants of Paracoccus denitrificans grown under various growth conditions. In addition, induction profiles of the three promoters in response to the presence of various nitrogenous oxides were determined. Transcription from the three promoters required the absence of oxygen and the presence both of the transcriptional activator NNR and of nitric oxide. The activity of the nnr promoter itself was halved after the cells had been switched from aerobic respiration to denitrification. This response was apparently not a result of autoregulation or of regulation by FnrP, since the nnr promoter was as active in the wild-type strain as it was in NNR- or FnrP-deficient mutants.