The cytochrome d complex is one of the two terminal oxidases in the aerobic respiratory system of Escherichia coli. This enzyme is not present in cells grown with high levels of dissolved oxygen in the culture medium but accumulates after mid-exponential growth, reaching high levels in stationary-phase cells. In this study, the transcriptional activity of the cyd operon, encoding the two subunits of the enzyme, was examined under a variety of growth conditions. This was accomplished by the use of a chromosomal operon fusion, cyd-lacZ, generated in vivo by a lambda plac-Mu hopper bacteriophage and also by the use of a cyd-lacZ protein fusion created in vitro on a plasmid, transferred onto a lambda transducing phage, and examined as a single-copy lysogen. Transcription of the gene fusions was monitored by determination of beta-galactosidase activity. The data clearly show that cyd is transcriptionally regulated and that induction is observed when the culture reaches a sufficient cell density so as to substantially reduce the steady-state levels of dissolved oxygen. The transcriptional activity is also regulated by other growth conditions, including the carbon source. The turn-on of cyd under semianaerobic conditions does not require the fnr gene product, cyclic AMP, or the cyclic AMP-binding protein.