The F0F1 proton-translocating ATPase complex of Escherichia coli, encoded by the atpIBEFHAGDC operon, catalyzes the synthesis of ATP from ADP and P1 during aerobic and anaerobic growth when respiratory substrates are present. It can also catalyze the reverse reaction to hydrolyze ATP during nonrespiratory conditions (i.e., during fermentation of simple sugars) in order to maintain a electrochemical proton gradient across the cytoplasmic membrane. To examine how the atp genes are expressed under different conditions of cell culture, atpI-lacZ operon fusions were constructed and analyzed in single copy on the bacterial chromosome or on low-copy-number plasmids. Expression varied over a relatively narrow range (about threefold) regardless of the complexity of the cell growth medium, the availability of different electron acceptors or carbon compounds, or the pH of the culture medium. In contrast to prior proposals, atp operon expression was shown to occur from a single promoter located immediately before atpI rather than from within it. The results of continuous-culture experiments suggest that the cell growth rate rather than the type of carbon compound used for growth is the major variable in controlling atp gene expression. Together, these studies establish that synthesis of the F0F1 ATPase is not greatly varied by modulating atp operon transcription.