In the absence of oxygen, Escherichia coli can use alternative exogenous electron acceptors, including trimethylamine oxide (TMAO), to generate energy. In this study, we showed that in contrast to the other anaerobic respiratory systems, the TMAO reductase (Tor) system was expressed during both aerobiosis and anaerobiosis. By using a torA-lacZ fusion and quantitative reverse transcription polymerase chain reaction, we established that the torCAD operon encoding the Tor system was induced in the presence of TMAO mainly during exponential phase, and that optimal induction required a certain level of DNA supercoiling. We also showed that the presence of oxygen prevented neither the biogenesis of the Tor system nor the reduction of TMAO. The physiological role of TMAO reduction during aerobiosis has not been yet established, but our experiments suggest that alkaline TMA production could enhance the growth conditions by increasing the pH of the culture.