Succinate dehydrogenase (SDH) of Escherichia coli, the sole membrane-bound enzyme of the tricarboxylic acid cycle, participates in the aerobic electron-transport pathway to generate energy via oxidative phosphorylation reactions. Previous studies have established that succinate dehydrogenase (SDH) synthesis is elevated by aerobiosis and suppressed during growth with glucose. To examine how the sdhCDAB genes that encode SDH are regulated by changes in the environment, sdh-lacZ fusions were constructed and analysed in vivo following cell growth under a variety of alternative culture conditions. Expression of sdh-lacZ was highest under aerobic conditions and was decreased 10-fold in the absence of oxygen. The fnr and arcA gene products are required for this oxygen control and each acts to repress sdhC-lacZ expression. Expression of sdh-lacZ also varied 10- to 14-fold depending on the type of carbon substrate used or the medium richness. This control was shown to be independent of the crp and fruR gene products, and indicates that some other regulatory element exists in the cell to adjust SDH enzyme levels accordingly. Iron and haem availability affected sdhC-lacZ expression by two- to three-fold. Lastly, sdhC-lacZ expression was shown to vary with the cell growth rate during aerobic and anaerobic conditions.