This chapter discusses the aerobic respiratory pathway of Escherichia coli and its energy transducing function. E. coli is a facultative organism capable of growth on a wide variety of substrates. Under anaerobic conditions, fermentative growth is supported by substrates such as glucose, galactose, and maltose. Several respiratory-chain-linked dehydrogenases have been purified from the membranes of E. coli. The adenosine triphosphate-dependent reaction has been demonstrated in E. coli membrane preparations. Energy is supplied to the energy-dependent processes as an “energized state” of the membrane. Both ubiquinone and menaquinone are found in membranes of E. coli. Nonheme iron is a major component of the respiratory pathway, being at a 10-fold higher level than the other respiratory chain components with the exception of ubiquinone. The total amount of quinone is generally in great excess over the other respiratory chain components with the exception of nonheme iron.