In Escherichia coli, several terminal reductases catalyze the reduction of S- and N-oxide compounds. We have used mutants missing either the constitutive dimethylsulfoxide (DMSO) reductase, dmsABC, and/or the inducible trimethylamine N-oxide (TMAO) reductase, torA, to define the roles of each reductase. These studies indicated that the constitutive DMSO reductase can sustain growth on DMSO, TMAO, methionine sulfoxide (MetSO), and other N-oxide compounds. Only one inducible TMAO reductase is expressed in E. coli, and this enzyme sustains growth on TMAO but not DMSO or MetSO. Characterization of a torA-, dms- double mutant revealed that adenosine N-oxide (ANO) reductase is specifically required for anaerobic respiration on ANO in this mutant.