Phages capable of transducing succinate dehydrogenase mutants (sdh) of Escherichia coli were isolated from pools of artificially constructed recombinant lambda phages using a selective casein digest medium. These phages produced characteristically dense turbid plaques, and as prophages they increased the aerobic growth efficiencies of sdh mutants on complex media but were unable to promote growth with succinate as sole carbon and energy source (an essential feature of sdh + strains). The phages were identified as fumarate reductase transducing phages (lambda frdA) by the presence of a characteristic 4.9 kilobase pairs R.HindIII fragment of bacterial DNA, the expression of a polypeptide with a relative molecular mass of 72,000 (the frdA gene product) and by comparing their transducing activities with authentic lambda frdA phages. In parallel studies a strain containing a ColE1-frd hybrid plasmid (pGS1 = pLC16.43) was characterized. Transfer of pGS1 to sdh mutants was accompanied by increased aerobic growth efficiencies on complex media and the ability to utilize succinate as sole carbon and energy source. It was concluded that fumarate reductase can replace succinate dehydrogenase but the extent of the reversal of the sdh lesion depends on frd gene dosage and the titration of the repressor which normally prevents aerobic synthesis of the reductase.