Two types of fumarate reductase transducing phages, lambda frdA, carrying the wild-type frdA gene but differing in the orientation of a R.HindIII fragment of bacterial DNA were isolated from populations of recombinant transducing phages by their ability to complement the lesions of frdA mutants of E. coli. In lysogens, the cloned frdA gene was controlled by its own promoter and was fully responsive to normal regulatory stimuli. The lambda frdA phages would not complement the defects of succinate dehydrogenase (sdh) mutants. Genetic studies showed that the R.HindIII fragment contains ampA, the cis-acting regulatory locus for the chromosomal beta-lactamase gene ampC. No evidence for the presence of other markers was detected but the bacterial segment could be extended to produce plaque-forming phage derivatives containing the amp operon and a gene concerned with bacteriophage morphogenesis, groE(mop). A physical map of the 4.9 kb R.HindIII fragment was constructed by restriction analysis and flanking fragments were identified by DNA:DNA hybridization analysis. The frdA region contained a single asymmetric R.EcoRI target 3.33 kb from one end and the orientation of the physical map with respect to the E.coli linkage map was established.