We demonstrated that a double-stranded (ds) gap in DNA is repaired by a gene conversion mechanism in an Escherichia coli recBC sbcA23 strain, as predicted by the ds break repair models for homologous recombination. The sbcA mutation is known to induce several gene products encoded on the Rac prophage present in most strains of E. coli K-12. These include exonuclease VIII (Exo VIII), a 5' to 3' exonuclease working from the end of a duplex DNA, and RecT, an annealing protein. We found that a rac- strain (lacking the Rac prophage) cannot support this repair. A plasmid carrying part of the Rac prophage supported highly efficient ds gap repair activity in a rac- strain, but two ExoVIII+ recT- plasmids did not. The recE159 mutation that blocks ds gap repair was found to be recT+, since these ExoVIII+ recT- plasmids complemented the recE159 mutation in repair of ultraviolet light damage. From these observations, we conclude that both ExoVIII and RecT are essential for ds gap repair. We discuss their possible roles in the ds break repair reaction.