The recA protein of Escherichia coli promotes the complete exchange of strands between full-length linear duplex and single-stranded circular DNA molecules of bacteriophage phi X-174, converting more than 50% of the single-stranded DNA into heteroduplex replicative form II-like structures. Kinetically, the reaction can be divided into two phases, formation of short heteroduplex regions (D loops) and extension of the D loops via branch migration. recA protein participates directly in both phases. D loops are formed efficiently in the presence of ATP or the nonhydrolyzable ATP analog adenosine 5'-[gamma-thio]triphosphate, whereas D-loop extension requires continuous ATP hydrolysis. Complete strand exchange requires a stoichiometric amount of recA protein and is strongly stimulated by the single-stranded-DNA-binding protein of E. coli.