RecA protein promotes homologous pairing by a reaction in which the protein first binds stoichiometrically to single-stranded DNA in a slow presynaptic step, and then conjoins single-stranded and duplex DNA, thereby forming a ternary complex. RecA protein did not pair molecules that shared only 30 bp homology, but, with full efficiency, it paired circular single-stranded and linear duplex molecules in which homology was limited to 151 bp at one end of the duplex DNA. The initial rate of the pairing reaction was directly related to the length of the heterologous part of the duplex DNA, which we varied from 0 to 3060 base pairs. Since interactions involving the heterologous part of a molecule speed the location of a small homologous region, we conclude that RecA protein promotes homologous alignment by a processive mechanism involving relative motion of conjoined molecules within the ternary complex.