The bacteriophage T4 uvsY protein is required for efficient recombination in T4-infected Escherichia coli cells. Previous in vitro work has shown that the purified uvsY protein is an accessory protein; it stimulates homologous pairing catalyzed by the phage uvsX protein (a RecA-like recombinase) under certain conditions. We show here that this effect can be traced, at least in part, to a UvsY-dependent stabilization of uvsX protein-single-stranded DNA complexes. These presynaptic filaments are one of the early obligatory intermediates in the strand exchange reaction between homologous single- and double-stranded DNAs. The mechanism of filament stabilization seems to involve a slower loss of UvsX subunits. A model that accounts for the data is presented in which both recombination proteins are incorporated into the presynaptic filament.