Homologous strand exchange is a central step in general genetic recombination. A multiprotein complex composed of five purified bacteriophage T4 proteins (the products of the uvsX, uvsY, 32, 41, and 59 genes) that mediates strand exchange under physiologically relevant conditions has been reconstituted. One of these proteins, the product of the uvsY gene, is required for homologous pairing but strongly inhibits branch migration catalyzed by UvsX protein, the phage RecA analog. Branch migration is completely dependent on the gene 41 protein, a DNA helicase that also functions in phage replication. The helicase is delivered to the strand exchange complex by the gene 59 accessory protein in a strand-specific fashion through direct interactions between the gene 59 and gene 32 proteins. These data suggest that strand transferases such as UvsX protein are essential for homologous pairing in vivo, but that a DNA helicase drives polar branch migration.