THE single-stranded-DNA-binding proteins (SSBs) are essential fur DNA function in prokaryotic and eukaryotic cells, mitochondria, phages and viruses(1,2). The structures of four SSBs have been solved(3-7), but the molecular details of the interaction of SSBs with DNA remain speculative. We report here the crystal structure at 2.4 Angstrom resolution of the single-stranded-DNA-binding domain of human replication protein A (RPA) bound to DNA. Replication protein a is a heterotrimeric SSB that is highly conserved in eukaryotes. The largest subunit, RPA70, binds to single-stranded (ss)DNA(8,9) and mediates interactions with many cellular and viral proteins(10). The DNA-binding domain, which ties in the middle of RPA70, comprises two structurally homologous subdomains oriented in tandem. The ssDNA lies in a channel that extends from one subdomain lo the other. The structure of each RPA70 subdomain is similar to those of the bacteriophage SSBs, indicating that the mechanism of ssDNA-binding is conserved.