The primary sequences were compared among several proteins: gene product 5 protein (GP5) from phage M13; PIKE from phage Ike; gene product 32 protein (GP32) from phage T4; RecA, SSB and SSF from Escherichia coli. These proteins bind strongly and cooperatively to single-stranded DNA with no sequence specificity. GP5 is the smallest in this group and its three-dimensional structure is well-characterized. Using the entire sequence of GP5 as a template we searched for the regions in other single-stranded DNA binding proteins yielding the best alignment of aromatic and basic residues. The identified domains show alignment of five aromatic and four charged residues in these proteins. The domains in PIKE, GP32 and RecA exhibit statistically significant sequence homology with GP5. These observations strongly favor the hypothesis that the protein-single-stranded DNA complex in this class of proteins is stabilized by the stacking interaction of the aromatic residues with the bases of the DNA, and by the electrostatic interaction of the basic residues with the phosphate groups of the DNA. We also find that the DNA binding domains of these proteins have similar secondary structural preferences, mainly beta structures. The triple-stranded beta-sheet may be a common motif in the DNA binding domains of these proteins.