The single-stranded DNA-binding protein (SSB) of phage GA-1 displays higher efficiency than the SSBs of the related phages ø29 and Nf. In this work, the self-interaction ability of GA-1 SSB has been analyzed by visualization of the purified protein by electron microscopy, glycerol gradient sedimentation and in vivo crosslinking of bacterial cultures infected with phage GA-1. GA-1 SSB contains an insert at its N-terminal region that is not present in the SSBs of ø29 and Nf. Three deletion mutant proteins have been characterized, DN19, DN26 and DN33, that lack the 19, 26 or 33 amino acids, respectively, that follow the initial methionine of GA-1 SSB. Mutant protein DN19 retains the structural and functional behavior of GA-1 SSB, while mutant proteins DN26 and DN33 no longer stimulate viral DNA replication nor display helix-destabilizing activity. Analysis of the mutant proteins by ultracentrifugation in glycerol gradients and electron microscopy indicates that deletion of 26 or 33 but not of 19 amino acids of the N-terminal region of GA-1 SSB results in the loss of the oligomerization ability of this protein. Our data support the importance of the N-terminal region of GA-1 SSB for the differential self-interaction ability and functional behavior of this protein.