The Escherichia coli transcriptional antiterminator protein BglG inhibits transcription termination of the bgl operon in response to the presence of β-glucosides in the growth medium. BglG is an RNA-binding protein that recognizes a specific sequence partially overlapping the two terminators within the bgl transcript. The activity of BglG is determined by its dimeric state which is modulated by reversible phosphorylation. Thus, only the nonphosphorylated dimer binds to the RNA target site and allows readthrough of transcription. Genetic systems which test dimerization and antitermination in vivo were used to map and delimit the region which mediates BglG dimerization. We show that the last 104 residues of BglG are required for dimerization. Any attempt to shorten this region from the ends or to introduce internal deletions abolished the dimerization capacity of this region. A putative leucine zipper motif is located at the N terminus of this region. The role of the canonical leucines in dimerization was demonstrated by their substitution. Our results also suggest that the carboxy-terminal 70 residues, which follow the leucine zipper, contain another dimerization domain which does not resemble any known dimerization motif. Each of these two regions is necessary but not sufficient for dimerization. The BglG phosphorylation site, His208, resides at the junction of the two putative dimerization domains. Possible mechanisms by which the phosphorylation of BglG controls its dimerization and thus its activity are discussed.