The Pseudomonas aeruginosa LasR protein functions in concert with N-3-oxo-dodecanoyl-l-homoserine lactone (3O-C₁₂-HSL) to coordinate the expression of target genes, including many genes that encode virulence factors, with cell density. We used a LexA-based protein interaction assay to demonstrate that LasR forms multimers only when 3O-C₁₂-HSL is present. A series of LasR molecules containing internal deletions or substitutions in single, conserved amino acid residues indicated that the N-terminal portion of LasR is required for multimerization. Studies performed with these mutant versions of LasR demonstrated that the ability of LasR to multimerize correlates with its ability to function as a transcriptional activator of lasI, a gene known to be tightly regulated by the LasR-3O-C₁₂-HSL regulatory system. A LasR molecule that carries a C-terminal deletion can function as a dominant-negative mutant in P. aeruginosa, as shown by its ability to decrease expression of lasB, another LasR-3O-C₁₂-HSL target gene. Taken together, our data strongly support the hypothesis that LasR functions as a multimer in vivo.