The gram-negative Xanthomonas campestris pv. campestris is the pathogenic bacterium that causes black rot disease in crucifers. The virulence determinants of this bacterium include extracellular enzymes, exopolysaccharides, and biofilm formation. Here, one transposon mutant of X. campestris pv. campestris strain 17 that affects biofilm formation was isolated, and subsequent analyses led to the identification of the lolA gene, which encodes an outer membrane lipoprotein chaperone.