The rpf gene cluster of Xanthomonas campestris pathovar campestris (Xcc) is required for the pathogenesis of this bacterium to plants. Several rpf genes are involved in the coordinate positive regulation of the production of virulence factors mediated by the small diffusible molecule DSF (for diffusible signal factor). RpfF directs the synthesis of DSF, and a two-component sensory transduction system comprising RpfC and RpfG has been implicated in the perception of the DSF signal and signal transduction. In L medium, rpfF, rpfG, rpfC, and rpfGHC mutants grew as matrixenclosed aggregates, whereas the wild type grew in a dispersed planktonic fashion. Synthesis of the extracellular polysaccharide xanthan was required for aggregate formation. Addition of DSF triggered dispersion of the aggregates formed by the rpfF strain, but not those of rpf strains defective in DSF signal transduction. An extracellular enzyme from Xcc whose synthesis was positively controlled by the DSF͞rpf system could disperse the aggregates produced by all rpf strains. The enzyme was identified as the single endo-␤-1,4-mannanase encoded by the Xcc genome. This enzyme had no detectable activity against soluble xanthan. The endo-␤-1,4-mannanase was required for the full virulence of Xcc to plants. On the basis of this model system, we propose that one role of the ␤-mannanase during disease is to promote transitions from an aggregated or biofilm lifestyle to a planktonic lifestyle in response to the DSF signal.