Xanthomonas campestris pv. vesicatoria is the causal agent of bacterial spot disease of pepper and tomato plants. Expression of its basic pathogenicity genes, the hrp genes, is induced in planta and in XVM2 medium and is dependent on the hrp regulatory gene hrpXv for five out of six loci in the 23-kb hrp cluster. Here we describe the isolation of a novel hrp gene, hrpG, that was identified after chemical mutagenesis and that is located next to the hrpXv gene. In a hrpG mutant induction of expression of the seven loci hrpA to hrpF, and hrpXv is abolished, suggesting that hrpG functions at the top of the hrp gene regulatory cascade. hrpG is the only gene in the locus and encodes a putative protein of 263 amino acids with a molecular mass of 28.9 kDa. The HrpG amino acid sequence shows similarity to response regulator proteins of the OmpR subclass of two-component systems, being mostly related to the ChvI proteins of Agrobacterium tumefaciens and Rhizobium spp., and TctD of Salmonella typhimurium. Expression of hrpG is low in complex medium, is increased in XVM2 by a factor of four, and is independent of other hrp loci. A model on hrp gene regulation in Xanthomonas campestris pv. vesicatoria is discussed.