In plant-pathogen interaction, the hypersensitive response is the host's resistance reponse which prevents pathogen spread and multiplication. This response results from the interaction between a pathogen with a dominant avirulence gene and a host with a specific complementary resistance gene. The adaptive significance of the pathogen avirulence gene to the pathogen in this host-pathogen relationship is not evident. This research presents preliminary data on the adaptive significance of avirulence genes in Xanthomonas campestris pv. vesicatoria.