A new bacterial leaf blight disease of parthenium (Parthenium hysterophorus L.) is described for the first time. The disease-causing bacterium was isolated and its morphological, physiological and biochemical characters were determined. The pathogenicity of bacterium is apparently limited only to parthenium. The pathogen was identified as Xanthomonas campestris pv. parthenii pathovar nov. on the basis of morphological, physiological, biochemical and pathogenic characteristics.