More than 120Xanthomonas campestrisstrains pathogenic for grasses and cereals were compared by polyacrylamide gel electrophoresis (SDS‐PAGE) of their whole‐cell proteins. Genotypic relationships between representative strains of the electrophoretic groups were determined by DNA:DNA hybridizations. Two major groups of bacteria were delineated. The first includedX. campestrispv.graminis, pv.arrhenatheriand some isolates fromBromus, which could be differentiated from each other by their protein fingerprints, and also the following pathovars which it was impossible to differentiate by SDS‐PAGE:cerealis, hordei, poae, secalis, translucensandundulosa.DNA:DNA hybridizations indicated that significant degrees of DNA‐binding (>60% D) exist between all these pathovars. In the second group, strains ofX. campestrispv.holcicola, pv.vasculorumand pv.oryzaewere related at 40–45% DNA‐binding, while strains of pv.oryzaeand pv.oryzicolawere genotypically highly related (85% D). All the pathovars of this second group could be differentiated from each other by their protein electrophoretic f