SUMMARY: The classification of plant-pathogenic coryneform bacteria has been rev-recd many times. All plant-pathogenic coryneform bacteria were once classified in the genus Corynebacterium. Currently, all are classified in the genera Arthrobacter, Clavibacter, Curtobacterium or Rhodococcus. Difficulties still remain regarding the characterization of species and subspecies within these groups. Analysis of cellular fatty acids by high-resolution capillary gas chromatography has recently become a standard method of bacterial identification in many laboratories. In the studies reported here, cellular fatty acid analyses were performed to examine the plant-pathogenic coryneform bacteria as an autonomous group. A total of 189 bacterial strains were profiled. Results from these studies support the division of plant-pathogenic coryneforms into four major groups (genera). Further subdivision was not required for Rhodococcus and Arthrobacter, as each genus contains only one species of plant-pathogenic bacteria. Some, but not all, Curtobacterium pathovars could be differentiated from one another based on fatty acid analysis alone. Infrasubspecific classification of plant-pathogenic Curtobacterium organisms appears to be appropriate. Most of the species or subspecies belonging to the genus Clavibacter could be differentiated using fatty acid profiles. Other chemotaxonomic methods also differentiate organisms within Clavibacter. Reclassification is warranted within Clavibacter michiganense. Clavibacter michiganense subspecies should be returned to full species status.