The Biolog GN MicroPlate system was evaluated for accuracy in identifying gram-negative and gram-positive phytopathogenic bacteria to genus, species, and pathovar. Approximately 1,000 strains of Agrobacterium, Clavibacter, Erwinia, Pseudomonas, and Xanthomonas were compared. Of 79 cultures representing 77 P. syringae strains and eight pathovars, identification to species and pathovar was correct in 100 and 2% of cases, respectively. Of 603 cultures representing 537 X. campestris strains and 28 pathovars, identification to species and pathovar was correct in 97 and 20% of cases, respectively. With E. carotovora and E. chrysanthemi, 22 and 62% of the strains, respectively, were correctly identified to species. After the library was amended by the addition of data on these strains, identification to pathovar was improved for many of the strains in the X. campestris group.