Whole-cell protein analyses of 75 Xylella fastidiosa strains by sodium dodecyl sulfate-polyacrylamide gel electrophoresis were compared, and variations in the protein banding patterns among the strains were observed. Based on the presence, absence, or difference in intensity of 10 protein bands within the 21.5 to 45.0 kDa molecular mass range, the strains could be subdivided into four distinct pathogenic groups and two miscellaneous groups whose members were pathogenic to various different hosts. Group I was the Pierce's disease of grapevine pathogenic group. Although 4 of these 45 strains had hosts of origin other than grapevine, they all produced Pierce's disease symptoms. Uniform, distinct protein profiles also occur-red with group 2 (elderberry leaf scorch strains), group 3 (oak leaf scorch strains), and group 4 (oleander leaf scorch strains). Groups 5 and 6 were made up of strains pathogenic to almond, blackberry, lupine, mulberry, periwinkle, elm, and plum. Thus, whole-cell protein analysis was shown to be a rapid and consistent method for identifying four pathogenic groups of X. fastidiosa strains.