Randomly amplified polymorphic DNA analysis was conducted with 14 primers to 17 strains of Xylella fastidiosa, There was a high degree of similarity among the seven Pierce's disease (PD) strains (S-xy > 0.93) and the seven oak leaf scorch (OLS) strains (S-xy > 0.96). However, the two groups were different, with a similarity index of 0.67, confirming the presence of a PD DNA cluster and suggesting the presence of a new OLS cluster. The control plum leaf scald strains (two strains) together with the periwinkle wilt strain had a much smaller similarity index (0.44) compared with the PD and OLS clusters.