RFLP analysis was used for the differentiation of races of Xanthomonas campestris pv. oryzae. The strains representative of Japanese,Indonesian and Philippine races of X. c. pv. oryzae were differentiated by RFLP patterns using the probe pJEL101, a pUC18 plasmid containing a 2.4kb Eco R1 -HindIII fragment derived from strain PXO86 of X. c. pv. oryzae. Japanese strains of X . c. pv. oryzae were grouped into certain categories by RFLP, and most of them coincided with races. Strains of the bacterium from different the geographic areas in South East Asia were also divided into some subgroups corresponding to geographic origin by RFLP patterns. RFLP patterns of chemically-induced mutants of X. c. pv. oryzae reflected the shift of the races. It is concluded that RFLP is a useful method for the differentiation of races and for genetic diversity analysis.