To investigate distributions of pathogenic strains of Xanthomonas oryzae in Japan, isolates collected from various areas of Japan in 1973 and 1975 were examined by the qualitative virulence test on four differential varieties of rice. Two hundreds and fifty-nine isolates were classified as group I, 128 as group II, 36 as group III, 3 as group IV, and one as group V. Group I and II were found to distribute widely in almost all areas. Group III distributed in Yamanashi and Nagano prefectures, and the western part of Japan; especially it was found frequently in Nagano, Kumamoto, and Kagoshima prefectures. In 1975, 3 isolates of group IV were found in Nago-city, Onna-village of Okinawa prefecture, and Nagasaki-city, while one isolate of group V in Gokase-twon of Miyazaki prefecture. In Kyushu-Okinawa region, all of the five groups distributed and among them group II was found most frequently. Such a high incidence of group II in this region may be attributed to predominant cultivation of Kogyoku group varieties which can be attacked by group II. It is readily assumed that cultivated varieties in certain regions are a major factor to determine the distributions of bacterial groups. However, the present results that groups II to IV which can attack Kogyoku group varieties were found in regions where Kinmaze group varieties had been cultivated extensively, suggest that some other factors must be involved in determining distributions of bacterial groups. Further studies should be done to examine effects of these factors such as environment and cultivation conditions on distributions of bacterial group. To investigate associations between virulences of bacterial isolates and their host varieties, 66 isolates collected from Kyushu region were further categorized, based on their host varieties. Among 42 isolates from Kinmaze group varieties, 18 isolates were identified as group I, 17 isolates as group II, 6 isolates as group III, and one isolate as group V, while among 24 isolates from Kogyoku group varieties, 20 isolates were identified as group II and 4 isolates as group III.