1. Present paper reports the results on the experiments on the variability of the virulence of Xanthomonas oryzae, the rice leaf-blight organism, successively infected the living tissue of Variety Kidama the resistant, or Variety Fukuoka-Asahi the susceptible. 2. The virulence increased gradually in the case of Kidama, while no change of virulence was observed in the case of Fukuoka-Asahi or it was decreased gradually. 3. By the interchange of these host varieties the virulence of the organisms was changed easily. 4. Increased virulence of the organism thus obtained by the successive passing the resistant host was reduced within several months when cultured on solid media. 5. No differences on the serological, cultural and on several physiological characters were observed between the virulence-increased strains and the original bacteria. 6. It seems likely that the variability of the virulence of Xanthomonas oryzae is the results of the environmental adaptation.