The experiments were conducted to examine the formation of chemical resistance of Xanthomonas oryze, the causal organism of rice bacterial leaf blight, to Agrepto in the succeeding geerations. The measurements of the bacterial growth were determined with the optical density as indirect method, and the results obtained were summarized as follows. 1. The Agrepto was effective in hindering the bacterial growth in the treatment of first generation, which was added in the broth even as low concentration as 30ppm. 2. when the successive four generations were treated with same concentrations, the bacterial growth increased conspicuously with succeeding generations and this phenomenon showed remarkable in the plots treated with 30ppm than in that of 10ppm. 3. When selections were repeated at treatment of 20 ppm, increasing the concentrations of the treatment in successive generations, the resistance increased with succeeding generations. 4. There was a tendency to acquire the higher rate of resistance in the progenies which selected from the treated plot of higher concentrations in previous generation than that of low concentrations.