1. Physiological characteristics of Xanthomonas oryzae, the causal organism of the bacterial blight of rice plant, have been studied by several authors, but there are some discrepancies in their results with regard to certain characteristics such as reaction in milk and reduction of methylene-blue. The present research was carried out to make these points clear and to elucidate the relationship between the physiological properties and pathogenicities of the bacterium. For this purpose, 20 isolates of different pathogenicities were selected from 118 isolates which had been collected from various districts of Japan.<br>2. All the isolates used in this experiment reduced methylene-blue, but not nitrate; produced ammonia and hydrogen sulfide, but not indole; gas production and diastase activity were negative; liquefied gelatin gradually; did not coagulate or digest milk; and discolored Litmus milk into blue. Thus the result differs from Ishiyama's original description as to the reduction of methylene-blue, the liquefaction of gelatin, and the production of hydrogen sulfide.<br>3. Isolates more active in liquefying gelatin were, with some exception, more pathogenic than those with less liquefying power, suggesting that there might be an intimate relationship between the liquefying power and the pathogenicity. There were some immunogenetical differences among the isolates in terms of the rate and strength of agglutination reaction, although the reaction was positive in all the isolates. The differences in the reaction, however, were correlated with the differences in the pathogenicity of the isolates.<br>4. The bacteria in culture were capable of decomposing xylose, arabinose, glucose, levulose, galactose, mannose, and sucrose, producing acids; and incapable of utilizing rhamnose, dextrin, starch, inulin, glycerol, mannitol, sorbitol, and salicin. Some of the isolates could not utilize arabinose.