The resistance reactions of rice varieties to bacterial leaf blight caused by Xanthomonas campestris pv. oryzae were grouped into three types: symptomless, browning and small yellow lesions. In the resistance reaction controlled by the resistance genes Xa-1, Xa-7 and Xa-10 no symptoms were observed, while the reaction controlled by Xa-2, Xa-4, Xa-4^a, xa-5 and xa-8 was characterized by small yellow to limited yellow lesions. The resistance reaction controlled by Xa-3, Xa-4^b, Xa-6, xa-9 was characterized by brown necrosis. The population levels of the incompatible strain were lower than 10⁶ cfu per inoculated leaf in the symptomless and browning type of resistance reactions. In the resistance reaction with small yellow lesions, the bacterium multiplied to the level of 10⁷ cfu per inoculated leaf. Histological observation showed that multiplication and translocation of the incompatible strains were very limited compared with those of the compatible ones. It is concluded that the infection types in the host-parasite interaction are caused by the degree of multiplication of the bacterium in the host tissues and ability of the host tissues to undergo necrosis.