The effects of the infection with filamentous phage Xf2 on the properties of Xanthomonas campestris pv. oryzae N5850 were investigated. The bacteria infected with Xf2 released the phage into medium to reach the maximum concentration of about 1×10¹¹ PFU/ml after 48hr of shaking culture. The multiplication of bacteria was considerably retarded by infection with Xf2 at temperatures lower than 33.5C. The infected bacteria, however, reached the same or higher level as compared with uninfected control later on. The infected cells grew even at 35C to some extent, while the uninfected cells did not. The Xf2-infected bacteria grown at different temperatures of 30, 33.5 and 35C for 72hr were ultrathin sectioned and their structural change was investigated with electron microscope. Although no remarkable difference was observed between Xf2-infected and uninfected cells in cytoplasmic structures, the cell wall of the former was vesiculated, denatured or partially stripped from the cells. The Xf2-infected bacteria multiplied in the inoculated rice leaves more slowly than uninfected bacteria during the first few days after inoculation. The former's population, however, exceeded the latter later on. The Xf2-infected bacteria incited symptoms on rice leaves earlier than uninfected bacteria. No difference was observed between Xf2-infected and uninfected bacteria in activities of xylanase, cellulase and lipase in culture filtrates. The amout of extracellular polysaccharide produced by Xf2-infected bacteria was higher than that produced by uninfected bacteria, suggesting some correlation with virulence.