Seasonal change of resistance of hosts against Xanthomonas oryzae of rice plants were studied. The varieties used were Kidama ?highly resistant, and Asahi (from Fukuoka Agr. Exp. Sta.) - susceptible. Inoculation experiments tried after the beginning of the reproductive stage of rice plant indicated that Asahi is highly susceptible while Kidama is strongly resistant. On the contrary, inoculation experiments tried during the nutritional stage (up to the 10th leaf), proved that Asahi is rather resistant than Kidama.