Population of Xanthomonas oryzae phage in water of irrigation channels and paddy fields was investigated periodically during the rice growing period in diseased area of Odayama and Ogame districts, Obama Town, Nagasaki Prefecture. Detection of natural phage was made by use of the phage type A strain of X. oryzae. There was a positive correlation approximately between the disease grade and the phage density. The time of first disease occurrence could be forecasted by first detectable time and following increase trend of phage population.