Several factors related to the growth and survival of pathogen of melon hairy root, A. rhizogenes were examined. The concerned bacteria could increase in number in sterilized soils at 20 and 30℃, and survive for a long time in naturally infested soils. The bacteria grew in soils at pH 4-8, and the optimum growth took place in the range of pH 6-8. The bacteria could grow when they infected sterilized soils with a density of 10⁶ cells/g of dry soil with a water content of 20-100% of water holding capacity. However, in case where 10³ cells/g of dry soils were infected, the bacteria died after the incubation for 7 days at a water content of 20% of the maximum water holding capacity. The bacteria did not change in number when they infected sterilized soils, while they decreased gradually in the non-sterilized soils, when their density was at the level of 10⁸ cells/g of dry soil. Their density eventually reached a level of 10⁴ cells/g of dry soil after the incubation for 250 days at 30℃.