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The inoculation experiments on pine seedlings and saplings with the pine wood nematode, Bursaphelenchus xylophilus, were conducted to clarify the relations between nematode numbers and disease development in host plant at the initial stage of pathogenesis.The branch inoculated with nematodes was cut off at various intervals after inoculation.Cutting the branch 1 day after inoculation resulted in the delayed disease development on 4-yearold Pinus densiflora and P. thunbergii and did not cause disease for 9-year-old P. thunbergii. Disease development in seedlings and saplings, whose inoculated branches were cut 3 days after inouculation or later, was similar to the controls which were left without cutting. There were no significant differences in the numbers of dispersing individuals over various parts of tree between the two removal times of inoculated branches; 1 day and 3 days after inoculation. More advanced pathological responses of the inoculated branch were histopathologically observed on 3 days or afterwards than on 1 day after inoculation.The present experiments suggest that the pathological responses caused by the nematode activities at the infection site prevailed in the tree and reached a certain extent enough to initiate disease development as early as 3 days after inoculation.