Abstract: The aseptic Pinus thunbergii seedlings were separately inoculated under aseptic conditions with aseptic pine wood nematodes [ Bursaphelenchus xylophilus (Steiner & Buhrer) Nickle] , bacterium isolate B619 isolated from the nematodes and the mixture of both the nematodes and the bacterium isolate. The results showed that the aseptic seedlings inoculated with the aseptic nematodes or bacterium isolate B619 did not wilt while those inoculated with the mixture of the aseptic nematodes and bacterium isolate B619 wilted. The findings indicated that the bacteria accompanying the nematodes were closely related to the pathogenicity of pine wood nematodes and there was some synergism between them in terms of the pathogenicity. Based on the results, it was suggested that the pine wilt disease was a disease complex involving the nematodes and bacteria.