Abstract: The interactions were studied between pine wood nematode (Bursaphelenchus xylophilus) and three bacterium strains isolated from the nematode, which are two strong-pathogenic bacterium strains (Pseudomonas fluorescens) GcMS-1A and (P. putida) ZpB1-2A, and a weak-pathogenic bacterium strains, (Pantoea sp.) ZM2C. The result showed that the strong-pathogenic (P. fluorescens) GcMS-1A strain and (P. putida) ZpB1-2A strain significantly increased fecundities, reproduction rate and the adults’ body volume of pine wood nematode. Meanwhile, pine wood nematode significantly promoted reproduction of the two strong-pathogenic bacterium strains. However, weak-pathogenic bacterium strains, (Pantoea sp). ZM2C, completely inhibited reproduction of pine wood nematode. Aseptic pine wood nematode significantly inhibited reproduction of the strain ZM2C (Pantoea sp.). The results indicated that mutualistic symbiosis existed between pine wood nematode and some pathogenic bacteria carried. The phenomena showed that the pathogenic bacteria carried by the nematode were not accidentally contaminated, rather than symbionts with pine wood nematode coevoluted for a along time. The part of mutulistic symbiosis playing in the process of pine wilt disease was discussed.