The pinewood nematode (PWN), Bursaphelenchus xylophilus , is the causal agent of pine wilt disease (PWD). This nematode is considered to be an indigenous to North America and was introduced to Japan in the late 19th century. Subsequently, it has spread throughout Japan and in many other countries, China, Taiwan, and South Korea. In 1999, B. xylophilus was discovered in Portugal, and in 2008 in Spain. So far the studies have revealed that the pathogenicity of B. xylophilus varies between different isolates. The conducted study compared the pathogenicity of five isolates of B. xylophilus , originating from different parts of Japan, to 3-year-old Pinus sylvestris, and their ability to reproduce in the seedlings. The results revealed diverse virulence of B. xylophilus resulting in plant mortality. Three isolates S10, Ka4, and T4 caused 100% mortality of plants within three months while at the same time, the other two isolates, C14-5 and OKD-1 did not cause any disease symptoms on plants. After seven months, some dieback occurred on two seedlings, but similar symptoms were also found on the control plant. Moreover, a significant positive correlation was found between nematode virulence and the number of nematodes reproducing on pine seedlings.