The phoretic relationships between the nematode Bursaphelenchus conicaudatus and the yellow-spotted longicorn beetle, Psacothea hilaris, collected at various districts in Japan were investigated. A high proportion of all ten subspecies of the beetle examined were infected with the nematodes, suggesting the universality of this phoretic association. Molecular analysis based on the partial base sequence of cytochrome oxidase subunit I in the mitochondrial DNA revealed that the phylogenetic relationships among ten subspecies of the beetle was similar to that among associated nematode isolates obtained from each beetle subspecies. Intraspecific vector replacements were supposed to occur in the isolates of B. conicaudatus on two islands. The phoretic association between the nematode and the beetle is assumed to have been established before the divergence of P.hilaris into subspecies.