In the onion thrips, Thrips tabaci Lindeman (Thysanoptera: Thripidae), the development of resistance to pyrethroid insecticides has become a problem worldwide. To identify this species' resistance mechanism, we analyzed partial nucleotides and deduced the amino acid sequences of the para-orthologous sodium channel gene by using 10 Japanese strains of T. tabaci that have a variety of susceptibility levels to pyrethroids. Six nucleotide substitutions were found in their cDNA sequences. Three of these create amino acid substitutions, M918T, T929I, and L1014F, which are well known to be associated with knockdown resistance in some pest insects. In the five highly resistant strains, two mutations, M918T and L1014F, were always detected with wild-type sequences, suggesting that these strains have both mutations heterozygously. A moderately resistant strain was homozygous for the T929I mutation. This is the first report to identify these three major mutations within the same species.