['Topoisomerase I (Top I) is referred as the cellular target of the camptothecins (CPTs) which are now being explored as potential pesticides for insect control. Three amino acid substitutions, including L530P, A653T and S729T, in Top Is of insects were found in our previous studies. In order to investigate the effect of these three substitutions, a comparative analysis was conducted between the wild-type and mutant Top Is in Spodoptera exigua Hübner.', 'The optimal salt concentration of A653T and S729T was 150 mm, which is consistent with that of the wild-type Top I, while the mutant L530P showed maximum relaxation activity at a lower KCl concentration (100 mm). The mutated L530P and A653T Top Is showed a higher relaxation efficiency owing to an increased relaxation velocity toward the negatively supercoiled plasmid pBR322 DNA, which rendered L530P and A653T resistant to CPTs, whereas mutant S729T exhibited sensitivity to CPTs as a result of a decreased relaxation activity toward plasmid pBR322 DNA.'] Do substitutions in Spodoptera exigua topoisomerase I modulate its relaxation activity and camptothecin sensitivity?