All major insecticidal genes ofBacillus thuringiensisvar.israelensis(Bti) are controlled by the sporulation-specific sigma factor Sigma E (sigE), whilesigEis negatively regulated by Sigma K (sigK). Therefore, knocking outsigKplays an important role in regulating the expression of insecticidal genes inBti. AsigKdeletion mutant ofB. thuringiensisvar.israelensisstrain Bt-59, Bt59(Delta sigK), was constructed by homologous recombination and characterized. ThesigKdeletion resulted in no mature spores and delayed mother cell lysis fromT(25)toT(60), while the genetically complemented strain, Bt59(HFsigK), had mother cell lysis atT(25). Compared to Bt-59, sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE) analysis indicated that the expression of Cry4Aa2/4Ba1 and Cyt1Aa1 proteins in Bt59(Delta sigK) increased approximately 1.67 and 1.21 times, respectively. However, there was no significant change in Cry11Aa1 protein expression between the two strains. Bioassay results showed that thesigKdeletion mutation slightly reduced the insecticidal activity of Bt-59 againstCulex pipiens pallensand did not obviously affect activity againstAedes albopictus.