Gut pH and pH optimum of amylase were measured in 4th-stage larvae of 2 pestiferous chironomid species, Chironomus crassicaudatus and Glyptotendipes paripes. The gut pH in both species was close to neutral, varying from 6.7 to 7.4 and 6.9 to 7.6 pH units for C. crassicaudatus and G. paripes, respectively. In both species, slightly alkaline pH values were measured in the anterior part of the mesenteron, whereas slightly acidic pH values were recorded in gastric caecae and the posterior part of mesenteron. Amylase pH optimum in both chironomid species remained between pH values of 5.5 and 7, being closer to pH value 7 than 5.5. This profile was consistent with the measured values of the gut pH. The recorded larval gut pH profile in the 2 species of chironomids in the present study is much lower than gut pH reported for mosquitoes or Lepidoptera larvae. This could be the reason for the relatively much lower susceptibility of chironomid larvae to Bacillus thuringiensis serovar israelensis toxin proteins than some other nematoceran Diptera, specifically mosquitoes.