Larvae of Psychoda and of Chironomus (diptera) maintained in solutions of appropriate indicators show that the typical acidities (pH) prevailing within the several regions of the digestive tract are: esophagus 7.1; cardiac chamber 6.2; mesenteron 7.5; the latter being functionally an intestine. The acidity of the hindgut, pH 6.4, is due to the discharge of the malphigian tubules.