The sand fly Lutzomyia longipalpis is the vector of Leishmania infantum (syn L. chagasi), the etiologic agent of visceral leishmaniasis in the New World. Despite its importance in public health, until now the internal anatomy of the immature forms was not yet described and little is known about their digestive processes. In nature, sand fly larvae feed on low nutrient food like organic detritus in the soil, ingesting constantly a relatively large quantity of material. The objective of this study was to describe the anatomy of the digestive tube, the pH inside the lumen as well as to investigate the proteases responsible for protein digestion and their localization. The larvae posses a short gut with a predominant well developed midgut. Ingestion of food containing indicator dies permitted the measurement of the intestinal pH.  A pH gradient was observed varying from higher or equal to 9, in the anterior midgut, to pH 6.5-7.0, in the posterior midgut. The endoproteolytic enzymes are synthesized in the anterior midgut and are able to digest azocasein over a large pH range. They are more effective in high alkaline environments presenting an optimal activity in pH 11. Studies with various inhibitors were performed and indicated that the endoproteases are trypsin-like and chymotrypsin-like serine proteases. These results were confirmed by using the substrates BApNA and BTpNA, specific for trypsin and chymotrypsin respectively. Aminopeptidases were also investigated using some p-Nitroaniline-derived substrates. These enzymes are located in the posterior midgut, bound to the membranes, presenting an optimal pH in the range of 6.5-7.5. The results presented are in accordance with the proposal that proteins are digested to small peptides in the anterior midgut inside the endoperitrophic space and undergo subsequent digestion in the ectoperitrophic space in the posterior midgut.