AbstractTrypsins constitute some of the most abundant midgut digestive proteases expressed by hematophagous insects upon blood feeding. In addition to their role in the digestion of the blood meal, these proteases also have been implicated in the ability of certain pathogens to infect their natural vector. In sand flies, digestive proteases including trypsins were associated with early killing of Leishmania and are believed to play a role in the species‐specificity dictating sand fly vectorial capacity. Our group is involved in studies of midgut digestive proteases in the sand fly Lutzomyia longipalpis, the principal vector of visceral leishmaniasis in Brazil. Here we report on the identification of two cDNAs, Lltryp1 and Lltryp2, which code for putative midgut trypsins in L. longipalpis. Analyses of RNA abundance using semi‐quantitative RT‐PCR show a different pattern of expression between the two genes. Lltryp1 expression remains undetected until blood feeding and reaches a peak at 12 h post‐blood meal (PBM), returning to pre‐blood meal levels at 72 h PBM. Additionally, Lltryp1 expression is undetected during larval development. Lltryp2, on the other hand, is constitutively expressed as high levels in the non‐blood fed female, but is reduced upon blood feeding. At the end of the digestive cycle, Lltryp2 regains its pre‐blood meal levels. This cDNA also is present in all developmental stages and in adult males. This pattern of expression is reminiscent of what is seen in mosquitoes and Old World sand flies, but has characteristics that are unique to L. longipalpis. Arch. Insect Biochem. Physiol. 66:53–63, 2007. © 2007 Wiley‐Liss, Inc.