The interaction of Rhodnius prolixus digestive enzymes with Trypanosoma cruzi could be important for parasite survival. We report herein the complete sequence of the messenger of a cathepsin L-like molecule (RpCat). The cDNA has 5'- and 3'- end UTRs and a methionine codon that corresponds likely to a translation initiation codon. In the deduced amino acid sequence, a region corresponding to an ERFININ domain, diagnostic of L-cathepsins, and a possible pro-peptide cleavage site were observed. At the C-terminus, a nine-amino acid sequence, almost identical to a secretion signal of human cathepsin L was found. RpCat messenger was expressed in intestines of R. prolixus adults, and from 1st to 4th but not in 5th instar nymph stages. In a similarity analysis, RpCat was grouped with L cathepsins forming a clear group separate of the B cathepsins.