Analysis of the Schistosoma mansoni peptidome for immunomodulatory molecules by solvent extraction and reverse-phase HPLC revealed a 27-amino-acid residue peptide from an extract of cercariae. Using matrix-assisted, laser desorption-ionization, time-of-flight mass spectrometry, the peptide yielded a protonated molecular ion [M + H]+ of m/z 2789. The unequivocal sequence was deduced by automated Edman degradation as: DLWNSIKDMAAAAGRAALNAVTGMVNQ. The peptide exhibited an 80.76% identity with dermaseptin 3.1 from the leaf frog Agalychnis annae, and was therefore named Schistosoma mansoni dermaseptin-like peptide (SmDLP). Immunocytochemical staining using a primary antidermaseptin B2 antibody located SmDLP in acetabular glands of cercariae, in and around schistosomula, and in adult worms and their eggs. Dot-blotting confirmed its presence in extracts (cercariae and worms) and excretion/secretion (E/S) products (transforming cercariae and eggs). This was corroborated by use of a MALDI-ToF spectra database of E/S products from cercariae. Functional characterization of the peptide indicated that SmDLP had typical amphipathic antimicrobial peptide properties, i.e., the ability to lyse human erythrocytes causing a decrease in the levels of nitric oxide produced by monocytic cells. This last function strongly suggests that SmDLP plays a vital role in the parasite's immunoevasion strategy. The possibility that schistosomes acquired this gene from amphibians has been discussed by constructing a phylogenetic tree.