This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues.Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. were identified in L. muta and L. stenophrys venoms. In addition, both venoms contained a large number of bradykinin-potentiating peptides (BPP) and a C-type natriuretic peptide (C-NP). BPPs and C-NP comprised around 15% of the total venom proteins. In both species, the most abundant proteins were Zn 2+ -metalloproteinases (32-38%) and serine proteinases (25-31%), followed by PLA 2 s (9-12%), galactose-specific C-type lectin (4-8%), L-amino acid oxidase (LAO, 3-5%), CRISP (1.8%; found in L. muta but not in L. stenophrys), and NGF (0.6%).On the other hand, only six L. muta venom-secreted proteins matched any of the previously reported 11 partial or full-length venom gland transcripts, and venom proteome and transcriptome depart in their relative abundances of different toxin families. As expected from their close phylogenetic relationship, the venoms of L. muta and L. stenophrys share (or contain highly similar) proteins, in particular BPPs, serine proteinases, a galactose-specific C-type lectin, and LAO. However, they dramatically depart in their respective PLA 2 complement. Intraspecific quantitative and qualitative differences in the expression of PLA 2 molecules were found when the venoms of five L. muta specimens (3 from Bolivia and 2 from Peru) and the venom of the same species purchased from Sigma were compared.These observations indicate that these class of toxins represents a rapidly-evolving Author's personal copy gene family, and suggests that functional differences due to structural changes in PLA 2 s molecules among these snakes may have been a hallmark during speciation and adaptation of diverging snake populations to new ecological niches, or competition for resources in existing ones. Our data may contribute to a deeper understanding of the biology and ecology of these snakes, and may also serve as a starting point for studying structure-function correlations of individual toxins.