The origin and evolution of venom in many animal orders remain controversial or almost entirely uninvestigated. Here we use cDNA studies of cephalopod posterior and anterior glands to reveal a single early origin of the associated secreted proteins. Protein types recovered were CAP (CRISP, Antigen 5 [Ag5] and Pathogenesis-related [PR-1]), chitinase, peptidase S1, PLA(2) (phospholipase A(2)), and six novel peptide types. CAP, chitinase, and PLA(2) were each recovered from a single species (Hapalochlaena maculosa, Octopus kaurna, and Sepia latimanus, respectively), while peptidase S1 transcripts were found in large numbers in all three posterior gland libraries. In addition, peptidase S1 transcripts were recovered from the anterior gland of H. maculata. We compare their molecular evolution to that of related proteins found in invertebrate and vertebrate venoms, revealing striking similarities in the types of proteins selected for toxic mutation and thus shedding light on what makes a protein amenable for use as a toxin.