The structures of purified "soluble" and "detergent-soluble" bovine caudate nucleus acetylcholinesterases were compared by peptide mapping on polyacrylamide gels. The digestion products generated from the two acetylcholinesterases on proteolysis by a given protease (Staphylococcus aureus V8 protease, alpha-chymotrypsin, or papain) are remarkably similar as judged from the electrophoretic band patterns. We conclude that the "soluble" and "detergent-soluble" acetylcholinesterases from bovine caudate nucleus share a common evolutionary origin.