A 26 residue peptide (Am 2766) with the sequence CKQAGESCDIFSQNCCVG-TCAFICIE-NH₂ has been isolated and purified from the venom of the molluscivorous snail, Conus amadis, collected off the southeastern coast of India. Chemical modification and mass spectrometric studies establish that Am 2766 has three disulfide bridges. C-terminal amidation has been demonstrated by mass measurements on the C-terminal fragments obtained by proteolysis. Sequence alignments establish that Am 2766 belongs to the δ-conotoxin family. Am 2766 inhibits the decay of the sodium current in brain rNav1.2a voltage-gated Na⁺ channel, stably expressed in Chinese hamster ovary cells. Unlike δ-conotoxins have previously been isolated from molluscivorous snails, Am 2766 inhibits inactivation of mammalian sodium channels.