A 14-residue peptide toxin has been isolated from the venom of the marine snail Conus magus. Its amino acid sequence, GlyArgCysCysHisProAlaCysGly LysAsnTyrSerCysNH2, is homologous with those of the previously described conotoxins GI, GII, and GIA from Conus geographus. The new peptide, conotoxin MI, is two to three times more active than the others, and is presumed to act as they do at the acetylcholine receptor of vertebrate neuromuscular junctions.