The α- and κ-neurotoxins are polypeptide antagonists of nicotinic acetylcholine receptors derived from snake venom. They are structurally very similar but differ in their specificity for receptor subtype and in their native aggregation state. While the α-neurotoxins are monomeric, the κ- neurotoxins occur as homodimers. The crystal structure shows that there is a correlation in the distance between essential arginine residues in the κ- bungarotoxin dimer and the distance between the acetylcholine binding sites in the pentameric receptor. This has lead to an investigation of the critical interactions at the dimer interface of κ-bungarotoxin. Mutations of residues that the crystal structure indicates participate in dimer interaction were found to fall into two general groups: those that do not affect the dimerization state or activity of κ-bungarotoxin as single mutants, and those that interfere with it to such an extent that the protein is no longer able to fold properly. In general, those residues that fall into the latter group are found to be invariant in κ-neurotoxins and not found in α-neurotoxins. The results suggest that the extent of both the main chain-main chain β-sheet hydrogen bond interaction and van der Waals interactions between Phe 49 and Ile 20 are required for dimer formation. These studies provide a basis for understanding why the κ-neurotoxins readily dimerize in solution and the α-neurotoxins do not and also suggest that there is u possible interrelationship between dimer formation and protein folding in κ-bungarotoxin.