BmK M4 is a neutral neurotoxin in the BmK toxin series.It is medially toxic and belongs to group III α-toxins.The purified sample was crystallized in rhombic space group P61.Using an X-ray diffraction technique,the crystal structure of BmK M4 was revealed by molecular replacement at 0.20 nm resolution.The model was refined.The final crystallographic R factor was 0.142 and the free R factor was 0.173.The root mean square deviation is 0.001 5 nm for the bond length and 1.753°for the bond angles.64 water molecules were added to the asymmetric unit.The refined structure showed an unusual non-prolyl cis peptide bond at residue 10.The structure was compared with group II α-toxin BmK M8 (an acidic,weak toxin).The potential structural implications of the cis peptide bond were discussed.