BmK M4 is a neutral neurotoxin in the BmK toxin series. It is medially toxic and belongs to group III alpha-toxins. The purified sample was crystallized in rhombic space group P6(1). 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 freeR factor was 0.173. The root mean square deviation is 0.001 5 nm for the bond length and 1.753 degrees 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 alpha-toxin BmK M8 (an acidic, weak toxin). The potential structural implications of the cis peptide bond were discussed.