Phospholipases in some snake venoms are potent neurotoxins that target their enzymatic action to the synaptic membrane. One of these is the heterodimeric neurotoxin, beta-bungarotoxin, which binds with a protease inhibitor-like subunit to a presynaptic potassium channel and then blocks neurotransmission with a second subunit that has phospholipase A2 activity. We have prepared and characterized well ordered crystals of the most toxic beta-bungarotoxin isoform, beta 1-bungarotoxin. The crystals are monoclinic, space group C2, with unit cell parameters: a = 176.5 A, b = 39.3 A, c = 92.7 A, and beta = 114.8 degrees. Rotation-function analysis of the Patterson function, as calculated from a 2.3-A data set, reveals an asymmetric unit composed of four heterodimers. These heterodimers appear to be associated as two crystallographically distinct (AB)4 tetramers, each having dihedral D2 symmetry. The two are positioned with equivalent molecular 2-fold axes coincident with crystallographic dyads, but rotated by 55 degrees relative to one another. X-ray analysis of these crystals will permit direct visualization of the specific structural motifs and chemical features that underlie phospholipase neurotoxicity.