Two-dimensional crystalline arrays of native tetanus toxin have been formed at the interface between a solution of the toxin and a phospholipid monolayer containing a ganglioside. Electron crystallographic analysis has been used to study these periodic arrays. The arrays obey the symmetry of plane group p12(1), with a = 126 A and b = 84 A, and a thickness of 90 A (1 A = 0.1 nm). The three-dimensional structure of tetanus toxin in negative stain is reconstructed to a nominal resolution of 14 A from multiple tilt images. The molecule presents an asymmetric three-lobed structure and could interact with the monolayer in two possible orientations.