Crystals of stonustoxin have been obtained and diffract to 3.4 Angstrom resolution. Stonustoxin is a protein lethal factor isolated from the venom of the stonefish, Synanceja horrida. The crystals belong to the tetragonal space group P422, with unit cell constants a = b = 109.0 Angstrom, c = 245.7 Angstrom. A native stonustoxin molecule has two subunits, designated alpha and beta, respectively, and there is one stonustoxin molecule per asymmetric unit.