Glycoprotein G of the vesicular stomatitis virus triggers membrane fusion via a low pH-induced structural rearrangement. Despite the equilibrium between the pre- and postfusion states, the structure of the prefusion form, determined to 3.0 angstrom resolution, shows that the fusogenic transition entails an extensive structural reorganization of G. Comparison with the structure of the postfusion form suggests a pathway for the conformational change. In the prefusion form, G has the shape of a tripod with the fusion loops exposed, which point toward the viral membrane, and with the antigenic sites located at the distal end of the molecule. A large number of G glycoproteins, perhaps organized as in the crystals, act cooperatively to induce membrane merging.