Glycoprotein G of the rhabdoviruses is involved in both receptor recognition at the host cell surface and membrane fusion via a low pHinduced structural rearrangement. G is an atypical fusion protein as there is a pH-dependent equilibrium between the pre- and post-fusion conformations of the protein. The atomic structures of the pre- and post-fusion conformations reveal that G is homologous to both glycoprotein gB of herpesviruses and gp64 of baculovirus and also that it combines features of the previously characterized class I and class II fusion proteins. Comparison of the structures of G pre- and post-fusion states reveals an extensive structural reorganization of the molecule that resembles that of paramyxovirus fusion protein F. It also allows to localize the fusion loops and to identify conserved key residues that constitute pH-sensitive molecular switches. Finally, the fusion properties and the structures of G also reveal some particularities that invite us to reconsider a few dogmas concerning fusion proteins.