Beta-barrel pores are found in outer membrane porins of gram-negative bacteria, bacterial toxins and mitochondrial channels. Apart from the beta-barrel the three groups show no close sequence or structural homology but these pores exhibit symmetrical voltage gating when reconstituted into planar lipid bilayers. The structures of several of these are known and many site-directed mutants have been examined. As a result it seems evident that the gating is a common characteristic of these unrelated large pores and is not generated by specialised structures in the pore lumen.