The X-ray structures of two prokaryotic pentameric ligand-gated ion channels have recently provided detailed insight into this important family of neurotransmitter receptors. These prokaryotic homologs share the overall architecture of their eukaryotic counterparts with conservation in functionally important residues. Although both structures are similar they show distinct conformations of the ion conduction pore. One structure depicts a nonconducting state of the channel with a narrow transmembrane pore that is interrupted by conserved hydrophobic residues. The second structure reveals a conducting conformation where the hydrophobic constriction has opened to an aqueous funnel-shaped channel. The two structures thus suggest a novel gating mechanism for the family, where pore opening proceeds by a change of the tilt of the pore-forming helices.