Voltage-gated Naxe2x81xba-channels are transmembrane proteins that are responsible for the fast depolarizing phase of the action potential in nerve and muscular cells. Selective permeability of Naxe2x81xba over Caxc2xb2xe2x81xba or Kxe2x81xba ions is essential for the biological function of Naxe2x81xba-channels. After the emergence of the first high-resolution structure of a Naxe2x81xba-channel, an anionic coordination site was proposed to confer Naxe2x81xba selectivity through partial dehydration of Naxe2x81xba via its direct interaction with conserved glutamate side chains. By combining molecular dynamics simulations and free-energy calculations, a low-energy permeation pathway for Naxe2x81xba ion translocation through the selectivity filter of the recently determined crystal structure of a prokaryotic sodium channel from Arcobacter butzleri is characterised. The picture that emerges is that of a pore preferentially occupied by two ions, which can switch between different configurations by crossing low free-energy barriers. In contrast to Kxe2x81xba-channels, the movements of the ions appear to be weakly coupled in Naxe2x81xba-channels. When the free-energy maps for Naxe2x81xba and Kxe2x81xba ions are compared, a selective site is characterised in the narrowest region of the filter, where a hydrated Naxe2x81xba ion, and not a hydrated Kxe2x81xba ion, is energetically stable.