SummaryWe present near-atomic-resolution cryo-EM structures of the mammalian voltage-gated potassium channel Kv1.2 in open, C-type inactivated, toxin-blocked and sodium-bound states at 3.2 Å, 2.5 Å, 2.8 Å, and 2.9Å. These structures, all obtained at nominally zero membrane potential in detergent micelles, reveal distinct ion-occupancy patterns in the selectivity filter. The first two structures are very similar to those reported in the related Shaker channel and the much-studied Kv1.2–2.1 chimeric channel. On the other hand, two new structures show unexpected patterns of ion occupancy. First, in the toxin-blocked channel α-Dendrotoxin, like Charybdotoxin, is seen to attach to the negatively-charged channel outer mouth, and a lysine residue penetrates into the selectivity filter. Penetration by α-Dendrotoxin is however deeper than with Charybdotoxin, occupying two of the four ion-binding sites. Second, a structure of Kv1.2 in Na+solution does not show collapse of the selectivity filter that was observed under similar conditions in the KcsA channel, but instead shows an intact selectivity filter with ion density in each binding site. We also attempted to image the Kv1.2 W366F channel in Na+solution, but the protein conformation was seen to be highly variable and only a low-resolution structure could be obtained. These findings present new insights into the stability of the selectivity filter and the mechanism of toxin block of this intensively studied, voltage-gated potassium channel.