POTASSIUM channels catalyse the permeation of K+ ions across cellular membranes and are identified by a common structural moth, a highly conserved signature sequence of eight amino acids in the P domain of each channel’s pore-forming alpha-subunit(1,2), Here we describe a novel K+ channel (TOK1) from Saccharomyces cerevisiae that contains two P domains within one continuous polypeptide, Xenopus laevis oocytes expressing the channel exhibit a unique, outwardly rectifying, K+-selective current. The channel is permeable to outward how of ions at membrane potentials above the K+ equilibrium potential; its conduction-voltage relationship is thus sensitive to extracellular K+ ion concentration. In excised membrane patches, external divalent cations block the channel in a voltage-dependent manner, and their removal in this configuration allows inward channel current, These attributes are similar to those described for inwardly rectifying K+ channels(3,4), but in the opposite direction, a previously uorecognized channel behaviour. Our results identify a new class of K+ channel which is distinctive in both its primary structure and functional properties. Structural homologues of the channel are present in the genome of Caenorhabditis elegans.