In the shark rectal gland (SRG), apical chloride secretion is electrically coupled to a basolateral K⁺ conductance whose type and molecular identity are unknown. We performed studies in the perfused SRG with 17 K⁺ channel inhibitors to begin this search. Maximal chloride secretion was inhibited by low perfusate pH, quinine, bupivicaine, and anandamide, consistent with the properties of an acid sensitive four transmembrane, two-pore-domain K⁺ channel (4TM-K2P). Using PCR with degenerate primers to this family, we identified a TASK-1 fragment in shark rectal gland, brain, gill, and kidney. Using 5'and 3' RACE PCR and genomic walking, we cloned the full length shark gene whose open reading frame encodes a protein of 375 amino acids that was 80% identical to the human TASK-1 protein. We expressed shark and human TASK-1 cRNA in Xenopus oocytes and characterized these channels using two electrode voltage clamping (TEVC). Both channels had identical current voltage relationships and a reversal potential of -90 mV. Both were inhibited by quinine, bupivicaine, and acidic pH. We identified this protein in SRG by Western blot and confocal immuno-fluorescent microscopy and detected the protein in SRG and human airway cells. Shark TASK-1 is the major K+ channel coupled to chloride secretion in the SRG, is the oldest 4TM 2P family member identified, and is the first TASK-1 channel identified to play a role in setting the driving force for chloride secretion in epithelia. The detection of this potassium channel in lung tissue has implications for human biology and disease.