By mediating the coupled movement of Na, K, and Cl ions across the plasma membrane of most animal cells, the bumetanide-sensitive Na-K-Cl cotransporter (NKCC) plays a vital role in the regulation of ionic balance and cell volume. The transporter is a central element in the process of vectorial salt transport in secretory and absorptive epithelia. A cDNA encoding a Na-K-Cl cotransport protein was isolated from a shark rectal gland library by screening with monoclonal antibodies to the native shark cotransporter. The 1191-residue protein predicted from the cDNA sequence has 12 putative transmembrane domains flanked by large cytoplasmic N and C termini. Regulatory phosphoacceptor residues in isolated peptides are identified as Thr-189 and Thr-1114 in the predicted sequence. Northern blot analysis identified a 7.4-kb mRNA in rectal gland and most other shark tissues; a 5.2-kb mRNA was restricted to shark kidney. Homology with an uncharacterized gene from Caenorhabditis elegans and with the thiazide-sensitive Na-Cl cotransporter of flounder urinary bladder was found over most of the coding region; shorter stretches of homology were found with a C. elegans cDNA and with an uncharacterized gene of cyanobacterium. Human HEK-293 cells have been stably transfected with the shark cDNA and shown to express Na-K-Cl cotransport activity with the bumetanide sensitivity of the shark protein. The expressed transporter is functionally quiescent in the host cells and can be activated by depleting the cells of chloride.