To clone new phosphate transporters from the kidney. EST bank was screened for new members of Na-dependent inorganic phosphate (P i) co-transporter. To obtain a full-length clone, a rat kidney cDNA library was screened with the EST clone as a probe. We isolated a cDNA that encodes a type I Na/P i co-transporter. Based on the sequence homology and tissue distribution, it will be the rat ortholog of human NPT4 (Solute Carrier family 17, member A3, SLC17A3) and is designated rNPT4. When expressed in Xenopus oocytes, Na/P i co-transport activity was demonstrated. In Northern blotting analysis, rNPT4 was selectively expressed in the liver and kidney. In immunohistochemical analysis, rNPT4 was localized at the apical membrane of the proximal tubules (S2, S3) in the rat kidney. The electron microscopic studies confirmed this finding. The present study revealed the expression of a new type I Na/P i co-transporter at the brush border membrane of the proximal tubules in the kidney.