The current studies were designed to isolate a cDNA encoding the control mouse renal sodium phosphate (Na+/P(i)) transporter and to determine mRNA levels encoding this transporter in control and (Hyp) mice. A 2.4 kb cDNA was isolated from a control mouse kidney cDNA library using a PCR-amplified 850 base pair (bp) fragment of rat cDNA. The cDNA encoding the mouse renal Na+/P(i) transporter shows sequence similarity to the rat and human Na+/P(i) transporters. The predicted protein exhibits 98% and 91% amino acid sequence identity with the rat and human proteins, respectively. A cRNA was synthesized from the cDNA and showed an expression of sodium-dependent phosphate transport in Xenopus laevis oocytes. Northern blot analysis of renal poly(A)+ RNA from (Hyp) and control mice showed a threefold decrease in mRNA level in the (Hyp) mice compared with control mice. In situ hybridization analysis localizes the Na+/P(i) transporter message to the renal proximal tubule, with message levels distinctly lower in the (Hyp) mouse. Also, genomic Southern blotting suggests that the gene encoding the sodium phosphate transporter is structurally identical in the control and (Hyp) mice. These studies suggest that we have cloned the cDNA encoding the renal Na+/P(i) transporter in the control [C57BL/6J+/ymale] mouse, localized the message to the renal proximal tubule, and shown that the mRNA level encoding the renal Na+/P(i) transporter is decreased in the hypophosphatemic mouse.