Background/Aim: Parathyroid hormone (PTH)-dependent inhibition of proximal tubular P_i reabsorption is mediated by protein kinase A and/or C and is associated with reduced border membrane expression of the type IIa Na/P_i cotransporter. The aim of this study was to analyze phosphorylation of the type IIa cotransporter protein. Methods: Opossum kidney cells were used as a ‘proximal tubular’ cell model. Protein phosphorylation was determined by immunoprecipitation of the type IIa Na/P_i cotransporter, followed by autoradiography. The transporter protein content was evaluated by Western blotting and transport activity by tracer P_i uptake. Results: Under control conditions (no PTH) the transporter was phosphorylated; upon treatment with PTH, a decrease in phosphorylation was observed. A protein phosphatase inhibitor (okadaic acid) was unable to prevent PTH-induced Na/P_i cotransporter inhibition but reduced transporter degradation. Conclusion: The type IIa Na/P_i cotransporter is a phosphoprotein, but alterations in its phosphorylation seem not to be involved in P_i transport inhibition.