The kidney plays an important role in ion regulation in both freshwater and seawater fish. However, ion transport mechanisms in the teleost kidney are poorly understood, especially at the molecular level. We have cloned a kidney specific SLC26 sulfate/anion exchanger from rainbow trout (Oncorhynchus mykiss) that is homologous to the mammalian SLC26A1 (Sat-1). Excretion of excess plasma sulfate concentration after Na₂SO₄ injection corresponded to significantly higher expression of the cloned SLC26A1 mRNA. Detailed morphological observation of rainbow trout renal tubules was also performed by light microscopy and transmission electron microscopy. According to the structure of brush border and tubular system in the cytoplasm, renal tubules of rainbow trout were classified into proximal tubule first and second (PI and PII) segments, and distal tubules. In situ hybridization revealed that SLC26A1 anion exchanger mRNA is specifically localized in the PI segment of kidneys from both seawater and freshwater adapted rainbow trout. Using immunocytochemistry, Na⁺, K⁺-ATPase and vacuolar type H⁺-ATPase (V-ATPase) were co-localized to the same cells and distributed in the basolateral and the apical membranes, respectively, of the cells where the SLC26A1 mRNA expressed. These findings suggest that the cloned kidney-specific SLC26A1 is located in kidney proximal tubules and is involved in excretion of excess plasma sulfate in rainbow trout.