Tight junction (TJ) constitutes the barrier by controlling the passage of ions and molecules via paracellular pathway and the movement of proteins and lipids between apical and basolateral domains of the plasma membrane. Claudins, occludin, and junctional adhesion molecules are the major three transmembrane proteins at TJ. This study focuses a newly identified mammalian TJ gene, claudin-19, in kidneys. Mouse claudin-19 composes of 224 amino acids and shares 98.2% and 95% amino acid homology with rat and human, respectively; the most evolutionary-related claudins are claudin-1 and -7, which share approximately 75% DNA sequence homology with claudin-19. Claudin-19 is abundantly expressed in the mouse and rat kidneys among the organs examined by Northern blots, and to a much less extent, also found in brain by RT-PCR. Claudin-19 and zonula occludens-1 (ZO-1) are localized at junctional regions of Madin-Darby canine kidney (MDCK) cells by immunofluorescent microscopy. In addition, ZO-1 is found in the claudin-19-associated protein complexes in MDCK cells by co-immunoprecipitation. Using aquaporin-1 and aquaporin-2 antibodies as markers for different renal segment, strong expression of claudin-19 was observed in distal tubules of the cortex as well as in the collecting ducts of the medulla. To less extent, claudin-19 is also present in the proximal tubules (cortex) and in the loop of Henle (medulla). Furthermore, intense claudin-19 immunoreactivity is found co-localized with the ZO-1 in kidneys from postnatal day 15, day 45, and adult rats and mice. Similar localizations of claudin-19 and ZO-1 are also observed in human kidneys. Since these renal segments are mainly for controlling the paracellular cation transport, it is suggested that claudin-19 may participate in these processes. In human polycystic kidneys, decreased expression and dyslocalization of claudin-19 are noticed, suggesting a possible correlation between claudin-19 and renal disorders. Taken together, claudin-19 is a claudin isoform that is highly and specifically expressed in renal tubules with a putative role in TJ homeostasis in renal physiology.