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Teleost fishes, living in fresh water, engage in active ion uptake to maintain ion homeostasis. Current models for NaCl uptake involve Na⁺uptake via an apical amiloride-sensitive epithelial Na⁺channel (ENaC), energized by an apical vacuolar-type proton pump (V-ATPase) or alternatively by an amiloride-sensitive Na⁺/H⁺exchange (NHE) protein, and apical Cl⁻uptake mediated by an electroneutral, SITS-sensitive Cl⁻/HCO₃⁻anion-exchange protein. Using non-homologous antibodies, we have determined the cellular distributions of these ion-transport proteins to test the predicted models. Na⁺/K⁺-ATPase was used as a cellular marker for differentiating branchial epithelium mitochondria-rich (MR) cells from pavement cells. In both the freshwater tilapia (Oreochromis mossambicus) and rainbow trout (Oncorhynchus mykiss), V-ATPase and ENaC-like immunoreactivity co-localized to pavement cells, although …