Objective: The urinary system of euryhaline fish, as well as the gills, is involved in ion regulation through the production of dilute urine in freshwater and isotonic urine in seawater. The low osmolality of the urine originates from an active reabsorption of ions by Ionocytes or mitochondria rich-cells (MRCs) present in certain parts of the urinary system. Mitochondria rich-cells possess a high density of Na+, K+-ATPase. Materials and Methods: Persian Sturgeon fry’s, adapted to freshwater and diluted Caspian Sea water (‰5 salinity) were fixed in Bouin’s solution after 24h. After the hydration with ethanol, the samples were paraffinaized and sectioned. Light microscopy and Hematoxiline-Fushin staining were used for histological examinations. Immunolocalization of Na+, K+-ATPase was observed through fluorescent microscopy (450-490 μm), using IgGα5 (as primary antibody) and FITC (as secondary antibody). Results: In both experimental conditions, maximum immunofluorescence of Na+, K+-ATPase (in mitochondria rich-cells) was found in distal and collective tubules. In both ureter and urinary bladder, immunostainings were found in dispersed cells with relatively weak intensity. In ‰5 acclimated fish, weak immunofluorescence was also observed in neck segment and proximal tubules, as well as in distal and collective tubules. Conclusion: Alternation in mitochondria rich-cells and Na+, K+-ATPase distribution in kidney tubules of ‰5 acclimated fry’s showed that the blood and osmolytes were nearly isotonic to environment but not isoionic. Thus the fish needs the absorption and excretion of some ions for the body homeostasis and osmoregulation.