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The localization of Na(+) , K(+) -ATPase (NKA) and the ultrastructural features of kidney were examined in larvae of the Persian sturgeon Acipenser persicus (L 31-41 mm total length and 182·3-417·3 mg). Investigations were conducted through light and electron microscopy and through immunofluorescence for NKA detection. The kidney nephrons consisted of a large glomerulus and tubules (neck, proximal, distal and collecting), which connected to the ureters. Posteriorly, ureters extended and joined together into a thin-walled ureter terminal sac. Ultrastructurally, the glomerular cells (podocytes) possessed distinctive pedicels that extended to the basal membrane. The proximal tubule (PT) showed two different cells. The cells lining the anterior part of PT possessed apical tall microvilli (c. 2·7 µm), a sub-apical tubular system, a basal nucleus and dense granules. Posteriorly in the cells, the sub-apical tubular system and granules were absent and round mitochondria associated with basolateral infoldings were found; the apical microvilli were reduced. Distal and collecting tubular cells showed the typical features of osmoregulatory cells, i.e. well-developed basolateral infoldings associated with numerous mitochondria. No immunofluorescence of NKA was detected in the glomeruli. A weak immunostaining was observed at the basolateral side of the cells lining the neck and PT. A strong immunostaining of NKA was observed in the entire cells of the distal tubules, collecting tubules and in some isolated cells of the ureters. In all immunostained cells, the basolateral region showed a much higher fluorescence and nuclei were immunonegative. In conclusion, the epithelial cells of kidney tubules had morphological and enzymatic features of ionocytes, particularly in the distal and collecting tubules. Thus, the kidney of A. persicus larvae possesses active ion exchange capabilities and, beside its implication in excretion, participates in osmoregulation.