Abstract: As a surface for respiratory gas diffusion, ion regulation and waste excretion, gill epithelium of Alosa Caspio Caspio has been investigated. Problem statement: Surely, morphological study of gill cells is the main route for giving the basic data which can be inductive for physiological and pathological investigations. Approach: Gills were removed from Alosa Caspio which is from euryhaline species and fixed in freshly prepared glutaraldehyde and osmium tetroxide. Thin sections were stained in Uranyl acetate-lead citrate and examined under LEO transmission electron microscope. Results: Flattened respiratory lamellae extend in two rows from the sides of the filaments. The pavement cell is the major cell covering the thin epithelia of lamellae, but mucous cell and chloride cell also can be seen on lamellar epithelia, however, most of them are observed in filamental epithelia. The other cell forms the wall of capillaries in lamellae is Pillar cell. Chloride cells which are as single cells without complex junction, are surrounded by the pavement cells, have high number of large mitochondria and extensive membranous tubular system, with a free and smooth surface. There is one type of chloride cell with a light cytoplasm. Conclusion: The main ultrastructure aspects of the major cells are similar in teleosts, although some features such as the localization of chloride cell on lamellae, one type of cell, small crypts and the smooth surface are special features of this cell in Alosa Caspio that resemble this cell to freshwater species.

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