A considerable amount of Persian sturgeon fingerlings has been cultured in hatcheries and releases into the Sea annually. High density of these fish in hatcheries and the importance of keeping their health for facing new environment and salinity conditions is a key to understanding the parasitology and pathologic studies. 100 number of 2-3g Persian sturgeon fingerlings were obtained from sturgeon hatchery. Based on histological observation of the gill, the unicellular parasite, Trichodina of Trichodinidae has been recognized. This parasite was observed in gill chamber, spiracle, buccal and pharyngeal chambers of the fingerlings. 15% of studied fish were contaminated to this parasite. Gill damaged by this parasite was especially to the capillaries by bleeding and clot making. Also due to the attachment of this parasite to gill lamellae and the tissue damages of the gill it could be interfere with respiration and osmoregulation of the infected fish.