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Clinoptilolite efficiency on absorption of ammonia in rainbow trout was studied. The fish specimens weighted 9.5-21g and were exposed to four different concentrations of total ammonia as N-NH4 including: 10, 15, 20, and .25mg/l. A group of 13 fish was considered as control. Lethal concentration was determined after 24 hours. Under stable temperature and pH conditions (T= 16±1 C, pH= 7.7±1), the lethal concentration of total N-NH4 was 25mg/l (as ionized N-NH3=0.44mg/l based on temperature and pH). In lethal concentrations of ammonia, different amounts of zeolite (2, 5, 10, 13 and 15g/l) were used. Application of 15g/l of the zeolite prevented mortalities in the fish. A significant difference was found in reduction of total ammonia and total hardness through application of zeolite after 24 hours (P<0.01). The maximum mortalities occurred in early time of experiment. The histopathological lesions of gill, kidney and liver were studied. *Corresponding author