This study aimed to investigate the toxic effects of atrazine herbicide on the fry of Caspian Kutum (Rutilus frisii kutum, Kamensky, 1901). First the 96-h LC50 of the fry were exposed to atrazine at the concentration of 24.95 ppm was determined. Then the toxicity of this herbicide on Caspian kutum fry exposed to the concentration of 12.47ppm (1/2 LC50), for four days was measured and compared with a control group. Comparison of the length, weight and condition factor showed no significant differences between atrazine exposed and control group. The concentration of Na+, K+, Ca2+, Mg2+ and Cl- in the whole body of fry in control and atrazine exposure groups were as the following order: Ca2+>K+> Na +> Cl- >Mg2+ and Ca2+>Na+>K+>Mg2+>Cl-, respectively. Results showed that the concentration of all these ions were higher in atrazine exposure group than control group, except for Cl-, and the only significant differences was found in Na+ concentration. Major histopathological effects of atrazine on the gills were hyperplasia and thickening of the filaments, separation of the pavement cells of the lamellae epithelium from the pillar cells and swelling of the epithelial cells. Results of the present study showed that atrazine could affect the ion composition of the body, and caused major damages in gill epithelium even at sublethal concentration and acute exposure, but had no effects on the growth parameters.