The effects of silver nanoparticles (AgNPs) and silver nitrate (AgNO3) on some histopathological lesions of gill and liver tissues in Zayandehrud Chub (P. esfahani) as a valuable ecological fish species were examined. To this aim, 100 individuals (average weight 26 g) were exposed to different concentrations (1 or 25 µg/L) of AgNPs (average hydrodynamic diameter 54.8 nm) or AgNO3under constant-renewal condition for 10 days. A control group containing 20 fish was also included in the study. At the end of the experiment, histopatological lesions of the tissues were assessed using classic histological techniques and quantification analysis was done using organ index calculation (I_org). Exposure to AgNPs or AgNO3, even at the low concentrations, could cause significant tissue lesions in fish in comparison to thethose in control group. Hyperplasia and edema in the gill tissue and hemorrhage and focal necrosis in the liver were the most severe lesions mainly after exposure to the highest doses of the chemicals. The lesions were significantly increased by the elevation of water-borne concentrations of both chemicals. More specifically, the highest I_org was observed in the fish treated with 25 µg/L AgNO3 followed by 25 µg/L AgNPs. The results showed relatively similar sensitivity of both tissues to the pollutants. However, the effects of AgNO3, especially at higher concentrations, was much stronger than that of AgNPs which consequentlyindicates higher toxicity effects of AgNO3 in comparison to AgNPs in Zayandehrud Chub.