The purpose of this research is to determine ecotoxicological effects of dodine (n-dodecylguanidini acetate) on aquatic environments. Though dodine is widely used as a fungicide in agriculture, but there is no much data about its ecotoxicology. In this regard, we investigated bioaccumulation levels and histological alterations on the tissues of muscle, liver and gills in Rainbow Trout (Oncorhynchus mykiss) against different doses (0.01, 0.1, 0.5 and 1mg/L) of Dodine exposure. The tissues of fish were extracted according to QUECHERS method and analyzed by mass spectrometer (LC-MS-MS). Neither of the applied dodine doses resulted in killing 50% of the total individuals in the experimental groups. However, 48hours after doses, behaviors such as instability, anomaly in swimming or sudden jumping movements were observed. Histological results of the study showed deteriorations of the radiological pattern of hepatocytes, sinusoidal dilatations, hemorrhages, edemas, mononuclear cell infiltrations, vascular congestions, hyperplasia and hypertrophy in liver, gill and muscle tissues. Accumulation of dodine in tissues correlated with increase of dose. The maximum level of active substance accumulation in tissues were measured 96hours after application of 1mg/L dodine dose -in order- in gills, muscles and liver. The accumulations were statistically significant (p<0.05) when compared with control group.