As an industrial synthetic chemical, bisphenol A (BPA) has the potential to have physiologically and histologically adverse effects on aquatic organisms. BPA causes the reproductive disrupting of all vertebrates due to its degradation on endocrine system. Therefore, the effect of BPA on fish with high economic value is an important issue. This study focused weekly on long-term BPA exposure on rainbow trout (Oncorhynchus mykiss). Hematological, biochemical, antioxidant activity and histopathological examinations were performed on O. mykiss exposed to 1000 mu g/L BPA concentration. Mortality was observed in the BPA group during the first three weeks. As a result of hematological studies, leukocyte count and hemoglobin in the BPA group were significantly higher in the first three weeks compared to the control group. Plasma cortisol level as a biochemical indicator showed a similar trend to leukocyte and hemoglobin. There was no significant difference between BPA and control groups in terms of superoxide dismutase and catalase. However, glutathione peroxidase activity in the BPA group was significantly lower than in the control group for four weeks. At the end of the study, many alterations were observed histologically in the gill tissues. While more intense hyperplasia and epithelial lifting were observed in the tissues in the BPA group compared to the control group, vacuolization, necrosis and hypertrophy were observed infrequently. In conclusion, this study argues that BPA causes negative effects on O. mykiss in terms of fish welfare and future study should be focused on its environmental adaptation with color preference patterns.