Heavy metals are major pollutants in ground, air, marine and fresh water and they cause toxic effects in fish tissues. This study was conducted to determine histopathological effects and changes in carbonic anhy- drase (CA) enzyme activity in liver and gill tissues of brown trout juveniles exposed to chromium (Cr3+). Fish were exposed to chromium concentrations (1 mg/L and 2 mg/L) for 24, 48 and 96h. It was found that CA was significantly inhibited at 48 and 96h of two concentrations (p<0.05). Besides, it was observed that the most common liver changes produced by Cr3+ were hyperemia and hepatocyte degeneration. The gill tissues of fish exposed to Cr3+ were characterized by lamellar hyperemia, lamellar edema, clumping, epithelial degeneration and lamellar atrophy. The present study indicated that Cr3+ inhibits carbonic anhydrase enzyme and causes histopathological damages in gill and liver tissues.