Chlorpyrifos is an organophosphate pesticide widely used in agriculture and aquaculture. This study investigated its effects on carbonic anhydrase (CA) enzyme activity and histopathology of rainbow trout gill and liver. The fish were exposed to 2.25 (25 % of 96 h LC ), 4.5 (50 % of 96 h LC ), and 6.75 μg L (75 % of 96 h LC ) of chlorpyrifos for 24, 48, 72, and 96 h. CA activity was measured in liver and gills and histopathological changes were examined by light microscopy. The most common liver changes at most of the chlorpyrifos concentrations were hyperaemia and degenerative changes. Gill tissues were characterised by lamellar hyperaemia, lamellar oedemas, clumping, cellular degeneration, hyperplasia, and lamellar atrophy. CA enzyme activity in the gills decreased at all concentrations at 48, 72, and 96 h after exposure to chlorpyrifos (p<0.05). Similarly, there was a time-dependent decrease in CA activity at all of the concentrations in liver tissues (p<0.05). The present study indicated that chlorpyrifos inhibits CA enzyme activity and causes histopathological damage in gill and liver tissues Klorpirifos je organofosforni pesticid široke primjene u poljoprivredi i ribarstvu. U ovome radu istražili smo njegov učinak na aktivnost enzima ugljikove anhidraze te histopatologiju škrga i jetre u kalifornijske pastrve. Ribe su bile izložene klorpirifosu u koncentracijama 2,25 μg L-1 (25 % 96-satnog LC50), 4,5 μg L-1 (50 % 96-satnog LC50) i 6,75 μg L-1 (75 % 96-satnog LC50) tijekom 24, 48, 72 i 96 sati. Aktivnost ugljikove anhidraze mjerena je u jetri i škrgama, a histopatološke promjene promatrane su svjetlosnom mikroskopijom. Najčešće promjene u jetri pri većini koncentracija bile su hiperemija i degenerativne promjene. Na tkivu škrga primijećeni su hiperemija i edemi u škržnim listićima, sljepljivanje i degeneracija stanica, hiperplazija te atrofija škržnih listića. Aktivnost ugljikove anhidraze u škrgama smanjila se pri svim koncentracijama nakon 48, 72 i 96 sati izloženosti (p<0.05). Također je uočeno i smanjenje aktivnosti ugljikove anhidraze u jetri ovisno o duljini izloženosti pri svim koncentracijama (p<0.05). Dobiveni rezultati upućuju na to da klorpirifos inhibira aktivnost ugljikove anhidraze i izaziva značajna histopatološka oštećenja u škrgama i jetri