It is known that heavy metals have toxic effects on fish. Insufficient measures are a serious problem in our country and around the world. This problem can threaten human health in areas where it is common for people to obtain nutrition from local bodies of water. In this study, the toxicological impacts of some heavy metals were investigated on carbonic anhydrase activity in gilthead gills. Carbonic anhydrase (CA) was purified from gilthead sea bream (Sparus aurata) gills with a specific activity of 2872.92 EU mg(-1) and a yield of 32.84% using affinity chromatography. The overall purification was approximately similar to 84-fold. SDS-polyacrylamide gel electrophoresis showed a single band, and the MW was approximately 30.5 kDa (Soyut et al., 2008, 2012; Soyut and Beydemir, 2008, 2012; Kaya et al., 2013). The kinetic and characteristic properties of CA such as the optimum pH, stable pH, optimum temperature, activation energy (E-a), activation enthalpy (Delta H), Q(10), K-m and V-max were determined. Cadmium (Cd2+), copper (Cu2+), nickel (Ni2+) and silver (Ag+) inhibited CA activity in in vitro conditions. K-i values were calculated for these metals. K-i values were 31.20 mM for cadmium (Cd2+), 161.96 mM for copper (Cu2+), 10.79 mM for nickel (Ni2+) and 0.0082 mM for silver (Ag+) based on Lineweaver-Burk plots. Except for cadmium, heavy metals had the same inhibition mechanism. Cadmium was competitive, and the others were noncompetitive. (C) 2015 Elsevier B.V. All rights reserved.