Ammonia is one of the most harmful water quality parameters restricting the growth and survival of aquatic living organisms. As a result, fish must adapt to this stressor by modifying physiological processes that are governed by gene expression regulation. The aim of the present study was to investigate the expression profiles of antioxidant related gene, catalase (CAT) in common carp (Cyprinus carpio) after exposure to 0.7 mg/l of unionized ammonia (UIA) in water. The relative gene expression was measured in liver, gills, and brain tissues at four time points (12h, 2d, 4d, and 7d of exposure). The expression level of CAT gene in the liver and brain peaked after 7d of ammonia exposure by 13.3 and 5.2folds, respectively, but in gills it upregulated only after 2d (2.7folds) and downregulated at the other time points. This study proves that exposure to ammonia affects the antioxidant status of common carp as indicated by the altered levels of expression of the CAT gene.