The potential toxic effects of mancozeb exposure on some antioxidant enzyme were investigated on heart tissue of zebrafish in this study. Zebrafish groups were exposed to different doses of mancozeb (Group A: 5 mgL(-1) and Group B: 7.5 mgL(-1)) for 120 hours. In this study, catalase (CAT) activity, malondialdehyde (MDA) level and total protein (TP) level were determined with spectrophotometer. Our results showed that CAT activity was found 2.541 +/- 0.771 mg L-1 in A group and 2.011 +/- 0.201 mg L-1 in B group in this study. CAT and MDA activity levels decreased in the experiment group according to control group. MDA levels were found 0.025 +/- 0.003 mg L-1 in A group and 0.025 +/- 0.003 mg L-1 in B group. TP levels were found 9.75 +/- 1.51 mg L-1 in A group and 10.18 +/- 0.32 mg L-1 in B group. TP levels increased in the all experiment groups according to control group. We observed that the changes in the CAT activity and MDA levels were time and as well as mancozeb dose dependent. As a result, mancozeb is a very toxic substance for zebrafish and other aquatic organisms.