The cytogenetic effect of heavy metal was studied in Clarias gariepinus using the micronucleus test, chromosomal aberrations and sister chromatid exchange. The fish were kept separately and treated with four different concentrations of mercuric chloride for a period of 7 days. For the micronucleus test blood samples were obtained from the caudal vein. The mean micronuclei frequencies were recorded as 0.20, 0.25, 0.50, 0.65 and 0.79 in controls and 1.0, 2.0, 4.0 and 6.0 ppm in groups treated

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... mercuric chloride, respectively. Kidney tissues were used for in vivo chromosome preparation. The mean frequencies of cells with chromosomal aberrations were 0.21, 0.32, 0.49, 0.70 and 0.97 in the control and 1, 3, 5 and 7 ppm in groups treated with mercuric chloride, respectively. Similarly, the mean frequencies of sister chromatid exchange were recorded as 0.10, 0.28, 0.47, 0.58 and 0.64 in controls and 1, 3, 5 and 7 ppm in groups treated with mercuric chloride, respectively. The findings of present experiment suggest that HgCl 2 caused genotoxic effects in fish.