Nile Tilapia,(Oreochromis niloticus L.,) is the most popular animal protein for Egyptian people due to their price and availability. Todays, a ll the production fish exposed to biotic and abiotic environmental stress, that cause huge damage for the fish and costumers. During the current research one common pesticide Lannat was used by different concentrations ie 0.8, 1.6 and 3.2 ppm, to investigate the possible genotoxic effect in aquatic organisms, measurement and alteration by the physicochemical properties of DNA and isolated chromatin from liver tissue and finally, investigating the chromosomal aberrations in vivo induction of sister chromatid exchange (SCEs). This work was carried out at the faculty of Agriculture Saba Basha during the period 2016-2017. The results showed that Mitotic activity in the gills of the control group was higher than in kidneys. In treatments, cells of gills displayed lower mitotic activity compared with that of kidneys. Different mitotic aberrations were recorded with high doses of Lannat such as stickiness and ring chromosomes, deletion, fragments, and Robertsonian Centric Fusion (RCF). Lannat showed that it can induce macro-DNA damage. Lysis of nucleus was also observed.