The study aimed to investigate the effect of fungicides chlorothalonil at different concentrations ( 0.1 , 0.5 , 5 , 25 , 50 ) × 10 - 5 M on some cytogenetic parameters of human peripheral blood lymphocytes . The genotoxicity parameters were estimated by the number of chromosomal aberrations (CAs) and their types and by estimating the induced micronuclei (Mn) . Cytotoxic effect recorded by estimating the mitotic index (MI) . Results revealed that the fungicide increased the CAs in dose – response pattern with positive correlation coefficient ( r = + 0.964) , there was a significant differences among the concentrations (P<0.01) . The major CAs records chromosomal breakage at concentrations. 0.5 , 5 , 25 , and 50 , while the lowest concentration ( 0.1) showed no abnormalities . Dicentric and ring chromosomes appeared at high concentration (25) and were (1 ± 0.06) which increased significantly upon duplication of concentration ( i.e., 50) in which another abnormality appeared and this was acentric chromosomes .Mn increased propotionally with increasing concentrations with positive correlation coefficient ( r = + 0.91 ) , but the value recorded for the lowest concentration (0.1) was non significant compared to control treatment .The percentage of MI were lower by increasing chlorothalonil concentration with a significant difference (P<0.01) although the decrease was not strongly