In recent years the simultaneous expression of morphological abnormalities of nuclei and micronuclei induction has received considerable attention as biomarker of clastogenicity. Employing MN induction the genotoxic potential of a broad spectrum carbamate Carbaryl pesticide in a freshwater fish Heteropneustes fossilis. LC50 of the pesticide was evaluated by probit method. It is 15.08mg/l. Fishes were exposed to 4 different concentrations (2mg/l, 4mg/l, 6mg/l & 8mg/l) of the test compounds for 24h, 48h,72h and 92 h of exposure. Blood samples of approximately were taken from each by cardiac puncture, fixed in methanol, treated with HCl and stained with schiff’s solution. It was found that treatment of fish with various concentrations resulted in dose and period of treatment related increase in the frequency of cells containing micronucleated erythrocytes. Statistically significant increase in the frequency of micronucleated erythrocytes, as compared to control, was recorded for all the treatments and duration of treatment except for 2mg/l.