The objective of the present study was to detect the erythrocyte alterations in freshwater teleost fish Heteropneustes fossilis (Bloch) exposed to different sub-lethal concentrations of organophosphate pesticide Nuvan on different time intervals. The 96 h LC50 value of Nuvan was estimated by using logdose probit regression line method. On the basis of LC50 value, the sub-lethal concentrations were determined as 0.26 mg/L, 0.32 mg/L and 0.43 mg/L which are 1/25, 1/20 and 1/15 of LC50 respectively. Well acclimated fishes were exposed to these different concentrations of Nuvan for 60 days. The fishes from both control and treated group were sacrificed after 7, 15, 30 and 60 days and blood samples were collected. Total erythrocyte count (TEC) was calculated by standard method and blood smear slides were prepared in both the groups. It was observed that total erythrocyte count (TEC) in pesticide treated fish decreases besides they show morphological changes when compared to their control sets.