Present study was designed to observe the effects of ⁶⁰Co gamma radiationin behavioral and histological changes in the gills of giant fresh water prawn Macrobrachium rosenbergii. The adult prawns were irradiated with four different dose levels (3 mGy,30 mGy,300 mGy and 3000 mGy) and the control group (without irradiation) was maintained separately. Behavioral changes like hyperactivity, loss of balance, reduced swimming rate, slower rate of food intake and convulsions were observed in higher dose levels of 300 mGy and 3000 mGy. The histological alterations such as accumulated haemocytesin haemocoelic spaces, abnormal gill tips, lifted lamellar epithelium, swollen and fused lamellae, hyperplasic, necrotic, clavate–globate and complete disorganization of lamellae were observed in ⁶⁰Co gamma irradiated prawns. Significantly more considerable histological alterations were observed in the highest dose level of 3000 mGy, but nomortality was evidenced. This study serves as biomonitoring tool to assess the radiation pollution in the aquatic environment.