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Combustion of fossil fuels and resultant emission of carbon dioxide has led to increased global temperature. Since cyanobacteria are an integral component of the paddy field microflora and contribute to nitrogen fixation, increase in temperature may adversely affect the nitrogen dynamics of the soil. Therefore, to understand the physiological and biochemical response of the mesophilic diazotrophic cyanobacterium Anabaena doliolum to elevated temperature, the organism was grown under three temperature regimes 30, 35 and 40 degrees C for 15 days. Exposure of the cyanobacterium to 40 degrees C resulted in severe reduction in growth and cellular constituents as compared to the cells exposed to 35 degrees C. The cyanobacterial cells also showed enhanced production of H2O2 and lipid peroxidation products in response to exposure to elevated temperature. Further, we observed increased activity of superoxide dismutase, catalase and peroxidase in A. doliolum exposed to elevated temperature. Increase in the temperature resulted in enhanced level of non-enzymatic antioxidants such as carotenoid, proline and ascorbate. Although, the number of heterocysts increased in response to temperature, the nitrogenase activity decreased significantly. The results have demonstrated the sensitivity of the cyanobacterium A. doliolum to elevated temperature.