Blue-green algae make a valuable contribution to the nitrogenous soil fertility by fixing atmospheric nitrogen and are of great agricultural importance as biofertilizer. Four commercial grade pesticides, Carbamate, Furadan and Sevin and Organophosphate, Rogor and Organochlorine, Endotaf were used to study their effect on the survivability and qualitative occurrence of blue-green algae by taking survival on the control plate as 100%. Stock solutions of these pesticides were prepared freshly for

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... xperiments in the sterilized BG-11 media and added to the culture media to obtain the desired concentrations of 100, 250, 500 and 1000 ppm. After 30 days of incubation, the blue-green algal forms appeared in the culture flask were identified using standard monographs. The pragmatic results indicated that, soil blue-green algae show variable resistance to pesticide treatments. The survival percentage of the tested blue-green algae was reduced upto 50 percent at 500 ppm of carbamate pesticides, Furadan and Sevin. While with Rogor only 10.52% and Endotaf 5.26% survivability was noticed at 500 ppm dose level. Furthermore, the organochlorine pesticide Endotaf was more toxic than the organophosphate, Rogor and the carbamates, Sevin and Furadan in sequence to the blue-green algae. It was cocluded that higher doses of pesticide application in the crop fields i.e. more than 100 ppm of Furadan, Sevin and Rogor and even at 100 ppm of Endotaf, qualitative and quantitative occurrence of heterocystous and non- heterocystous blue-green algae was decreased considerably. This suggests that, indiscriminate use of these pesticides may cause adverse effects on the nitrogen fixing blue-green algae of various crop fields, which has a direct influence on total productivity. In general, the sensitiveness of different blue-green algae to pesticide application was found to be more in sheathless heterocystous and unicellular forms than the heterocystous ensheathed and non- heterocystous ensheathed forms.