Samples were collected from paddy fields in Corum-TURKIYE. Nitrogen-free BG-11 medium was used for isolation of nitrogen fixing cyanobacteria. Acetylene reduction technique was used to determine the effects of different chemical agents on the nitrog enase activities of t he cyanobacteria, which wer e identified at the genus level. Nodularia showed the highest nitrogenase activity (0.006 ethylene µl/ mg.h) at 10mM nitrate concentration. At 25mM phosphate concentration, Nodularia showed the highest nitrogenase activity (0.006 ethylene µl/ mg.h ). The highest tolerances for the herbicides were present in Nodularia (0.06 ethylene µl/ mg.h) for bensulphuron (50µg /ml) and Nostoc 6 ethylene µl/ mg.h (for molinate 100µg / ml)