The goal of this study was to determine the effect of bispyribac-sodium on the nitrogenase activities and growth of cyanobacteria isolated from paddy fields. Ten cyanobacterial species were used in this study. Cyanobacterial species were isolated from soil and water samples obtained from rice fields in Corum, Turkey. Among all Anabaena strains, the maximum activity was determined in Anabaena sp. O-22 (2.26 mu l ethylene/mg.h) whereas; the lowest activity was shown in Gloeothece sp. O-Y (0.04 mu l ethylene/mg.h). The maximum inhibition was seen in Anabaena sp. O-22, Synechocystis sp. O-X and Anabaena sp. O-16 in 100 mu g/ml bispyribac-sodium concentration. Although low bispyribac-sodium concentrations somewhat stimulated growths of Anabaena sp. O-X2, O-C, O-4, O-16, O-8 and Synechocystis sp. O-X, the biomass of all cultures were severely inhibited at higher concentrations. The growths of Anabaena sp. O-22 and Synechocystis sp. O-X completely repressed at 100 mu g/mL and at higher bispyribac-sodium concentrations, whereas, Anabaena O-X2, O-6, O-4 and O-16 completely suppressed at 500 mu g/mL bispyribac-sodium concentration. The end of the study Anabaena sp. O-22 has been proposed as biofertilizer. The results obtained may be useful for the production of rice.