A variety of responses were noticed when examining the influence of the rice field herbicide benthiocarb (saturn) on Gloeocapsa sp. The herbicide affected growth and cellular Chl a content of the Cyanobacterium. This suggested investigations on some other metabolic activities. Oxygen evolution was reduced by 20-55% in response to treatment with low herbicide concentrations (4-I0 ppm, respectively). The process was time dependent. The rate of protein synthesis was highly sensitive to benthiocarb at low concentrations, whereas high concentrations caused an impairment of the protein synthesis process and a reduction in the nitrogen fixation ability of the Cyanobacterium. The results are of considerable significance, since the applied concentrations are less than normal field application.