This study was carried out to investigate the toxic action of herbicide, paraquat(1,1`-dimethyl-4,4`-bipyridyliumdichloride), against microorganisms. The toxic effect of paraquat was observed mainly using Escherichia coli(KCTC 1039), as follows; Growth of aerobic microorganisms which comprise 4 strains of bacteria and 2 strains of yeast and 4 strains of mold was inhibited drastically in the presence of 1.0 mM paraquat. But the growth of anaerobic bacteria was not affected by the chemical. When actively growing cells of E. coli were exposed to the paraquat at the concentration higher than 1.0 mM, they rapidly lost their ability to form colony and clearly formed inhibitory zone by well test. More than 50% of the cells were killed by 1.0 mM paraquat treatment, even at immediate addition of paraquat to the medium.