The potential effect of herbicide quinclorac on culturable microorganisms was investigated in a flooded paddy soil added with 0.33, 0.67, 1.00, 1.33, and 2.00 microg quinclorac x g(-1) dried soil, by a 10-fold serial dilution plate technique for total soil aerobic bacteria, actinomycetes and fungi, by three-tube anaerobic most-probable-number (MPN) method with anaerobic liquid enrichment media for anaerobic fermentative bacteria (AFB), denitrifying bacteria (DNB) and hydrogen-producing acetogenic bacteria (HPAB), and by the rolling tube method in triplicate for methanogenic bacteria (MB) and nitrogen-fixing bacteria (NFB). The results showed that there were some differences in the aerobic heterotrophic bacteria, AFB and DNB between soils supplemented with quinclorac and non-quinclorac at the early stage of incubation, but none of them was persistent. The numbers of fungi and DNB were increased in soil samples treated with lower than 1.33 microg quinclorac x g(-1) dried soil, while the CFU of fungi and HPAB were decreased in soil samples treated by higher than 1.33 microg quinclorac x g(-1) dried soil. The population of actinomycete declined in negative proportion to the concentrations of quinclorac applied after 4 days, and the application of quinclorac greatly stimulated the growth of AFB and NFB. MB was more sensitive to quinclorac than the others, but its number in the soil samples with lower concentrations of quinclorac was nearly equal to that in the control on the 33rd d. It could be concluded that quinclorac was safe to the soil microorganisms when applied at normal concentrations (0.67 microg x g(-1) dried soil).