Microbial population in soils was studied in relation to defluorinaton of organic fluorine compounds, monofluoroacetate (FA) and monofluoroacetamide (FAA). No significant difference in microbial pooulation was observed between the soils collected from FAA-sprayed and non-sprayed citrus orchards. Soils of the FA-producing factory contained more bacteria which are capable of growing in a medium containing FA or FAA as a sole source of carbon. The soils sprayed with relatively large amount of the pesticide ingredients contained more ingredient resistant bacteria, by 30 days after spraying, suggesting that a temporary seleection to the ingredient degradation occurs in soils. The defluorination in soils, however, was not as high as had been expected.