Examination was made of the fluctuations of numbers of the total bacteria and copper (Cu)-resistant bacteria with soluble/exchangeable Cu (Ex-Cu) fraction in three types of soils spiked with Cu at four concentrations. Drastic increase in Cu-resistant bacteria was observed in three soils spiked with 20 mmol Cu kg(-1) after 2 weeks of incubation, indicating the strong selection of individuals originally resistant to Cu. Adaptation and proliferation of bacteria were also observed in the soil environment under the long-term exposure to extremely high concentration of Cu (800 mg kg(-1) soil of Ex-Cu), deriving from the development of Cu resistance. These bacterial fluctuations and the toxic effects of Cu depended on soil types, due to the chemical forms in which Cu occurs. It was also found that the ratio of Cu-resistant bacterial number to total bacteria was significantly correlated with the amount of Ex-Cu in the soils. This sensitivity-resistance index seems to be useful for evaluating the toxic effects of Cu on the soil bacterial community. Whereas the toxicity of Cu depended on the soil properties, they also changed with time. This phenomenon can be explained by the decrease in the most labile Cu phase, Ex-Cu, with time in the soils.