The occurrence of heterotrophic CO2 fixation by soil microorganisms was tested in several mineral soils differing in PH and two artificial soils (a mixture of silica sand, alfalfa powder, and nutrient medium inoculated with a soil suspension). Soils were incubated at ambient (similar to 0.05 vol%) and elevated (similar to 5 vol%) CO2 concentrations under aerobic conditions for up to 21 days. CO2 fixation was detected using either a technique for determining the natural abundance of C-13 or by measuring the distribution of labeled C-14-CO2 in soil and bacteria. The effects of elevated CO2 on microbial biomass (direct counts, chloroform fumigation extraction method), composition of microbial community (phospholipid fatty acids), microbial activity (respiration, dehydrogenase activity), and turnover rate were also measured. Heterotrophic CO2 fixation was proven in all soils under study, being higher in neutral soils. The main portion of the fixed CO2 (98-99%) was found in extracellular metabolites while only similar to 1% CO2 was incorporated into microbial cells. High CO2 concentration always induced an increase in microbial activity, changes in the composition of the microbial community, and a decrease in microbial turnover. The results suggest that heterotrophic CO2 fixation could be a widespread process in soils.