The research results presented in this chapter concern the possible usage of microorganisms in measurements of CO₂ in subsurface environment. Microbiological tests of soil samples from the Muszyna region, which is an area of natural CO₂ exhalations (southern Poland), and the Ketzin region, which is an area containing an underground CO₂ storage facility (near Berlin, Germany), were run. Quantitative and qualitative compositional analysis of microflora present in investigated samples allowed the selection of bacteria whose proliferation reflects the availability of CO₂. Clostridium kluyveri bacteria and the nitrification process can be applied as indicators to estimate the amount of CO₂ in soil gas. Thiobacillus bacteria also appear to be valuable as indicators of increased amounts of CO₂ in soil gas. Based on changes in the Thiobacillus population present on pyrite surfaces, we suggest herein that a biosensor can be constructed to measure the quantity of CO₂.