Effects of carbon dioxide partial pressuree(PCO₂) and substrate compositions on the bacterial population, methane production rate and matter degradation in anaerobic digestion were investigated by using anaerobic chemostat type reactors at 35±1℃, at the HRT of 7 days. Two kinds of single substrate containing carbohydrate and protein, and mixed of these were used in this study. At PCO₂ of 0.5 atm, the specific methane production rate and specific substrate removal rate reached the maximum rates at the protein substrate reactor. At PCO₂ of 0.5 atm, the methane production rates in the reactors fed by carbohydrate, protein and mixed substrate were 20%, 29% and 26% higher than those obtained under the controlled condition, respectively. The number of acetate consuming methanogenic bacteria enumerated by the MPN(most probable number) method, decreased when PCO₂ exceeded 0.7 atm. Hydrogen consuming methanogenic bacteria and homoacetogenic bacteria increased as PCO₂ increased from 0.1 to 0.6 atm, however, decreased slightly at PCO₂ above 0.7 atm. The number of hydrolytic bacteria, sulfate-reducing bacteria and H₂-producing acetogenic bacteria were not much influenced by the change of PCO₂. The potential methanogenic activity reached the maximum at PCO₂ 0.5 atm, however, decreased significantly when PCO₂ exceeded 0.7 atm. It was considered that the inhibition of specific methane production rate at PCO₂ above 0.7 atm would depend on free CO₂ concentration in solution.