This work describes the effect of mixed culture on reduction of carbon dioxide into methane during biogas production. Various mixed cultures, such as IMS, IWT, IRP, and IMW1, were isolated from marshy soil, decaying woody tissue, rice paddy soil, and mixture of activated sludge from municipal waste treatment plant along with a cow dung-based biogas plant (IMW1), respectively, and separately grown in cow dung slurry. Combinations of these mixed inocula were used for biogas production at 35 degrees C in a 1.5 L batch reactor. A relatively high average yield of methane (0.345 Lg(-1) volatile solid), low average yield of carbon dioxide (0.131 gL(-1) VS), and maximum 72% methane and 23% carbon dioxide content in biogas during 25-30 days was obtained with inoculum IRP. Whereas a corresponding average yield of methane (0.313, 0.315, 0.308 Lg(-1) VS), carbon dioxide (0.164, 0.158, and 0.167 Lg(-1) VS), and average methane content (70, 68, and 68% during 25-30 d) were observed with inocula IMS, IWT, and IMW1, respectively. Experimental results found that a 21.5% reduction in the average yield of carbon dioxide along with a 12% increase in average yield of methane and 4% increase in methane content (25-30 d) in biogas was observed with IRP, whereas a 1.8 and 5.4% reduction of carbon dioxide was found with a small increase in average methane yield 2 and 2.3% and in methane content 2% and no change (25-30 d) was observed with IMS and IWT mixed cultures as compared to IMW1 inoculum. These results of the experiments indicate that mixed microbial culture (IRP) can be used for improved quality biogas (72% methane and 23% carbon dioxide) production.