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Ruminococcus albus represents a significant proportion of culturable rumen bacteria and contribute to fiber degradation and various substrates utilization in the rumen. This study was to investigate the interaction between fibrolytic R. albus KU-F152 and non-fibrolytic S. ruminantium S137 on the improvement of fiber digestibility and fermentation products of rice straw and para grass. In the present study, dry matter (DM) and neutral detergent fiber (NDF) digestion, ammonia-nitrogen (NH3-N) and volatile fatty acids (VFA) products were examined in the basal medium and mixed rumen microflora using rice straw and papa grass as substrates for 72 h incubation. The data analysis was used to 2×5 factorial in completely randomized design. The results showed that coculture of R. albus KU-F152 with S. ruminantium S137 had higher DM and NDF digestibility of rice straw and para grass compared with monoculture (P<0.01). In addition, coculture of R. albus KU-F152 with S. ruminantium S137 showed significantly higher NH3-N concentration difference for all the fiber sources than R. albus KU-F152. Bacterial monocultures significantly lowered acetate production (P<0.01) and no differences were found (P>0.05) in VFA concentrations between rice straw and para grass of basal medium and mixed rumen microflora. This finding suggests that the combination of R. albus KU-F152 with S. ruminantium S137 can improve fiber digestibility and increase the fermentation product. However, further studies are required to develop and apply coculture of fibrolytic R. albus KU-F152 with nonfibrolytic S. ruminantium S137 in in vivo study.