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Studies were conducted to determine whether intestinal cellulolytic bacteria could be introduced into the bovine rumen or pig large intestine. In the first study, the ruminal fluid of three cows was evacuated and replaced with 20 liters of buffer and 6 liters of the ruminal or swine cellulolytic organism Clostridium longisporum or Clostridium herbivorans, respectively. The introduced organisms were the predominant cellulolytic bacterium in the fluid (>107 cells ml21) at 0 h. C. longisporum was still the predominant cellulolytic organism after 5 h, at 0.55 3 107 cells ml21; however, after 24 h the count of C. longisporum decreased to 0.05 3 107 cells ml21 compared with 2.8 3 107 cells ml21 for the total cellulolytic organisms. After 48 h, C. longisporum was no longer detectable. C. herbivorans was identified in only one of the three cows after 24 h and was not detected at 72 h. In a second study, when C. longisporum (50 ml; 107 cells ml21) was infused into the terminal ileum of seven pigs, it was not recovered when fecal samples were evaluated at 24, 48, or 72 h after infusion. These studies emphasize the competition that must be overcome to successfully introduce organisms into an intestinal ecosystem. Furthermore, these studies suggest that C. longisporum is a transient organism in the bovine rumen; however, C. herbivorans is part of the normal intestinal flora of some pigs, although the role that it plays in fiber degradation in these pigs is unclear.