The first anaerobic cultivation of the gastrointestinal bacteria Eubacterium ramulus in a disposable bag reactor system is described. E. ramulus was cultivated in batches up to 10 L culture volume in a novel commercial disposable rocking motion bag reactor system for the first time. Optical densities about 5-fold higher compared to previously used bottle cultivations could be obtained. Produced biomass per batch (10 L bag cultivation) yielded up 60 to 70 g wet cell mass. Comparative batch cultivation with the same volume in a common stirred tank reactor was performed. It showed results similar to the cultivation in the bag reactor system regarding optical density and wet cell mass.