In order to investigate effect of glucooligosaccharide (GOS) prepared by extrusion process as a bifidogenic factor, cultivation of Bifidobacterium sp., Bacteroides fragilis and Clostridium perfringens was done and analyzed. B. fragilis and C. perfringens were able to utilize only 16% and 11% of the oligosaccharides in GOS, respectively, whilst Bifidobacterium sp. FBD-22 could utilize 38%. Especially, many kinds of oligo saccharides in GOS were able to be utilized selectively only by Bifidobacterium sp.. In case that GOS, as a carbon source, was used in the co-cultivation by Bifidobacterium sp., B. fragilis and C. perfringens, growth of Bifidobacterium sp. was not influenced by the existence of B. fragilis and C. perfringens. Bifidobacterium sp. showed advantage on carbon source competition for GOS with B. fragilis. Acetic acid, antimicrobial agent in the intestine, was produced two times more from GOS than glucose in co-cultures of three strains. Therefore, it is suggested that GOS can be a potent bifidogenic factor which proliferates the population of Bifidobacterium sp. and may finally improve the intestinal environments of human.