We estimated and compared the action of three selected strains of bifidobacteria in a semi-synthetic medium for different degrees of polymerization of fructo-oligosaccharides contained in three commercial products derived from chicory inulin: Fibrulose F97 (shorter chains), Fibruline Instant (native chains), Fibruline LC (longer chains). Biomass and production of lactate and acetate were greater when the substrate contained mostly shorter chain fructo-oligosaccharides. Shorter chains were first to be consumed, and one strain could use longer chains. As the degree of polymerization increased, residual fructo-oligosaccharides increased after growth of the strains, and the rate of consumption of fructo-oligosaccharides decreased.