Lactobacillus delbrueckii TU-1, which apparently takes intact inulin into its cells and then degrades it intracellularly, was co-cultured in vitro with L. paracasei KTN-5, an extracellular inulin degrader; or L. plantarum 22A-3, a strain that is able to utilize fructose but not inulin; or both in order to prequalify inulin as a prebiotic agent in vivo. When L. delbrueckii TU-1 was co-cultured with L. paracasei KTN-5 on fructose or inulin, the growth of L. delbrueckii TU-1 on inulin was markedly higher than that of L. paracasei KTN-5, whereas the growth of L. delbrueckii TU-1 on fructose was much lower than that of L. paracasei KTN-5. These results suggest that L. delbrueckii TU-1 and L. paracasei KTN-5 were efficient at utilizing inulin and fructose, respectively. When L. plantarum 22A-3 was co-cultured with L. delbrueckii TU-1 on inulin, the growth of L. plantarum 22A-3 was enhanced by L. paracasei KTN-5 but not by L. delbrueckii TU-1, suggesting that the fructose moiety that L. paracasei KTN-5 released temporarily into the medium was "scavenged" by L. plantarum 22A-3. Thus, L. delbrueckii TU-1, L. paracasei KTN-5, and L. plantarum 22A-3 were then cultured altogether on inulin. The growth of L. delbrueckii TU-1 was unaffected but that of L. paracasei KTN-5 was markedly suppressed. This evidence suggests that prebiotic use of inulin supported the selective growth of intracellular inulin degraders such as L. delbrueckii rather than extracellular inulin degraders such as L. paracasei in the host microbiota.