The effect of levanheptaose produced by levanase from Streptomyces sp. 366L on principle intestinal microflora was investigated. The reaction product, levanheptaose, was used as a carbon source for various intestinal microflora. As a results, Bifidobacterium adolescentis, Lactobacillus acidophilus, and Eubacterium limosum grew effectively in the in vitro experiment, whereas Clostridium perfringens, E. coli, and Staphylococcus aureus did not. Therefore levanheptaose seems to promote selectively the growth of B. adolescentis and L. acidophilus. In the in vivo experiment, the effect of levanheptaose on the growth of intestinal microflora, $eta$-fructosidase activity, pH, and butyrate concentration were examined in rats. Apparently, the number of fecal Bifidobacteria, the amount of butyrate, and $eta$-fructosidase activity were increased, whereas total aerobes and pH were reduced in rats fed by levanheptaose diets, compared with those of control diets. We concluded that those effects may be beneficial in improving gastrointestinal health.