Characteristics of an exopolysaccharide (EPS) produced by a mutant strain of Lactobacillus plantarum, strain 301102S, including yield from whey, monosaccharide analysis, and prebiotic activity, were investigated. The EPS production in whey was higher at 25°C than at 30 and 37°C. The supplementation with yeast extract and glucose in whey gave a high EPS yield of 145 mg/L, and the EPS contained glucose and mannose (molar ratio, 1:2). Prebiotic activities of galactooligosaccharide, inulin and the EPS with 37 lactic acid bacteria strains were investigated, and prebiotic activity of the EPS was high with the parent strain. This suggests that the EPS produced in whey by the mutant strain is easier utilized by strain 301102 than the other strains, and the EPS produced by this strain is capable for use as a prebiotic.