The aim of this study was to determine the effect of a probiotic, i.e. fermented milk prepared with Lactobacillus casei strain Shirota, on indigenous Lactobacilli in the pig large intestine. This fermented milk was given as a probiotic to experimental pigs for 2 weeks. The fecal organic acid concentration increased with the fermented milk; acetate and propionate increased significantly (p<0.05). At the same time, lactate and butyrate tended to increase. The fecal pH was significantly reduced by the fermented milk (p<0.05). Although the number of bacteria of strain Shirota in the intestinal contents was much smaller than those of indigenous Lactobacilli, 104 vs 10⁸ (cfu/g), the numbers of indigenous Lactobacilli and Bifidobacteria in the pig intestine appeared to increase with the fermented milk, In addition, the phenotypic diversity (phenotypic group numbers) of indigenous Lactobacilli increased from 3 to 8 with the fermented milk supplementation. Thus the fer-mented milk affected the indigenous Lactobacillus population and constitution.