To elucidate the effect of yogurt containing Bifidobacterium lactis BB-12 (4×109 cfu/100 g, BB-12 yogurt) on the defecation for intestinal microflora,a placebo control cross-over trial was conducted. Twenty nine female adults (average age 22.0 years) were divided into two groups, and either of BB-12 yogurt or placebo yogurt without Bifidobacterium lactis BB-12 was administered at 100 g/day for 14 days. The defecation frequency (days) obviously increased during BB-12 yogurt period compared to placebo yogurt period in the subjects with mild constipation. The fecal microflora of 20 adults selected from 29 adults were studied. Increase of the number of Bifidobacterium and ratio of Bifidobacterium to total bacteria, decrease of the number of Bacteroidaceae and ratio of Bacteroidaceae to total bacteria, decrease of ratio of Enterobacteriaceae to total bacteria, improvement of fecal shape were obviously observed during BB-12 yogurt period compared to placebo yogurt period in the subjects with mild constipation. These results suggest that the yogurt containing Bifidobacterium lactis BB-12 shows to improve the defecation, intestinal microflora and intestinal environment.