Forty healthy individuals with a low defecation frequency were selected, and the effects of intake of a fermented milk beverage that contains Lactobacillus casei strain Shirota (LcS) at 4 × 10⁹ bacteria/bottle for 2 weeks (1 bottle/day) on the defecation frequency and intestinal microflora were evaluated by the placebo-controlled double-blind cross-over scheme. Defecation frequency both in times per week and days per week significantly increased in the LcS beverage period compared with the frequency before the beginning of intake. The differences were more notable in those with a stronger tendency to constipation (frequency of defecation before intake ≤ 4.0 times/week, n=21), and the frequency of defecation in the LcS beverage period was significantly higher than in the placebo period. The stool smell and feeling of completion of voiding improved significantly in the LcS beverage period compared with the placebo period, and in those with a stronger tendency to constipation, the stools were significantly softened compared with the state before intake. The number of bifidobacteria and their percentage in the total number of fecal bacteria in the LcS beverage period increased significantly compared with the levels before intake and were significantly higher than the values in the placebo period. No marked change due to the intake of the LcS beverage was observed in the other components of the microflora, the organic acid contents, stool pH, water content, or contents of putrefactive metabolites. These results suggest that intake of the probiotic fermented milk beverage conditions the intestines by improving the state of bowel movements and stool quality and increasing the fecal population level of bifidobacteria.