A partially hydrolyzed guar gum preparation (PHGG, average molecular weight: 20,000), obtained as a water-soluble dietary fiber by digestion of guar gum with beta-D-endomannanase, was administered as a beverage (11 g a day, bid) to 15 constipated women for 3 weeks. Defecating frequency, pH, weight, moisture, and bacterial flora of the feces were investigated and compared with the control periods. Average total dietary fiber taken from food was 9.7 +/- 0.1 g/day during the experiment. PHGG caused an increase in the defecating frequency from 0.46 +/- 0.05 (frequency/day, M +/- SE) to 0.63 +/- 0.05. Fecal moisture significantly increased from 69.1% in the control period to 73.8% by ingestion of PHGG. Fecal moisture content also increased consistent with lowering the pH of feces (r = -0.478). The frequency of Lactobacillus spp. occurrence in feces significantly increased (p < 0.05) compared with the control period. These results clearly indicate that PHGG softens and improves the output of feces.