The effects of the fermented soybean product "Natto" on the composition and metabolic activity were studied in seven healthy volunteers (22-49 years of age) who ingested 50g of Natto/day for 14 days. During Natto consumption, the counts of Bacillus subtilis (B. natto; p<0.001) and Bifidobacterium (p<0.05) were significantly increased, except for the numbers of Bifidobacterium in two volunteers, whereas the counts and the frequency of occurrence of lecithinase-positive clostridia (p<0.05), including Clostridium perfringens were significantly decreased, when compared to the values before the consumption.The decreased tendency in the counts of Enterobacteriaceae and the increased tendency in the detection rate of B. subtilis were observed during the consumption, compared to the values before the consumption.No detectable changes occurred in the counts of other organisms throughout the experimental periods.The amounts of fecal acetic acid (p<0.05) during the consumption, and total organic acids and succinic acid (p<0.05) on day 14 of the consumption were significantly increased when compared to the values before and after the consumption.Fecal concentrations of phenol, ethylphenol, and skatol(p<0.05) were significantly decreased during the consumption. Fecal ammonia and cresol (p<0.05) were significantly decreased on day 14 of the consumption. Fecal pH values (p<0.05) were significantly decreased on day 14 of the consumption. The odor of the feces was slightly reduced during the consumption.