Development of intestinal flora in newborn human-flora-associated (HFA) mice was compared with that in newborn conventional (CV) mice. Facultative anaerobes were detected from the first day after birth in both CV and HFA mice but anaerobes were not detected in the first week. Anaerobes rapidly increased from the 2nd week after birth and became predominant in newborn intestine. Most of the intestinal bacteria in adult CV and HFA mice were colonized in the intestine of CV and HFA mice, respectively, within 3 weeks after birth. The human intestinal flora established in the intestine of HFA mice finally reproduced without any remarkable change in composition in the intestine of newborn HFA mice. The development of intestinal flora in HFA mice was similar to that in CV mice but not that in human infants. These results indicated that human flora associated in HFA mice could be transferred from mothers to their offspring although HFA mice could not simulate the development of intestinal flora of the human infant.