In monoxenic mice, vegetative cells and spores of Bacillus licheniformis were enumerated and in vivo antibiotic production was measured at various levels of the digestive tract and in the feces. The apparent independence of vegetative cell and spore populations in the cecum and feces, as well as the marked fluctuations of these two populations in the stomach and small intestine, suggested that sporulation of B. licheniformis and production of antibiotic occur only in the upper levels of the digestive tract. Study of dixenic models wherein B. licheniformis was inoculated after Clostridium perfringens or Lactobacillus sp. or before Lactobacillus sp. revealed the simultaneous disappearance of B. licheniformis from the stomach and antibiotic from the feces. This strain persisted in the cecum, but only in the form of vegetative cells. These models demonstrate that the activity of the microflora in the upper segments can affect the equilibrium of the microflora throughout the digestive tract.