The importance of the colonic microflora in health and nutrition is well known, but how they colonize and become established in the colon is not well understood. We therefore characterized the quantitative and qualitative changes of the colonic microflora during the first 120 days of postnatal development. Unlike previous studies, changes were defined for individual pigs using in situ samples collected anaerobically and aseptically from the distal colon. Although the colons were sterile at birth, they were rapidly colonized, and within 12 h bacterial densities had stabilized at 10−9-1010 bacteria/g colonic content. Facultative anaerobes, notably coliforms, initially dominated the microflora, but were supplanted within 48 h after birth by obligate anaerobes, which constituted greater than 90% of the microflora thereafter. Bacteroides spp., the predominant anaerobes in the adult colon, did not markedly increase in abundance until after weaning and were still increasing by postnatal day 120. Shifts in the relative abundances of different bacterial populations throughout the first 120 days after birth confirm previous reports that the establishment of the adult colonic microflora is a gradual, sequential process, and highlight the need to focus research on anaerobic groups. © 1993 S. Karger AG, Basel.