Horses, like all animals, are born without the symbiotic microbes that occupy the gastrointestinal tracts of mature animals. As grazing animals, horses rely on these microbes to fully utilize the grasses and other cellulosic feeds that they consume. Thus, colonization of the foal's gastrointestinal tract must occur between birth and weaning. The feces of nine mare and foal pairs were sampled from the day of parturition until 12 weeks of age, and the samples were analyzed by polymerase chain reaction amplification of the bacterial 16S rRNA gene and denaturing gradient gel electrophoresis (PCR-DGGE). The gels from feces of day (d) 0 foals had no or very few (x = 3, n = 6) bands, which indicates that species richness was low. The number of bands increased during the first 4 days of life, and by d 14 the foals and mares had similar numbers of bands (x = 28, n = 23). Some bands were present in young foals, but not in mares or in foals on d 42 or d 84, which indicated succession of bacterial species. When the PCR-DGGE profiles were compared with Dice's algorithm, all mare-foal pairwise similarities on d 14 and later were as great as the pairwise similarities between mares. These results are consistent with the idea that foals are born with a sterile gut, colonization proceeds rapidly, and a mature microbial community is present in the first few weeks of life.