Escherichia coli K-12 Hfr and F- strains were successively implanted in axenic chicks. Conjugation with exchanges of chromosomal genes occurred with high frequencies in the gut of the chicks and could continue as long as fertile strains coexisted in this environment. Almost all of the expected recombinant types were recovered in the feces under these experimental conditions. Furthermore, these recombinants were analogous to those obtained after conjugations in vitro. Recombinants formed in the gut were more numerous when the F- strain was seeded before the Hfr strain. The recombinants showed no apparent selective advantage over the parental strains in the intestinal medium. They were maintained throughout the experimental period and represented more than 10% of the total intestinal flora. The chick gut is usually rapidly colonized by other bacterial types under natural conditions. The possible effects of other components of the bacterial flora on conjugation of E. coli in holoxenic animals will require subsequent work with more complex microbiological conditions.