Germ-free chicks hatched in flexible film gnotobiotic isolators were dosed orally at 2 days of age with a pure culture of Lactobacillus acidophilus. The chicks monoassociated with L. acidophilus were killed at 2 weeks of age to obtain gastrointestinal tract specimens for histological sectioning. Both light microscopy and transmission electron microscopy were performed to determine colonization and adhesion of lactobacilli. This strain of L. acidophilus was found associated with epithelium from three segments of the gastrointestinal tract (crop, proventriculus, and duodenum) of the gnotobiotic chick. The electron micrographs showed not only a close relationship between the Lactobacillus organism and the crop epithelia, but also an attachment through physical contact.