Eight strains of lactobacilli isolated from conventional mice, rats, fowls, and pigs were tested for their ability to colonize squamous epithelia in the gastrointestinal tracts of gnotobiotic animals. Two of the strains isolated from rodents could colonize the nonsecretory epithelium of the stomach of gnotobiotic rodents, and one strain isolated from a fowl could colonize the crop epithelium of gnotobiotic chickens. The specificity of colonization of squamous epithelia by lactobacilli was demonstrated by these in vivo studies; strains of lactobacilli isolated from rodents could only colonize rodent epithelium; the strain from a fowl could only colonize crop epithelium.