We grouped 20 isolates of lactobacilli from the stomach of conventional rats, 21 isolates from pig stomachs, and 19 isolates from the crop of fowls according to their ability to ferment N-acetylglucosamine, dextrin, cellobiose, gum arabic, and xylan. Most of the isolates did not resemble previously describedLactobacillus species. Representative group A isolates were associated with germ-free mice. Only a rat isolate was able to colonize the keratinized squamous epithelium of the stomach of gnotobiotic mice.