Reference strains of A. israelii were orally implanted in germ-free rats after multiple inoculations. Early colonization was associated with areas of interdental hair and bedding impactions. Largest populations of organisms were isolated from teeth, as compared to the tongue, cheek, or palate. Examination of molar teeth with the scanning electron microscope revealed bacterial accumulations in the occlusal fissures, but not on smooth or approximal surfaces. Extensive plaque formation after 100-150 d was accompanied by root-surface caries and bacterial invasion of the pulp. There was no evidence of bacterial penetration of the dentin underlying surface enamel.