Lactobacilli comprise a small percentage of the normal oral microbial flora of humans and are isolated commonly from saliva and frequently from an active caries lesion. We have compared the pathogenesis and colonization pattern of Lactobacillus casei with that of Streptococcus mutans strain 6715 in gnotobiotic rats. Of the two L. casei strains tested, L. casei strain ATCC 4646 caused slightly more caries than L. casei strain ATCC 11578. However, the level of caries induced by either L. casei strain was significantly lower (P less than 0.01) than that observed in similar-aged rats monoassociated with S. mutans strain 6715. When groups of rats were infected with mixtures of L. casei strain ATCC 4646 and S. mutans strain 6715, or with L. casei followed by S. mutans, higher numbers of L. casei than S. mutans were found associated with the tongue and in saliva; S. mutans always predominated in plaque. The level of caries observed in these groups of rats was similar to that seen with rats monoassociated with S. mutans except when L. casei comprised greater than 1% of the plaque microflora. In this latter situation, the level of caries was significantly lower (P less than or equal to 0.05) than that obtained in S. mutans-monoassociated rats. The results of this study suggest that L. casei colonizes sites in the oral cavity (including the tongue and saliva) other than the tooth surface in rats. The effect of L. casei in plaque toward reduction of S. mutans-induced dental caries in rats is discussed.