The levels of selected cariogenic and obligately anaerobic bacteria have been compared in carious dentine taken from fissures, smooth surfaces, and root surfaces. The numbers of infected dentinal tubules were determined by immunohistological staining using species-specific antisera against selected bacteria. Selective localization was observed; mutans streptococci were the predominant bacteria in dentine from fissures and smooth surface coronal caries, but not from root surface caries. The proportion of mutans streptococci was higher in the shallow and middle layers of dentine from fissures and smooth surface coronal carious lesions than from the deep layers. In root surface caries, Actinomyces spp. were the predominant bacteria, and were present at higher levels in the deep layer of root lesions than in the shallow and middle layers. The proportion of lactobacillus spp. was relatively low despite its high detection frequency in all layers of the three types of carious lesion. Immunohistological staining with species-specific antisera was able to reveal the distribution and the localization of various bacteria in carious dentine.