In 47 patients plaque samples were taken from sound and carious root surfaces. The carious surfaces were categorized as soft or hard. In total, 70 sound, 29 hard carious and 36 soft carious surfaces were sampled and analyzed for the presence of Streptococcus sanguis, Streptococcus mutans, Actinomyces viscosus/naeslundii, Lactobacillus and Capnocytophaga species. Samples from soft carious surfaces contained significantly higher numbers of S. mutans as compared to samples from sound surfaces. The numbers of S. mutans on hard carious and sound surfaces were approximately equal. For S. sanguis and A. viscosus/naeslundii no significant differences were found. The isolation frequency of Capnocytophaga was comparatively low. Lactobacilli were only occasionally found. The results indicate that, from the above species, only S. mutans is closely related with root surface caries.