The intraoral establishment and proportional distribution of suspected periodontal pathogens Actinomyces viscosus and Actinomyces naeslundii were studied using a recently developed differential plating medium, CNAC-20. Saliva and dental plaque samples were collected from 108 subjects ranging in age from infants to young adults; tongue and buccal mucosa samples were collected from only the adult subjects. Catalase-negative A. naeslundii was isolated from 40% of the predentate infants' and almost all other subjects' saliva samples. It predominated among CNAC-20 isolates in the saliva of subjects of all age groups, in the plaques of young children, and in the adult tongue samples. In contrast, catalase-positive A. viscosus was not isolated from predentate infant samples, and its frequency of isolation increased slowly with age (greater than 50% detection by age 7). A. viscosus was isolated in highest relative proportions from dental plaque and buccal mucosa samples. The two closely related species A. viscosus and A. naeslundii apparently differ in respect to factors determining the host age at which they colonize and their relative intraoral distribution in humans.