Oral colonization by A. viscosus, strain T-6 was studied in Osborne-Mendel rats which were fed either sucrose, glucose, starch or casein alone ad libitum by mouth. Essential nutrients were given to the rats twice daily by stomach tube. All three carbohydrates supported implantation by Actinomyces to varying degrees. Implantation did not occur when the animals were fed casein. Casein did not have any antimicrobial effect on Actinomyces already established in the rat mouth; it was impossible to eliminate an established Actinomyces flora. Feeding the animals all the dietary components through the stomach tube did not result in eradication of Actinomyces from an established flora. The Actinomyces was considerably diminished when coprophagy was prevented.