The effect of bacteriocins produced by strains of Streptococcus mutans on the microbial composition of dental plaque was studied in gnotobiotic rats. In one set of tests, using S. mutans strains T2 and SW31, rats were simultaneously infected with the bacteriocinogenic parent (bac(+)) and corresponding bac(-) mutants, and the relative proportions of each strain present in three different tooth sites were determined at various time intervals after inoculation. Some animals were monoinfected with bac(+) and bac(-) strains, and in vivo properties, such as plaque fermentation end products, amount of plaque formed, and cariogenicity, were measured. In another series of tests with strains T2 bac(+) and T2 bac(-), animals were sequentially inoculated with a time interval of 1 week. On simultaneous inoculation, the bac(-) mutant, which was sensitive in vitro to the bacteriocin produced by the parent, was totally unable to establish in a plaque consisting of the bac(+) strain even when the challenging dose was increased 100-fold. In contrast, the bac(+) strain was successful not only in invading an established bac(-) plaque but also in eventually almost eliminating the mutant. In monoinfected animals, there were no significant differences in plaque-forming ability, fermentation end products, or cariogenicity between the two strains. Also, bacteriocin activity was detected in plaque harboring the bac(+) but not the bac(-) strain. The results not only indicate that bacteriocin activity can occur in vivo but also suggest that bacteriocinogeny might confer an ecological advantage upon a strain attempting to establish itself in an ecosystem.