The experiments were designed to study the effects of fluoride on the competition between a fluoride-sensitive strain of Streptococcus mutans (C180-2) and a fluoride-resistant mutant (S. mutans C180-2FR) in rat dental plaque. Competition was induced either by superinfecting rats or by co-caging rats infected with either strain with non-infected rats. Sodium fluoride was applied in diet and drinking water (60 ppm F), as daily brushings with a 1% F solution, or as a combination of both administrations. The fluoride applications favoured S. mutans C180-2FR, but not enough to outcompete the parent strain successfully under all conditions. Fluoride protected rat dental plaque harbouring S. mutans C180-2FR against colonization by the parent strain. In rats not previously infected with S. mutans, the fluoride in diet and drinking water in combination with the fluoride brushings favoured S. mutans C180-2FR in colonizing rat dental plaque. However, the fluoride applications did not benefit S. mutans C180-2FR in colonizing plaque in which the parent strain was already present. Human dental plaque may be colonized by fluoride-resistant S. mutans strains after spontaneous mutation in response to a fluoride pressure. Our data suggest that this pressure may also provide an ecological advantage to this mutant to colonize the dental plaque.