This study examined the prevalence of Streptococcus mutans in dental plaque from carious and noncarious sites before and after routine dental treatment in five young men in the US Navy. S mutans was isolated in 96.3% of the samples from carious sites and in 63.3% of the samples from noncarious sites before treatment. After all carious lesions had been restored, the prevalence of S mutans was significantly reduced, but many of the carious and noncarious sites still had detectable levels of S mutans. A further reduction in S mutans was observed after conventional 8.9% stannous fluoride paste prophylaxis and 10% stannous fluoride topical application; however, in no instance was the organism completely eliminated. About 12 to 18 months after the initial examination, new carious lesions were seen in four men who were available for reexamination. Plaque samples of new lesions had high levels of S mutans. Therefore the restorations of lesions and the stannous fluoride prophylaxis treatments given under the conditions of the study were effective but incomplete measures for eliminating S mutans from the tooth surfaces of Navy personnel and were inadequate procedures for arresting the carious process.