An epidemiological investigation was carried out to evaluate the prevalence of Streptococcus mutans in a group of 134 adult patients. Markedly higher frequency of isolation was observed in caries-active subjects than in caries-inactive or caries-free subjects, indicating a significant association between the prevalence of the microorganism and the caries status. Moreover, the presence of the microorganism appeared to have a significant association with the extent of caries experience evaluated by the DMF score. These findings are in agreement with those reported previously for school children in other areas of Italy. Isolation of S. mutans was compared among patients groups with different caries activity in relation to culture times of dental plaque samples in a transport medium (Colorimetric Broth Medium). S. mutans was most frequently isolated from caries-active subjects when the medium was incubated for 48 h after inoculation with dental plaque samples.