Streptococcus mutans is a primary etiologic agent of human dental caries. S. mutans ability to form a biofilm, acidogenic and aciduric properties are its main virulence factors. The current study aimed to investigate some virulence traits of S. mutans isolated from dental plaque of caries-free indivduals, plaque of caries active patients and from dental caries lesions. The isolates were tested for the antimicrobial susceptibility patterns against diverse classes of antimicrobial agents. The interaction of S. mutans with other members of microbial oral flora such as Candida albicans was studied. The result of the study revealed that there were differences in the antimicrobial susceptibility between the three groups of isolates. The ability of isolates to form biofilm was the highest among the isolates from soft caries lesion followed by isolates from plaque of caries-free individuals followed by the isolates from the plaque of caries active patients. Candida albicans enhanced the biofilm formation capability of S. mutans isolated from the three groups. The isolates from soft caries lesion showed the highest acid tolerance ability followed by the isolates from the plaque of caries active patients, then the isolates from the plaque of caries-free individuals. The drop in pH of media of the isolates from soft caries lesion showed similar results with the isolates from the plaque of caries-free individuals but not isolates from the plaque of caries active patients. In conclusion, there were notable differences in results obtained from the isolates from the plaque of caries-free, caries active and caries lesion.