Background: Antimicrobial agents have been used as a chemical plaque control agent to prevent oral disease and to improve oral health. The aim of the study was to evaluate the antimicrobial activity against Streptococcus mutans in vitro of four commercially available mouthwashes. Materials and Methods: Four commercially available mouthwashes containing chlorhexidine, triclosan, tea tree leaves, and Aloe vera were selected. Two different techniques were used: (1) Agar disc diffusion method to determine the zone of inhibition in mm and (2) contact test method to determine the bacterial count of S . mutans in vitro . The samples were incubated anaerobically for 24 h. Comparisons among various mouthwashes were tested using analysis of variance. All analyses were done at 5% level of significance. Results: Triclosan mouthwash was most effective in terms on zone of inhibition having mean diameter of 21 1.41 mm. Significant difference ( P P Conclusion: Chlorhexidine mouthwash showed excellent antimicrobial activity against S. mutans . Triclosan and tea tree leaves mouthwashes had intermediate level of efficacy.