Aim: To evaluate the anti microbial activity against S mutans in vitro of four commercially available mouth washes. Materials and methods: Four commercially available mouth washes- chlorhexidine, triclosan, tea tree leaves and aloe vera were selected. Two different techniques were used: Agar Disc diffusion method to determine the zone of inhibition in mm and Contact test method to determine the bacterial count of S mutans in vitro. The samples were incubated anaerobically for 24 hours. The survival of bacterial colony in regard to baseline was calculated and compared using paired t test. Comparisons among various mouthwashes were tested using analysis of variance. All analyses were done at 5% level of significance. Conclusion: Chlorhexidine mouth wash showed excellent antimicrobial activity against S mutans. Triclosan and Tea tree leaves mouth washes had intermediate level of efficacy. While Aloe vera was the least effective mouth wash among all.