Purpose: The aim of the present study was to assess and compare the antibacterial effect of garlic extract with those of chlorhexidine and negative control mouthwashes against Streptococcus mutans. Materials and Methods: The present study was carried out in two phases. In Phase 1, the zone of inhibition of various concentrations of garlic extract against S. mutans was determined using the cup and plate method. The minimum concentration at which a zone of inhibition appeared was further employed to prepare a mouthwash that was used in Phase 2. This phase included 45 dental students whose baseline salivary S. mutans level was assessed. They were randomly divided into three groups: '1' representing students using garlic extract mouthwash (garlic extract + water + sorbitol + spearmint oil), '2' representing those using chlorhexidine (0.2%) mouthwash and '3' representing those using a negative control (water + sorbitol + spearmint oil). All of the subjects were advised to use 10 ml of the assigned mouthwash once daily after their last meal for a duration of 7 days. On day 8, the post-treatment salivary S. mutans counts were assessed, and the data were analysed and compared by performing appropriate statistical tests. Results: Phase 1: the 3% concentration was the minimum concentration at which a zone of inhibition was observed. Phase 2: a reduction in post-test S. mutans counts in all three groups was found. The mean difference that was observed in the garlic extract group was 5.23 x 10⁵ CFU/ml, in the chlorhexidine group 2.63 x 10⁵ CFU/ml and in the negative control group 1.18 x 10⁵ CFU/ml. The differences among all three groups were statistically significant (P 0.05) and that between the negative control and the garlic group was highly significant (P 0.001). Conclusions: Garlic extract is effective against S. mutans when tested both in vitro and in vivo. As S. mutans is one of the primary aetiological organisms in dental caries development (Loesche, 1986), and in the present study garlic extract has been shown to be effective against S. mutans, garlic extract mouth rinse might be used as an effective remedy in the prevention of dental caries. Keywords: chlorhexidine mouthwash, garlic mouthwash, Streptococcus mutans