Objective: To determine antibacterial activity of chewing mastic gum against the salivary levels of Streptococcus mutans , the total number of viable bacteria, and lactobacilli in patients undergoing therapy with fixed orthodontic appliances. Materials and Methods: In this study, the levels of S mutans , lactobacilli, and total cultivated bacteria were measured before and after chewing mastic gum. The antibacterial effects of chewing mastic gum against these microorganisms in saliva were compared with a placebo gum. The counts for orthodontically treated patients were evaluated before chewing gum; just after chewing gum; and after 45, 75, 105, and 135 minutes. Saliva samples taken from the patients were inoculated onto trypticase-yeast-cystine-bacitracin agar for mutans streptococci and onto Rogosa agar for lactobacilli. The agar plates were incubated for 48 hours anaerobically at 37°C. The total number of viable bacteria was then counted. Results: Just after chewing the mastic gum for 15 minutes, a significant decrease of total bacteria and S mutans was observed ( P < .001). The reduction in lactobacilli was not significant at later first stage ( P > .05). However, at the end of 135 minutes, there were significantly fewer S mutans ( P < .001), total viable bacteria ( P < .001), and lactobacilli ( P < .001) in the oral cavity after chewing mastic gum than after chewing paraffin ( P < .001). The results show that chewing mastic gum decreased the total viable bacteria, S mutans , and lactobacilli in saliva in orthodontically treated patients with fixed appliances. Conclusion: Chewing mastic gum might be useful in preventing caries lesions.