The purpose of the present study was to investigate whether the use of a chitosan mouthrinse could be efficacious in reducing plaque and saliva mutans streptococci level. A randomized crossover clinical trial was performed to evaluate the effect of a rinse with 0.5% chitosan for 14 days on plaque formation and mutans streptococci counts in saliva. Twenty-four subjects were randomly assigned either the chitosan rinse or a placebo rinse in addition to their usual oral hygiene procedures. Following the baseline examination, each subject was given a prophylaxis. They were instructed to rinse with 20 ml of the mouthrinse twice daily for 30 seconds. Plaque scores were measured after a 14-day rinsing period, and mutans streptococci counts in saliva were also determined at the start and the end of the each rinsing period. The procedures were repeated with the alternate rinse after a 14-day washout period. Rinsing with 0.5% chitosan was significantly more effective in plaque reduction using the Quigley & Hein Index (chitosan: 1.44, placebo: 1.62, p<0.001) and Plaque Severity Index (chitosan: 0.138, placebo: 0.186, p=0.003). The mutans streptococci count in saliva was less after the chitosan rinsing ( χ²_cal=13.51, p=0.035) than placebo rinsing. In conclusion, the chitosan rinsing was effective in reducing plaque formation and counts of salivary mutans streptococci after a 14-day rinsing period. These results would appear to warrant further investigation into the potential value of chitosan as an effective anti-plaque agent for use in oral hygiene products.