Objective: The purpose of this double-blinded, placebo-controlled, randomized, cross-over study was to determine the short-term efficacy of an essential oil mouthrinse (EOM LISTERINE Antiseptic) on the levels of oral microorganisms in supragingival plaque. Methods: After IRB approval, 98 individuals with moderate gingivitis were screened and 30 were qualified to participate. Subjects were instructed to brush with a fluoride dentifrice for 2-weeks to normalize oral hygiene. At baseline, subjects were asked to refrain from oral hygiene, eating, drinking and smoking for at least 12 hours, but no more than 24 hours before the baseline plaque was collected at the site. Plaque samples were collected from the supragingival surfaces of four molars (#3 buccal, #14 lingual, #19 buccal and #30 lingual) for cultural analysis. Subjects were randomly assigned to either EOM or a control treatment and instructed to rinse twice daily for 2 weeks to start Phase I of the cross-over study. After 2 weeks, subjects returned to the site for plaque collection and culture to complete Phase I. Subjects then started a 2-week wash-out period and returned for baseline plaque collection for Phase II. Subjects then used the alternate mouthrinse for 2 weeks, after which plaque collections were repeated to complete Phase II. Percent reductions for total anaerobes colony counts and Streptococcus sp between baseline and 2-week post-treatment were assessed by ANCOVA. Results: In the EOM treatment group, the levels of supragingival plaque were decreased by 55.7% from baseline for total anaerobic bacteria (p< 0.001) and by 51% for total Streptococcus sp (p <0.001). Control treatment showed a 2.9% reduction for anaerobes and 1.7% increase for Streptococcus sp (p=0.01). Conclusions: These results confirm the short-term effect of LISTERINE Antiseptic against oral bacteria, in subjects with moderate gingivitis, when used daily as directed, for a 2-week period.