Objective: The purpose of this study was to determine in vitro the inhibitory activity of the ethanolic extract of Cyperus rotundus (Cajamarca - Contumazá) against a standardize strain of Streptococcus mutans (ATCC®25175™). Materials and methods: This study was an experimental in vitro study, which consisted of determining the inhibitory effect of three concentrations of the ethanolic extract of Cyperus rotundus: 250 mg/ml, 500 mg/ml, and 1000 mg/ml against a strains of Streptococcus mutans (ATCC®25175™). Ten tests were performed for each concentration, having 0.12% chlorhexidine as a positive control for Streptococcus mutans plates and 10% DMSO as a negative control. To evaluate the inhibitory effect, the disk diffusion method or Kirby-Bauer test was used, reading the results at 48 hours after initial sowing. Results: None of the three concentrations of the ethanolic extract of C...