The objective of this work was to determine the in vitro antibacterial effect of Tropaeolum majus extract "mastuerzo" in strains of Streptococcus mutans ATCC 25175. The extract was obtained by maceration with ethanol at 40º for one week. The susceptibility test was performed, using the disk diffusion method. Strains were seeded on plates with Mueller Hinton agar, embedded discs with concentrations of 10%, 20%, 30% and 40% of the extract. The plates were incubated at 37 ° C by anaerobiosis, the halos being measured after 24 hours; All the discs presented inhibition halos, whose size increased in relation directly proportional to the concentrations used, evidencing a greater diameter of inhibition halo at 40% of the extract of 16 mm. To determine the minimum inhibitory concentration (MIC), the macrodilution method was used in tubes using 10 concentrations, from the extract that showed th...