Aim: To compare the number of colony forming units (CFU) and agar adherence of S. mutans on amalgam (AM) and resin composite (RC) occlusal restorations. Sixty-five healthy patients older than 18 years with high caries risk who had at least one occlusal AM and RC restorations (n=130) were selected. Methods: The restorations were evaluated according to the alpha Ryge criteria (Cohen-Kappa 0.8). For each patient, a biofilm sample was taken using an impression tray technique with previously loaded with solid trypticase yeast extract cysteine sucrose with bacitracin agar placed over the AM restorations and RC restorations in the same patients. The tray was placed inside an oven at 37 °C for 48 h, and the S. mutans count was then performed. Data were analyzed with the test Wilcoxon with a 95% confidence level. Results: RC restorations had statistically significant higher number of CFU of S. mutans than AM restorations (p<0.05). Conclusions: In adult patients with high caries risk, RC occlusal surfaces showed greater agar adherence of S. mutans count than AM restorations.