Aim & Objectives: This study assesses the quantity of streptococcus mutans group and lactobacillus in interproximal contact of posterior teeth with proximal amalgam and resin composite restorations. Methods: Twenty patients were divided into two groups (n=10) according to the type of restorative material they presented: amalgam (AM) or resin composite (RC). In the same patient, one interproximal Class II amalgam or resin composite restoration and one interproximal contact without restorations (control group – C) were studied. The material was collected from the interproximal with the aid of a piece of sterilized dental thread, and stored in phosphate buffered saline solution. The material was seeded on plates containing Mitis Salivarius Bacitracin Sucrose agar and Rogosa agar culture medium and incubated at 37°C for 48-72h. The colony forming units (Log10 CFU/mL) were counted and the data were submitted to the Student’s-t test (5%). Results: There was significant difference between Groups C and AM (p=0.019) with regard to lactobacillus count. For streptococcus mutans group, there was significant difference between Groups C and RC (p=0.015). Conclusion: There is significantly greater growth of streptococcus mutans group in RC Class II restorations and lactobacillus in AM restorations when compared with interproximal contact without the presence of restorations.