Objectives: The demand among patients for esthetic restorations is one main cause for the increased use of condensable composite resins. These resins were designed for use in load bearing areas of posterior teeth as an alternative for amalgam. As such, they should have suitable mechanical properties. Methods: In the present study, the potential of the oral bacteria S. Mutans and S. Fecalis to alter the mechanical properties of condensable composite resins was investigated. Two materials were selected P 60 (3M Espe) and Tetric Ceram HB (Ivoclar-Vivadent). Complete cylindrical set samples (6 mm x 3 mm) were prepared and placed in bacterial suspensions or in control solutions and incubated at 37°C for 30 and 90 days. Stiffness, diametral tensile strength (DTS), and hardness (KHN) of the samples were examined at time 0, 30 days, and 90 days after storing in dry or wet conditions. Results: The data showed that P60 had higher KHN, higher DTS, and higher stiffness than Tetric Ceram HB. All specimens tested in dry conditions showed higher values of DTS, stiffness and KHN (p<0.05). In moisture, there was an increase in KHN with time (p<0.05) where a decline in this parameter was found when specimens were stored in dry conditions. There was a decline in DTS with time (p<0.05). There was no difference with stiffness during time or different treatments (p>0.05). Conclusions: The mechanical properties of composite resins should be tested in moisture and the bacteria tested did not cause any significant deterioration in the mechanical properties of the tested materials.