Objectives: The microgap at the tooth-restoration interface is inevitable and may allow bacterial penetration that could lead to failure. The purpose of this in vitro study was to assess the antibacterial potential of 4 flowable composite restorative materials. Method and Materials: The antibacterial potential of Aeliteflo (Bisco), Filtek Flow (3M ESPE), Tetric Flow (Ivoclar Vivadent), and Dyract Flow (Dentsply) was tested against Streptococcus mutans. Agar diffusion test (ADT) and direct contact test (DCT) were the methods used. For ADT, wells were punched in S mutans-inoculated plates. The materials were placed in the wells and polymerized. Inhibition zones were measured after 48 hours' incubation at 37°C. In the DCT, 8 samples of each tested material were placed on the side walls of wells in a 96-microtiter plate and polymerized. A suspension of S mutans was placed on the surface of each sample. Bacterial growth was monitored by optical density changes at 650 nm every 30 minutes for 16 hours. The experiment was repeated after the samples were aged in phosphate-buffered saline for 1 and 7 days. Results: In both tests, only Dyract Flow showed inhibition of S mutans growth. Except for Dyract Flow samples, aged samples did not statistically differ in S mutans inhibition when compared to their 1-hour control counterparts. Dyract Flow samples lost their S mutans inhibitory potential after 24 hours. Conclusion: The flowable composites tested do not possess effective long-term antibacterial ability. Keywords: antibacterial, direct contact test, flowable compomer, flowable composite resins