The role of sulfate-reducing bacteria (SRB) in the cycling of Fe and S was studied in a young constructed wetland located in Kanata, Ontario, Canada. The wetland is a surface-flow system composed of three consecutive cells. Sediments and water samples were collected over the course of 1 year within each cell. Sediments were analyzed for the presence of SRB (using a lactate-rich medium), whereas surface and porewaters were analyzed for their concentrations of dissolved Fe and sulfate and for pH, Eh, and dissolved organic carbon. Lactate-using SRB were present at all three sites within the wetland, and the populations were largest (1010colony-forming units per gram of sediment) during the cold winter months, where the temperature of the water was 1°C. The presence of high-SRB populations also corresponded to highly anoxic conditions within the sediments and to a decrease of sulfate concentrations, suggesting that cold temperature did not affect the activity of SRB. Our results indicate that Fe and S cycling in the young constructed wetland was active throughout the year, especially in the cold winter months, where large SRB populations were encountered. This suggests that Fe removal in wetlands can be effective in temperate climates, even though the temperature of the water decreases drastically during the winter.