The current research was undertaken to evaluate the performance of a top loading vertical flow submerged bed treatment system (TLVFSBTS) treating primary sewage effluent. Both pollutant elimination and microbial processes were measured. The wetland system is located just west of the Hudson River in upstate New York. The system consists of four 232-m² wetland cells, currently operating in series. Two cells are planted with Phalaris arundineacea and two with Phragmites communis. Treating CBOD₅ to permit levels (< 4 mg L^-1) was not difficult to accomplish and occurred in the first wetland cell during much of the study period. At no time during the 17-month evaluation period did measurable CBOD₅ leave the wetland, although lack of carbon source (CBOD₅) for microorganisms in subsequent wetland cells (cells 3 and 4) may have been a limitation for denitrification. Ammonium oxidation in the first wetland cell was greater than in the second wetland cell (70.8% and 39.2%, respectively). Nitrite accumulation in the first wetland cell (2.204 mg L^-1 maximum value) appeared to be seasonal, and not directly related to nitrite oxidation. Both nitrite and nitrate leave the wetland system at levels below primary drinking water standards (1 and 10 mg L^-1, respectively. The ammonium concentration leaving the system was at or below permit level (2.2 mg L^-1) during much of the study period. Nitrification potential and denitrification enzyme activity in the wetland system, especially the first wetland cell, exceeded published values for natural wetlands, tropical soils, and both marine and freshwater sediments. These findings, however, demonstrate the ability of TLVFSBTS wetlands to remove the various nitrogen constituents once the microbial population becomes acclimated to the influent wastewater.