The integrated fixed-film activated sludge process is an advantageous alternative for an effective nutrient removal process without increasing reactor or clarifier basin size. Attached biofilms in the IFAS system allow for anoxic layers to be formed in aerobic environments, leading to simultaneous nitrification and denitrification. This paper discusses simultaneous nitrification and denitrification studied at a full-scale IFAS WWTP. A computer process model was developed using BioWin software. Calibrated to the fullscale, denitrification rates determined by the model were compared to experimental batch studies from the full-scale biofilm samples. Dissolved oxygen concentrations increased throughout the full-scale basin from 1.0 mg/L through saturation. However, denitrification was observed and verified with the process model. Modeling results and field sampling observations indicate denitrification rates increase throughout the length of the aerobic basin. This paper compares full-scale denitrification rates (determined experimentally) with those produced by a process model. Conclusions are provided to explain denitrification in an aerobic IFAS basin.