Nitrification inhibition due to aniline was investigated in completely mixed suspended-growth, batch, and continuous processes. Synthetic wastewater was used with aniline as the carbon source. The experiments were conducted at aniline concentrations inhibitory to nitrifier organisms. In the batch tests, degradation took place rapidly (within 4 to 6 hours) for initial aniline concentrations below 100 mg/L, with nitrification picking up as soon as the aniline concentration decreased to less than 3 to 4 mg/L. For initial aniline concentrations of 250 mg/L and higher, complete nitrification did not take place even though the aniline concentration decreased to less than 3 to 4 mg/L. This observation indicated nitrifier inhibition due to aniline. In the continuous experiments, a hydraulic residence time of 8 to 24 hours and a solids retention time of 8 to 24 days were maintained. Complete nitrification took place at bulk aniline concentrations less than 0.5 mg/L, while at higher aniline concentrations, nitrification inhibition took place. The inhibitory effect of aniline on the nitrification process was modeled using uncompetitive inhibition kinetics. Modeling of batch processes yielded the value for the inhibition constant for aniline, Ki, as 3.3 mg/L. Modeling of continuous processes yielded criteria for stable process operation for nitrification under inhibitory conditions, which were also confirmed through experimental results.