The present work aims at identifying and analyzing the main parameters governing the value of the maximum nitrification rate in the extended aeration activated sludge process. An experimental monitoring of an activated sludge pilot plant operated under controlled conditions, a series of dynamic simulations achieved with the Activated Sludge Model n°1 (ASM1), and a theoretical approach based on the equations of growth and decay of nitrifying biomass have been undertaken to reach this objective. At a given temperature, a minimum duration per day of oxygen presence is required to prevent the nitrifying micro-organisms from being washed out. Beyond this limit, the maximum nitrification rate is proportional to the nitrogen volumetric loading rate. It also depends on other operating parameters such as the COD/TKN ratio of the influent, the sludge retention time, the fraction of the total sludge contained in the intermittently aerated tank.