The nitrifier growth rate constant, µA,max, is a critical parameter for design and performance of nitrifying activated sludge systems. Although many investigations have been devoted to study µA,max, only a few have dealt with the effect of the reactor configuration on this important kinetic parameter. Bench-scale and full-scale trials were devised to study the impact of the internal mixed liquor recycle (IMLR) on the nitrifier growth rate constant. The nitrifier growth rate constant for an existing activated sludge plant was determined at different operational conditions using the High F/M test and by process model calibration. Overall, the results obtained during this study indicate that high IMLR values have a negative effect on µA,max. Based on the results obtained during this investigation, a 15-percent decrease in µA,max was observed at IMLR of 4Q or higher. It is surmised that at high IMLRs, the reactor behavior shifts from a plug-flow configuration to a “quasi” complete mixed configuration influencing either the species selection in activated sludge population or at least adaptation of specific species. These results have a tremendous impact on the design on activated sludge processes that incorporate IMLR for denitrification such as the Bardenpho, MLE, UCT and A2/O.