A three year-long investigation sponsored by the Water Environment Research Foundation and many supporting agencies addressed the need for simple methods of measuring critical parameters in IAWQ-type activated sludge models. One of the critical parameters identified in this project was the maximum specific nitrifier growth rate (µA) because of its impact on sizing and control of nitrifying activated sludge systems. Methods of measuring (µA) were reviewed, evaluated, developed and modified. This paper describes the evaluation of four methods of measuring (µA) at the City of Phoenix’s 91st Avenue WWTP. Three of the methods were bench-scale and one depended upon collection of a large amount of field data to calibrate an activated sludge model. The paper presents short descriptions of the methods, and the equipment and labor required to implement the methods. Relatively high values of (µA) were obtained ranging from 0.88 to 0.96 day-1. There was relatively good agreement in values measured between the methods although it was necessary to adjust values of the nitrifier endogenous decay coefficient bA and the temperature coefficient for (µA), è.