Nitrifier growth is a common problem in drinking water distribution systems where chloramines are used for disinfection. Nitrifier growth in distribution systems is supported by five principle mechanisms that supply ammonia as an initial substrate: (1) excess ammonia remaining from chloramine formation reactions; (2) autocatalytic decay of chloramines; (3) oxidation of organic matter by chloramines; (4) chloramine reactions with nitrite; (5) chloramine reactions with pipe surfaces. The Comprehensive Disinfection and Water Quality (CDWQ) model is a dynamic water-quality model that was developed to account for these ammonia-producing reactions and the dynamics of suspended and fixed bacteria, including nitrifiers, in full-scale distribution systems. The results of this study show that increasing chloramine levels to disinfect nitrifiers is ineffective. The ammonia released through chloramine decay reactions fuels nitrifier growth, outpacing chloramine disinfection dynamics. The only effective strategy to limit nitrifier growth is to reduce substrate availability by increasing the initial chlorine-to-nitrogen ratio used to form chloramines and the use of booster disinfection. Copyright ASCE 2004.