Ammonia-oxidizing bacteria (AOB) in chloraminated drinking water are problematic in that they degrade the quality of water by rapidly deteriorating the disinfectant. Consequently, water quality is compromised and potentially subject to opportunistic pathogens. Elevated heterotrophic bacteria and even coliforms have been reported to have occurred simultaneously during nitrification episodes. This paper discusses two cases in which nitrification occurred in the distribution system of utilities in southern California. In order to determine the extent and source of the nitrification episodes, water and biofilm samples were collected at each facility. The nitrification episodes, the role of biofilms in supporting nitrifying bacteria, and control strategies are described.