The purpose of this paper is to present the results of a five-month pilot testing program in Tucson, Arizona to investigate the factors promoting nitrification and means of control. Twelve parallel pilot treatment trains each with plug-flow reactors were tested. The reactor systems reproduced the nitrification process in a manner similar to nitrification events in full-scale distribution systems. Previous laboratory studies supported by Tucson and full-scale systems in Texas have indicated that the presence of chlorite ion in water prevented the nitrification process and stabilized ammonia concentrations. These pilot studies confirmed the previous findings with chlorite ion and have opened new avenues of research for control of nitrification with minimal use of additional chemicals. The following conclusions can be drawn from this study: plug flow reactors designed for this study successfully reproduced nitrification conditions representative of nitrification episodes in distribution systems; continuous chlorite ion feed prevented nitrification from ever getting established, even at a concentration as low as 0.1 mg/L; in systems that already had serious nitrification underway, intermittent chlorite ion feeds as low as 0.2 mg/L stopped nitrification completely for several weeks after their application; an intermittent dose of only 0.05 mg/L of chlorite ion damaged the ammonia oxidizing bacteria (AOB) population, which was able to re-establish itself in a few weeks, and a serial intermittent feed of 0.2 mg/L on the same reactor system badly damaged the AOB and effectively stopped nitrification; in other pilot units studied, AOB regrowth was destroyed with an intermittent dose of 0.8 mg/L; and, in full-scale distribution systems using chloramines, it appears that nitrification can be prevented by short-term and regularly scheduled applications of chlorite ion at concentrations as low as 0.2 mg/L. Includes 8 references, tables, figures.