Symptoms indicative of nitrification occurred in the Ann Arbor, Michigan, system during the late summer of 1990. The episode was characterized by decreases in monochloramine residual and increases in heterotrophic plate count (HPC) bacteria and nitrite. The cause of the nitrification was most probably a high dosage of ammonia combined with granular activated carbon filtration and discontinuation of the hydrant flushing program. The incident was ultimately halted by free chlorination of the distribution system. During free chlorination, an unexpected increase in HPC bacteria was observed in areas not affected by nitrification. Recommendations for future control of nitrification include more stringent control of ammonia, increased monitoring of nitrite and nitrate, development and implementation of a hydrant flushing program, and experimentation with the effects of pH. Includes 21 references, tables, figures.