Chironomid larval populations contaminating a municipal water system were monitored over a 24-month period between June 1987 and June 1989. Larval drift within distribution lines occurred throughout the year, and larval suspension was uniform at all depths within open reservoirs. Draining and cleaning reservoirs to rid them of chironomid populations was of questionable value because larval populations recovered fully within weeks, and larval drift occurred throughout the system.