Automated biomonitoring systems provide continuous, real-time monitoring of changes in water quality and can rapidly identify toxicity associated with a wide range of chemical contaminants and increase public confidence in drinking water quality. Although widespread in Europe, biomonitor use is rare in the United States. Using case studies of a biomonitor that continuously monitors fish ventilatory patterns, this article illustrates how biomonitors can contribute to an early warning monitoring system for source and finished water protection.The case studies provide a context for a discussion of considerations important for biomonitor implementation, including toxicant responsiveness, event confirmation, implementation of biomonitors in a decision-making process, and cost. Recommendations are also provided for biomonitor use at raw water intake and distribution systems. An online real-time biomonitor for contaminant surveillance in water supplies 108 FEBRUARY 2007 | JOURNAL AWWA • 99:2 | PEER-REVIEWED | MIKOL ET AL 2007