Legionella pneumophila is detected sporadically in treated drinking water, and is suited for survival under conditions typical of distribution systems. Although data are abundant on the potential for Legionella occurrence in buildings and cooling towers, there are little data on Legionella occurrence in drinking water distribution systems. This project will collect and analyze data on the occurrence of L. pneumophila in distribution systems. In addition, the research will explore the relationship between secondary disinfectant concentrations and other system features and the occurrence and concentration of L. pneumophila. Research partners: American Water Works Association and Association of State Drinking Water Administrators.