The Metropolitan Water District of Southern California (Metropolitan) collects and analyzes approximately 900 bacteriological compliance samples monthly, routinely meeting the 5% maximum contaminant level (MCL) under the Total Coliform Rule (TCR). However, certain operational practices and events (e.g., open reservoirs, low disinfectant residuals, heavy rains) in the early to mid-1990s resulted in elevated levels of coliform-positive samples. Although these positive samples did not result in an MCL violation, they placed a greater burden on the almost 200 water agencies receiving Metropolitan's water to meet the TCR. Since then, Metropolitan has developed and implemented a program to reduce the number of coliform-positive samples within its distribution system and to ultimately assist in reducing the coliform levels in member agencies' systems. This program included identifying the historically problematic sites, field inspections, bacteriological monitoring, collaboration with the operations and engineering staff for corrective actions, disinfectant residual adjustments, modifications to structures in the distribution system, and additional training of field staff. By implementing an aggressive and proactive approach to improving the bacteriological quality of water in the distribution system, the occurrence of coliforms in the distribution system can be significantly reduced. Includes 9 references, tables, figures.