Hillsborough County, Florida, has detected coliform species within both the Northwest and South Central regions of its distribution system over the past six years. Furthermore, during the past two years, the detection of total coliforms has exceeded 5% of the sample prevelance. However, the occurrence showed a seasonality, peaking in late summer to early fall, which coincided with the rainy season and lower water consumption. This project was designed to study the microbial ecology within the distribution system, as well as any public health issues. Analysis included speciating heterotrophic plate count and coliform bacteria using the Analytical Profile Index system and Biolog. The project also included analysis of biofilms in meters removed from the distribution system. The meters were chosen based upon the service record of the area from which they were taken, as well as upon the rationale of achieving a general overview of the system. The presence or absence of coliforms, pathogenic bacteria, such as Mycobacterium, and potential pathogens like Aeromonas and Pseudomonas were investigated using standard biochemical tests and polymerase chain reaction. Ecologically important bacteria, such as nitrifying and sulfate-reducing groups, were also investigated. The data show that the Northwest region is more affected by individual coliform species entering the system. The South Central region seemed to be more affected by biofilm formation, including the detection of sulfate-reducing bacteria within biofilms. Aeromonas hydrophila was detected in both regions. The data will be combined with data from the Hillsborough County database in order to provide more consistent water quality to the customers of Hillsborough County. Includes 2 references.