The bacterium Aeromonas hydrophila is a known fish and opportunistic human pathogen commonly occurring in surface waters supplying drinking water distribution systems. The major concern of government and drinking water providers is that A. hydrophila may invade and become established in the biofilm of a distribution system, thus potentially leading to outbreaks of disease. The purpose of this study was to survey source water, distribution system biofilm, and to establish a simulated distribution system to explore the possibility of A. hydrophila invading and becoming established under normal and disrupted treatment conditions. A. hydrophila (AH) medium and the API-20E system were used for identification, while Ampicillin-Dextrin Agar (ADA) was used for enumeration. Presumptive counts were high in source water approaching 103 CFU/ml during summer months. Biofiim from an actual distribution system showed the presence of A. hydrophila in 10 % of the samples. In the simulated distribution system A. hydrophila was never found in the bulk water or biofilm under normal treatment condition, showing disinfectant efficiency. Under disrupted treatment conditions A. hydrophila was not able to colonize a pre-established biofilm over a 14 week period. / Department of Biology