Phosphate-based anticorrosion treatments have been found to causea decrease in bacterial density in distribution systems. However, some studies have shown that the addition of phosphate may promote bacterial regrowth. The major objectives of this study were: to investigate, under semi-controlled conditions, the influence of phosphate on the composition of a bacterial community in an established biofilm; to test the effects of phosphate on the efficiency of disinfection of an established bacterial biofilm; and to determine which of several subpopulations is the most sensitive to disinfectants. Includes 26 references, tables, figures.