This study investigated the effects of phosphorus on biofilm formation via annular reactor systems in terms of biofilm cell growth, exopolysaccharide (EPS) production, biofilm structure and cell metabolic potential. Drinking water biofilms were developed in annular reactors with supplement of carbon and different levels of phosphorus. The biofilm formation was monitored over a period of 30 days. Biofilm related parameters were examined by various methods, which included heterotrophic plate count, total carbohydrate content, confocal laser scanning microscopy and GN2 microplate assay. Our results showed that phosphorus addition can promote the biofilm cell growth (cell count increased about 1 log with addition of 30 and 300 mu g l(-1) of phosphorus). However, the addition of 30 and 300 mu g l(-1) of phosphorus caused 81% and 77% decrease in EPS production, respectively. The results of biofilm structure analysis showed that the addition of 30 and 300 mu g l(-1) of phosphorus can induce thicker and less homogeneous biofilms with more biomass. Furthermore, the addition of 30 and 300 mu g l(-1) of phosphorus dramatically increased the biofilm cell metabolic potential. The addition of 3 mu g l(-1) of phosphorus was found to have minor effects on the parameters examined. The results indicate phosphorus addition to drinking water distribution system (DWDS) has a complicated effect on the biofilm formation. As the addition of phosphorus at certain levels can affect the biofilm growth in DWDS, care should be taken when phosphate-based corrosion inhibitors are used in the DWDS.