Bacterial Regrowth Potential (BRP) and biodegradable organic carbon (BDOC) measurement were used to investigate the limiting effect of phosphorus on the bacterial growth in raw water. Addition of phosphorus to improve organic substance removal in raw water with bio-ceramic filters as the biological pre-treatment was studied. The raw water used in the experiments was from a reservoir located in northern China. Two parallel pilot-scale up-flow bio-ceramic filters were used in the experiments. 25ug of PO4 3--P liter-1 (as H3PO4) was added continuously to the influent of one bio-ceramic filter for 60 days while another was operated as a reference without phosphorus addition. This investigation indicates the importance of phosphorus for biological treatment of drinking water. Includes 19 references, tables, figures.