This article evaluates the capacity of slow sand filters to remove natural organic matter (NOM) and organic precursor materials as quantified by dissolved organic carbon, UV absorbance, and trihalomethane formation potential from source waters with varying levels of NOM. Municipal facilities were sampled to provide baseline data and to evaluate the effects of two different filter cleaning techniques, normal scraping and harrowing, on NOM and particulate removal. Pilot-scale filter studies were conducted to evaluate the influence of two different flow rates on NOM removal, particulate removal, and filter biomass development. Includes 47 references, tables, figures.