The limited efficacy of traditional chemical disinfection for inactivating pathogens such as Cryptosporidium parvum emphasizes the critical role of physico-chemical processes such as granular media filtration for effective water treatment. Increasingly stringent regulations such as the anticipated Long-Term 2 Enhanced Surface Water Treatment Rule (LT2ESWTR) may require the demonstration of treatment performance or the incorporation of additional treatment processes for pathogen removal and/or inactivation. Since indigenous C. parvum concentrations are typically low, studies are often performed utilizing seeded concentrations of C. parvum oocysts or surrogate indicators. With the use of high seeded concentrations, however, comes the question of whether it is appropriate to extrapolate the C. parvum removal capacities of filters treating waters with low influent concentrations of oocysts from studies based on extremely high influent oocyst concentrations. C. parvum removals by granular media and diatomaceous earth (DE) filtration were investigated during experiments utilizing high and low seeded C. parvum and polystyrene microsphere concentrations (approximately two orders of magnitude) during several operating conditions. Excellent C. parvum removals by both granular media and DE filtration during stable operation were observed. Over the range investigated, seeded concentrations did not appear to impact C. parvum removal by either granular media or DE filtration. The experiments also suggested some limitations in the applicability of polystyrene microspheres as quantitative surrogates for C. parvum removal by filtration. Includes 9 references, figures.