The size and surface characteristics of a surrogate particle and Cryptosporidium parvum oocysts are important in determining the ability of the particle to mimic the behavior of C. parvum oocysts in filtration and particle transport experiments. The ζ potential, hydrophobicity, and filterability of a surrogate particle, 5 μm carboxylated latex microspheres, and oocysts were compared for a variety of solution conditions. C. parvum oocysts had a slightly negative ζ potential (-1.5 to -12.5 mV) at pH 6.7 over a wide range of calcium concentration (10-6-10-1 M), while the fluorescent microspheres were more negatively charged under the same conditions (-7.4 to -50.2 mV). After exposure to 5 mg of C/L of Suwanee River natural organic matter (NOM), the ζ potentials of both particles became significantly more negative, with the microspheres consistently maintaining a more negative ζ potential than the oocysts. Alum was able to neutralize the negative ζ potentials of both particles when in the presence of NOM, but nearly twice the dosage was required for the microspheres. NOM also affected the hydrophobicity of the particles by increasing the hydrophobicity of the relatively hydrophilic oocysts and decreasing the hydrophobicity of the relatively hydrophobic microspheres. A bench-scale filtration system removed less microspheres (40.3 ± 1.5%) than oocysts (49.7 ± 2.9%) when 0.01 M CaCl2 was supplied as coagulant. After preexposure to 5 mg of C/L of NOM, the removals of both particles declined significantly, and the removals of microspheres (13.7 ± 1.5%) and oocysts (16.3 ± 1.5%) were similar. Finally, the removal efficiencies of microspheres and oocysts in the presence of NOM increased to 69.3 ± 3.5% and 67.7 ± 6.4%, respectively, when alum was supplied as coagulant at the optimum dosage needed to destabilize the oocysts. These experimental results suggest that microspheres can be used to provide a conservative estimate of oocyst removal in filters containing hydrophilic negatively charged filter media.