The removal of waterborne viruses by packed bed filtration was examined using a model system consisting of two different bacteriophages (MS2 and lambda) and saturated beds of ultrapure quartz grains. The majority of these experiments were conducted at a solution pH of 5, where the viruses and quartz possess a net negative surface charge. On the basis of a simple model that correctly predicts the isoelectric point (pI) of MS2, the surface charge on this virus appears to originate from the ionization of amino acid residues located on the exterior of the virus particle. The deposition rates of both MS2 and lambda at pH 5 are sensitive to the ionic composition of the suspending fluid, with more rapid filtration occurring at the higher salt concentrations, The filtration rate of lambda approaches the theoretical value estimated using the Smoluchowski-Levich (S-L) approximation at pH 5 and high salt concentrations (300 mM NaCl) or at the pi of the virus, suggesting that electrostatic repulsive forces effectively dominate the filtration dynamics of lambda. The filtration rate of MS2, on the other hand, is at least 800% less than the theoretical S-L value at high salt or at the pi of this virus, suggesting that both electrostatic and nonelectrostatic repulsive forces influence the filtration of MS2. We investigate the possibility that this nonelectrostatic force is steric in nature, arising from hydrophilic polypeptide loops which extend a maximum of 1 nm off the MS2 surface.