Experiments were conducted with a laboratory-scale variable-aperture dolomite rock fracture to compare the transport of virus-sized microspheres and viruses. Transport tests were conducted using bromide, two sizes (20 and 200 nm diameter) of carboxylate-modified latex (CML) microspheres, and two bacteriophages (MS2 and PR772) that are often used as pathogenic virus surrogates. Retention of negatively charged MS2 was two to three times lower than retention of similarly sized negatively charged 20 nm microspheres, in contrast to expectations from Derjaguin-Landau-Verwey-Overbeek theory. Retention and transport of MS2 and PR772 were much closer to that of the 200 nm microspheres, with approximately 20% greater retention of MS2 compared to PR772 for all conditions tested. Microsphere transport was more significantly affected by ionic strength than the bacteriophage transport, particularly the 20 nm microspheres. Impacts of calcium chloride were significant and different for microspheres compared to bacteriophages. Application of a one-dimensional advection-dispersion transport model with two types of kinetic attachment and detachment terms gave good fits to microsphere and bacteriophage transport data. However, blocking was required to accurately simulate transport of the 20 and 200 nm microspheres. Blocking was not a significant process for MS2 or PR772 under most conditions tested. Overall, results indicate that 20 nm CML microspheres are not suitable surrogates for transport of MS2 and PR772 in fractured dolomite, and therefore, likely not suitable as surrogates for other viral pathogens. The 200 nm microspheres would be more suitable as surrogates but show different responses to changes in solution chemistry compared to bacteriophages.