We compare for the first time the electrokinetic and aggregation properties of MS2 phage (pH 2.5 to 7, 1 to 100 mM NaNO 3 electrolyte concentration) with those of the corresponding virus-like particles (VLPs), which lack entirely the inner viral RNA component. In line with our previous work (J. Langlet, F. Gaboriaud, C. Gantzer, and J. F. L. Duval, Biophys. J. 94:3293-3312, 2008), it is found that modifying the content of RNA within the virus leads to very distinct electrohydrodynamic and aggregation profiles for MS2 and MS2 VLPs. Under the given pH and concentration conditions, MS2 VLPs exhibit electrophoretic mobility larger in magnitude than that of MS2, and both have similar isoelectric point (IEP) values (ϳ4). The electrokinetic results reflect a greater permeability of MS2 VLPs to electroosmotic flow, developed within/around these soft particles during their migration under the action of the applied electrical field. Results also support the presence of some remaining negatively charged component within the VLPs. In addition, MS2 phage systematically forms aggregates at pH values below the IEP, regardless of the magnitude of the solution ionic strength, whereas MS2 VLPs aggregate under the strict condition where the pH is relatively equal to the IEP at sufficiently low salt concentrations (<10 mM). It is argued that the stability of VLPs against aggregation and the differences between electrokinetics of MS2 and corresponding VLPs conform to recently developed formalisms for the stability and electrohydrodynamics of soft multilayered particles. The differences between the surface properties of these two kinds of particles reported here suggest that VLPs may not be appropriate for predicting the behavior of pathogenic viruses in aqueous media.Understanding the behavior of virus particles of enteric pathogens (e.g., norovirus, hepatitis A virus) in terms of aggregation or adhesion is mandatory for addressing appropriately the processes involved in, e.g., viral dissemination or water treatment (9, 11). In that respect, much effort is now devoted to the analysis of the so-called surface properties of viruses, which are impacted by solution pH, ionic strength, or the presence of organic matter (20). The importance of viral surface properties within the framework of water treatment is well illustrated by the relationship between the efficiency of membrane filtration for removing viruses and the charge and degree of hydrophobicity of these biocolloids (11).According to standard formulations of Derjaguin-LandauVerwey-Overbeek (DLVO) representation and electrokinetic theories for hard (impermeable) particles, a parameter commonly used as an indicator for the sign and magnitude of charge carried by a virus in a solution of given pH is the difference between the pH and the so-called isoelectric point (IEP) of the virus, defined as the pH value at which virus electrophoretic mobility is zero (14). Within the regime of partial dissociation of charges located at the surface, the higher (or lower) the solution pH is than ...