Aim: The aim of this study was to determine whether aggregation processes in aqueous phase may explain the decrease in plaque forming unit (PFU) counts for pH close to the isoelectric point (pI) of viral particles (MS2 phages). Methods and Results: Loss in PFU was observed for pH ≤ pI (pIMS2 = 3·9): for example, at pH 2·5, loss was approx. 3 log10 PFU. Particle size analysis combining results of dynamic light scattering and flow particle image analysis was then applied to determine the aggregate state of viral suspensions by recording size distributions. The size of major population significantly changed to 30 nm at neutral pH to more than several micrometres when passing below the isoelectric point. Conclusions: Our study shows that MS2 phages exhibit significant aggregation processes for pH ≤ pI leading to aggregate with sizes of few micrometres. This aggregation process can largely explain the decline in PFU counts. Significance and Impact of the Study: It is clear that viral aggregation can be a source of significant bias for PFU assays because in the presence of an aggregate the PFU count can be less than the sum of its constituent particles. Therefore, cautions should be taken in terms of conditions of storage (pH far from pI) to avoid such aggregation artefact.