Recent data have shown that a temporary release in the transmembrane pressure can cause a significant increase in virus transmission through a number of commercial virus filters. The objective of this work was to study the effect of a pressure release on retention of the bacteriophage phi X174 by the Ultipor DV20 membrane, with phage capture within the membrane visualized by confocal microscopy. Phage challenge tests showed a significant transient increase in phage transmission (typically by an order of magnitude) immediately after a pressure release. This transient increase was not due to any damage to the filter or to the presence of a back-flow (from permeate to feed). Confocal images demonstrated that the pressure release caused the migration of previously captured bacteriophage further into the depth of the filter. Data were analyzed using a modified internal polarization model, with the results providing important insights into the factors controlling virus retention during virus filtration. Biotechnol. Bioeng. 2014;111: 545-551. (c) 2013 Wiley Periodicals, Inc.