Self-assembling biological systems hold great potential for the synthetic construction of new active soft nanomaterials. Here we demonstrate the hierarchical bottom-up assembly of bacteriophage P22 virus-like particles (VLPs) that encapsulate the thermostable CelB glycosidase creating catalytically active nanoreactors. The in vivo assembly and encapsulation produces P22 VLPs with a high packaging density of the tetrameric CelB, but without loss of enzyme activity or the ability of the P22 VLP to undergo unique morphological transitions that modify the VLPs internal volume and shell porosity. The P22 VLPs encapsulating CelB are also shown to retain a high percentage of the enzyme activity upon being embedded and immobilized in a polymeric matrix. © 2012 The Royal Society of Chemistry.