In situ affinity foam fractionation is a potential powerful tool for continuous, selective removal of products from bioprocesses. When evaluating its applicability to cellulase production by Trichoderma reesei fermentation, we encountered the difficulty of significant removal of fungal mycelia along with the cellulase. To solve this problem, cell immobilization using cut pieces of hydrophilic polyurethane (PU) foam was evaluated. Five commercial PU foams with different pore sizes and porosities were tested. Two were found to support good cell growth, cellulase production, and cell loading (about 0.6 g dry cells per g PU). The PU-immobilized mycelia were successfully retained in the foaming process.