Although the use of fungi in the production of antibiotics, organic acids, end enzymes by conventional fementation techniques is well known, immobilized viable filamentous fungal systems have been little studied. In the present work, the tendency of fungi to attach firmly in nature on surfaces has been utilized in immobilizing the production organism on a solid, inert carrier for the production of lignocellulolytic enzymes by Phanerochaete chrysosporium and Trichoderma reesei. For the immobilization, nylon-web and polyurethane foam were employed as carrier materials. No release of biomass from the carrier into the medium during the fermentation process was observed in either case. Both with P. chrysosporium lignin peroxidase and with T. reesei endoglucanase and xylanase, the immobilized biocatalyst system resulted in higher enzyme activities in comparison to the free mycelium, and the immobilization made repeated batch fermentations possible. Under optimal conditions the highest enzyme activities of 940 nkat ml^-1 for endoglucanase, 8020 nkat ml^-1 for xylanase, and 21 nkat ml^-1 (1251 U 1^-1) for lignin peroxidase were obtained. Several factors affecting enzyme production by immobilized filamentous fungi are discussed.