Abstract Fungal–bacterial interaction on decomposing beech leaves was investigated in a microcosm experiment. Sterilised 1 yr old leaves were inoculated with microbial populations isolated from beech forest soil: bacteria (inoculum B), fungi and bacteria (inoculum F+B), a cellulolytic Humicola sp. (inoculum H) and this fungus together with bacteria (inoculum H+B), respectively. The data for mineralization rates and fungal activities, the latter determined by a new enzymatic tool, suggested antagonistic interaction between fungi and bacteria. Carbon transformation in the microcosms, estimated as C mineralized plus net dissolved organic carbon (DOC) formation, was consistently higher when fungi were present in the inoculum as compared to a diverse community of bacteria acting alone. DOC quality was determined as the ability of the carbon to support denitrification with excess nitrate. The DOC quality was dependent on the inoculum type: B and F+B treatments resulted in similar DOC quality, but when the Humicola sp. was present (H and H+B treatment) the quality was low. Thus, in spite of the higher fungal carbon transformation a fungal-mediated flush of DOC made available to heterotrophic bacteria was not observed. Taking all data into account, the antagonistic interaction between fungi and bacteria was most likely explained by competition for C substrate.